text stringlengths 1 1.11k | source dict |
|---|---|
c#, reinventing-the-wheel, queue
There's another bug in Resize. Suppose your user says that they expect the capacity to be 1000. Fine; we allocate a thousand items for the array. Then they do 250 enqueues and a dequeue. What happens? The capacity is reset to 500!
The user said that they had an expectation of how much storage they'd need, in order to cut down on the number of resizes on the way there. But you possibly give them a resize on the way to 1000 items.
Keep track of the user's capacity preference, and use that as the minimum array capacity; don't shrink to less than that. They said they're cool with using that much memory, so honour that request.
There is no logic in here that will help with debugging or testing. For example, it would be nice to see something like an implementation of ToString that dumps out a comma-separated list. | {
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c++, pointers, polymorphism, object-detection
// #1
using machine_types = std::variant<
MachStrato,
MachFloat,
// ... add a new machine here
>
// #2
static constexpr auto _create(std::string_view s)
{
// whatever the logic to determine which machine to create from a
// string, put it here
if (name.contains("float"))
return MachFloat{s};
else // the default?
return MachStrato{s};
}
machine_types _mach;
};
// usage:
auto mach = Mach{"strato"};
mach.common_action();
// Accessing specific interface
if (user_wants_strato_action)
{
mach.as<MachStrato>.specific_strato_action(); // will throw std::bad_variant_access if wrong type
}
if (user_wants_float_action)
{
if (not mach.is<MachFloat>())
throw std::runtime_error("Need a float machine for this!");
mach.as<MachFloat>.specific_float_action();
} | {
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oscillators, differential-equations
Alternatively, we can decide that no real $\omega$ means we don't have a solution of the correct form, and we should look for other solutions. We already have a hint from above.
$$ x = A \exp(-rt)$$
Plug in our guessed solution to the differential equation
$$mx'' + bx' + kx=0$$
$$mr^2x+b(-r)x + kx=0$$
which gives the constraint on $r$
$$mr^2-br + k=0$$
$$r = \frac{-(-b) \pm \sqrt{b^2 - 4mk}}{2m}$$
So now we have the exact opposite constraint $b^2-4mk\ge 0$. Our imaginary oscillatory rate is now a decay rate!
Since we can add solutions to get a new solution, the most general is:
$$ x = A \exp(-r_1t) + B \exp(-r_2t)$$
Where $r_1,r_2$ are the two solutions to the constraint on $r$ (given by the $\pm$ in the quadratic formula). | {
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homework-and-exercises, forces, vectors
Title: Component of force field tangent to a curve I recently had to solve the following problem:
A force $F=10\hat x- 8\hat y$ is applied to an object that is constrained to travel towards increasing values of x along the path defined by $y=x^2$ and $z=0$. Find the component of $F$ that is tangent to this path at the point $(2,4,0)$. | {
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ros2, colcon, windows10, compilation, windows
Also, my build finishes now. Only a few connext related packages report a stderr output. I guess, I can ignore those as I do not want to use connext?
Comment by Dirk Thomas on 2018-08-31:
Thank you for sharing the logs. I am pretty confident that the PR will fix this case: https://github.com/colcon/colcon-output/pull/13#issuecomment-417714567
Comment by Dirk Thomas on 2018-08-31:
Yes, you can ignore the warnings about Connext if you don't want to use it.
There are also reports of the latest version of Visual Studio 2017 (15.8) having some new compiler errors. There is an issue here: https://github.com/osrf/osrf_testing_tools_cpp/issues/15
I can confirm that Bouncy compiles on Windows 10 with Visual Studio 2017 (15.0), try downgrading if possible. | {
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automata, nondeterminism
$$\delta'((a,b),x) = (\delta(a,x),\delta(b,x))$$
The only elements that will be accepted here are those for which there's a run from $s\rightarrow q$ and $q\rightarrow f$.
Now we have no way of knowing whether the 'midway point' of $xx$ while going through $A$ is indeed the state $q$. However, because of nondeterminism, we don't need to guess - we can make one copy of this for each state $q$. Then my hypothesis is that the accepted words of the union of all these NFAs are those which are doubly accepted by $A$. | {
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ros, ros-comm
The launch file hasn't changed so I doubt anything wrong with launch.
The file reported (module?) as missing is really not on the PC: defusedxml.xmlrpc. I do not know if it was ever on the computer and was never needed before, or it was there and is now gone.
The file is part of python, but the file that calls it is a ROS file and it does not make note of py version. This is the util.py file requesting the file.
"""
Utility routines for rosmaster.
"""
try:
from urllib.parse import urlparse
except ImportError:
from urlparse import urlparse
try:
from xmlrpc.client import ServerProxy
except ImportError:
from xmlrpclib import ServerProxy
from defusedxml.xmlrpc import monkey_patch
monkey_patch()
del monkey_patch | {
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algorithms, graphs, optimization, np-complete, weighted-graphs
For a set of items $I$ and the number of bins $k$, create a complete bipartite graph of size $k + |I|$.
Each left vertex corresponds to a bin and has weight $0$ (or an appropriately small number if strictly-positive weight is required).
Each right vertex corresponds to an item and has the item's weight.
Finally, a root vertex is added and connected to all $k$ left vertices.
A proper subtree of a spanning tree of the constructed graph corresponds to a set of items packed in a bin. The subtree weight corresponds to the total item weight. A bin packing solution with bin capacity $C$ corresponds to a spanning tree with maximum subtree weight $C$. | {
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nlp
Distance between any pair is the same, and effectively by knowing the vector of "rose" we gain no information about the related "tulip" or unrelated (or less related) "dog" and "cat", hence the name "localist".
But if we happen to find a distributed representation like
dog = [1.0, 0.1]
cat = [1.1, 0.2]
rose = [0.0, 1.2]
tulip = [0.1, 1.3]
The vector of "rose" alone gives us clues about other flowers like "tulip", which are closer, and also about other entities like animals, which are farther, hence a name like "globalist".
Two notes:
Being "localist" is not a zero / one thing. The more clues a representation gives about other data points, the less it is localist.
The notion of "relatedness" is task dependent. For example, in the task of "grouping things with similar colors", a red car is more related to a red rose than a blue car.
Here is a random image of those entities for fun!
If all pairs look equally different, your brain works with "localist" vectors, thus has no clue ;) | {
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ssh
Originally posted by Tully with karma: 1145 on 2011-09-09
This answer was ACCEPTED on the original site
Post score: 2
Original comments
Comment by tfoote on 2011-09-10:
ROS uses ports allocated to it by the system, so they're not known ahead of time which makes setting up the forwarding hard, unless you basically forward all ports.
Comment by Ibrahim on 2011-09-09:
That's what I had thought, but after figuring it out I'm still a little curious as to whether it's possible to do it via ssh tunnels or not. But I guess because ROS nodes use a lot of different ports it'd be hard to do. | {
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ros, turtlebot, build, pointcloud-to-laserscan, make
I have ROS-Hydro and Ubuntu 13.10
Thank you in advance.
Originally posted by Wojciech on ROS Answers with karma: 66 on 2013-11-10
Post score: 0
In ROS Hydro, all ROS messages were removed from PCL. This requires that code be updated to use pcl_conversions, see the migration guide for details: http://wiki.ros.org/hydro/Migration#PCL
You might check if there is a version already updated for Hydro, or if you are using the Turtlebot, check out the replacement depthimage_to_laserscan which offers significantly better performance
Originally posted by fergs with karma: 13902 on 2013-11-11
This answer was ACCEPTED on the original site
Post score: 2 | {
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algorithms, graphs, optimization, dag
Find one root $r$ of $G$ by repeatedly going to the ancestor of a node. Mark $r$ as an wanted vertex.
Remove all vertices reachable from $r$.
Go to step 1 unless there is no more vertex.
return all vertices that are marked as wanted.
Exercise. Given a directed graph (that may have cycles), describe an algorithm that finds a minimum set of vertices from which all vertices can be reached. (Hint, strongly connected components are useful now. Try reducing the problem to the current question.) | {
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statistical-mechanics, collision, plasma-physics, stochastic-processes
Title: What is the Fokker-Planck collision operator and how is it derived? On page 7 of Goldston and Towner (1981) they state that "The Fokker-Planck collision operator for pitch-angle scattering is given by
$$\left.\frac{\partial f}{\partial t}\right|_c=\frac{\nu_{ii}}{2}\frac{\partial}{\partial \xi}\left(1-\xi^2\right)\frac{\partial}{\partial \xi}f,$$
where $\xi =v_{||} / v$."
Do you know how this equation is derived? Are there any books or lecture notes you would recommend which explain and give a background to the Fokker-Planck collision operator? To answer your question I first state some general formulas that you can find e.g. in the classic paper https://doi.org/10.1103/PhysRev.107.1 but also in most books on plasma kinetic theory.
Fokker-Planck Operator in a Plasma
Generally, a Fokker-Planck collision operator has the form
\begin{align} | {
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c#, object-oriented, parsing, functional-programming, youtube
Title: Collecting YouTube links from a page I wrote code for my needs (i.e. collecting YouTube links from a page) and it is simple, but now I just want to know what can I do just to make it a better and properly written one. For example, I think I should have try/catch blocks/exceptions in my methods, or get/set properties. What tests can I write to test the code? I don't know to be honest, but how would the code look if you had written it?
public class Parser
{
public static void Main()
{
Console.WriteLine("Enter the url of the page:");
var url = Console.ReadLine();
var numOfPages = GetNumberOfPages(url);
var numOfLinks = 1; | {
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general-relativity, fluid-dynamics, tensor-calculus, stress-energy-momentum-tensor
$$T^{\alpha\beta} = (e + p)u^{\alpha}u^{\beta} + pg^{\alpha\beta}.$$
Assuming units where $c = 1$. As you have correctly pointed out
$$T^{00} = (e + p)u^{0}u^{0} - p = e,$$
where you have correctly shown $u^{0}u^{0} = 1$.
Now, the mistake comes from the way you have derived the four-velocity from the expression for the metric. The correct way to derive the four-velocity here is to first write the four-velocity as
$$u^{\alpha} = \frac{\mathrm{d}}{\mathrm{d}\tau}(x^{0}, x^{i})^{\mathsf{T}},$$
which gives
$$u^{0} = \frac{\mathrm{d}t}{\mathrm{d}\tau} = \gamma.$$
Then of course, in the fluid rest frame we get $(u^{0})^{2} = 1$ as before.
I hope this helps. | {
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• @ABlumenthal I actually noticed the pun, after I made it $:)$. – Ali Aug 20 '13 at 1:43
$e^x=1+x+\dfrac{x^2}{2!}+\cdots+\dfrac{x^k}{k!}+\cdots$
Differentiating both sides,
$e^x=0+1+\dfrac{2x}{2!}+\cdots+\dfrac{kx^{k-1}}{k!}+\cdots$
Multiplying both sides by $x$,
$xe^x=x+\dfrac{2x^2}{2!}+\cdots+\dfrac{kx^k}{k!}+\cdots$
Again differentiating both sides,
$(x+1)e^x=1+\dfrac{2^2x}{2!}+\cdots+\dfrac{k^2x^{k-1}}{k!}+\cdots$
Now putting $x=1$ gives,
$2e=\dfrac{1^2}{1!}+\dfrac{2^2}{2!}+\cdots+\dfrac{k^2}{k!}+\cdots$ | {
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neural-networks, optical-character-recognition
When the network returns this output, my program will tell me that he identified the image as number 3.
Now by looking at the values, even though the network recognized the image as 3, the output value of number 3 was actually very low: $0.1015$. I am saying very low, because usually the highest value of the classified index is as close as 1.0, so we get the value as 0.99xxx.
May I assume that the network failed to classify the image, or may I say that the network classified the image as 3, but due to the low value, the network is not certain?
Am I right thinking like this, or did I misunderstand how does the output actually works? From the values you have provided I can easily guess your output layer has the sigmoid (do clarify!) activation function. For sigmoid activation function this can be a quite normal occurrence. Also maybe the number of training epochs is not high enough. | {
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Hint : If $n$ is some integer, then
$n=0 \mod{4} \implies n^2=0 \mod{4}$
$n=1 \mod{4} \implies n^2=1 \mod{4}$
$n=2 \mod{4} \implies n^2=0 \mod{4}$
$n=3 \mod{4} \implies n^2=1 \mod{4}$
Apart from @Vincent's answer, here's another way of looking at it $$x=\sqrt{2-4q}$$ Clearly, $2-4q$ is divisible by $2$ but not by $4$ for any integer $q$. Thus, it cannot be a perfect square.
Let $x \in \Bbb Z$ be a solution. Of course $x$ must be even, since an odd $x$ would make $2-x^2$ odd too.
Now let $x=2y$, then $2-x^2=2-4y^2=2(1-2y^2)$. But $1-2y^2$ is odd, so $4$ can't divide $2-x^2$.
This shows there doesn't exist solutions for that equation in $\Bbb Z$
This answer builds completely on GoodDeed's excellent answer, but I think going into further detail is useful.
Consider that
$$x=\sqrt{2-4q} = \sqrt{2 (1 - 2q)} = \sqrt 2 \sqrt{1 - 2q}$$
$1 - 2 q$ is odd for all $q \in \Bbb Z$. This means that $x$ must be irrational because $\sqrt{1 - 2q}$ will never have a factor of $\sqrt 2$. | {
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optics, astronomy, reflection, metals, telescopes
As you can probably tell from the soft background and such it was taken with a lens with a very large aperture (50mm f/1.4), and as you can (hopefully not) see the different wavelengths of light have actually gone different paths through the lens. Notice:
As you can see, "glass" isn't even very good at treating the visible wavelengths the same way. A picture is worth a thousand words.
I used a good lens, which is why the effect is so small, but this shows the principle behind it.
Exhibit D
This is a microwave door - it is opaque to microwaves, but as you can see, lets visible light through. (See Faraday Cage)
Exhibit E
WiFi. It can pass through walls and doors.
It should be clear now that light doesn't exactly behave like what our brain calls "light"
Finally
I hope this helps. As you can see - WAAAY to long for a comment. | {
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large-hadron-collider
The particles in the LHC are ultra-relativistic and move at 0.999997828 times the speed of light at injection and 0.999999991 the speed of light at top energy
Energy in beam.
Total beam energy at top energy, nominal beam, 362 MJ
$2808 \,\text{bunches} * 1.15\times 10^{11}$ protons @ 7 TeV each. = $2808*1.15*10^{11}*7*1012*1.602*10^{-19}$ Joules = 362 MJ per beam
The bunches are generally separated by about 7.5 m or 25 ns. (this is actually 10 RF buckets - with an RF frequency of around 400.8 MHz). However, there are some holes in the bunch structure.
The biggest is the beam abort gap of 3 microseconds (900 m.) - this is there to give the beam dump kickers time to get up to full voltage. There are also other smaller gaps in the beam which arise from similar needs from the SPS and LHC injection kickers.
A rough idea of how the bunches are inserted. | {
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ros, package.xml, cmake
Original comments
Comment by gvdhoorn on 2016-01-31:
Related: How to move packages from build to devel folder.
Comment by gvdhoorn on 2016-01-31:
Don't post answers if you're not answering your own question. If you want to update your question (to provide us with more information fi), then please use the edit button/link.
Comment by gvdhoorn on 2016-01-31:
re: deadlock: there is no deadlock. As the documentation states, just add all your add_library(..) and add_executable(..) statements after the catkin_package(..) statement.
The only obvious thing that I would warn you about is the ordering; see my answer to: http://answers.ros.org/question/225447/how-to-move-packages-from-build-to-devel-folder/
The ordering requirement is the last item on this page: http://docs.ros.org/jade/api/catkin/html/howto/format2/catkin_overview.html and is also documented on the wiki: http://wiki.ros.org/catkin/CMakeLists.txt | {
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physical-chemistry, acid-base, equilibrium
Now, many problems that I've read on the calculation of $\mathrm{pH}$ of weak acids that doesn't take $\ce{H+}$ due to dissociation of water into account. I speculated that maybe in the expression of $K_\mathrm{a}$, the $\ce{[H+]}$ is the total hydrogen ion concentration (including hydrogen ions from water). Is that true? Or is that an approximation we have to make?
Which one of the following correctly represents acid dissociation in water?
$$\ce{H2O + HA <=> H3O+ + A-},\tag{R1}$$
$$\ce{HA <=> H+ + A-}?\tag{R2}$$
Does the first reaction imply that water also dissociated and the $\ce{H+}/\ce{H3O+}$ concentration takes into account the contribution from water? Your question and your formulas are really asking about two different things.
Let's look at your formulas first.
$$\ce{H2O + HA <=> H3O+ + A-},\tag{R1}$$
$$\ce{HA <=> H+ + A-}?\tag{R2}$$
For R1 the equilibrium equation should be:
$$\mathrm{K_a} = \dfrac{\ce{[H3O+][A-]}}{\ce{[H2O][HA]}}\tag{Eq-1}$$ | {
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D = xcorr2(M1,M2);
DD = D(0+r2,2+c2)
DD =
585
Given a matrix of size and a matrix of size , their two-dimensional cross-correlation, , is a matrix of size with elements
is the trace and the dagger denotes Hermitian conjugation. The matrices and have size and nonzero elements given by
and
Calling xcorr2 is equivalent to this procedure for general complex matrices of arbitrary size.
Create two complex matrices, of size and of size .
X = randn([7 22])+1j*randn([7 22]);
H = randn([6 17])+1j*randn([6 17]);
[M,N] = size(X);
m = 1:M;
n = 1:N;
[P,Q] = size(H);
p = 1:P;
q = 1:Q;
Initialize and .
Xt = zeros([M+2*(P-1) N+2*(Q-1)]);
Xt(m+P-1,n+Q-1) = X;
C = zeros([M+P-1 N+Q-1]);
Compute the elements of by looping over and . Reset to zero at each step. Save time and memory by summing element products instead of multiplying and taking the trace.
for k = 1:M+P-1
for l = 1:N+Q-1
Hkl = zeros([M+2*(P-1) N+2*(Q-1)]);
Hkl(p+k-1,q+l-1) = H;
C(k,l) = sum(sum(Xt.*conj(Hkl)));
end
end | {
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"url": "http://se.mathworks.com/help/signal/ref/xcorr2.html?requestedDomain=se.mathworks.com&nocookie=true"
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algorithms, graphs, shortest-path, linear-programming
I'd really love your help with understanding what does $x_e$ represent. Is it the number of times I use $e$ in order to find all the shortest paths in the graph?
I don't understand why does the above condition for this linear program is as at it, why does $\sum\limits_{e\in B(v)}x_e-\sum\limits_{e\in F(v)}x_e=b_v$ for all $v \in V$ should be $1$ for every node and $-(n-1)$ for the all the root? If I think of a $3$ nodes tree for a graph,for the middle node we get that the condition equals to $1$, which makes me think that I might be misunderstood what $x_e$ stands for. This formalization takes Dijkstra's algorithm and formalizes it as a network flow problem. | {
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"openwebmath_score": null,
"tags": "algorithms, graphs, shortest-path, linear-programming",
"url": null
} |
c++, node.js, memory-management, curl
inline MemWrapper* ClientToMemWrapper( void* client ) {
return reinterpret_cast<MemWrapper*>( static_cast<char*>( client ) - MEMWRAPPER_SIZE );
}
void AdjustMem( ssize_t diff )
{
Nan::AdjustExternalMemory( static_cast<int>( diff ) );
}
void* MallocCallback( size_t size )
{
size_t totalSize = size + MEMWRAPPER_SIZE;
MemWrapper* mem = static_cast<MemWrapper*>( malloc( totalSize ) );
if ( !mem ) return NULL;
mem->size = size;
AdjustMem( totalSize );
return MemWrapperToClient( mem );
}
void FreeCallback( void* p )
{
if ( !p ) return;
MemWrapper* mem = ClientToMemWrapper( p );
ssize_t totalSize = mem->size + MEMWRAPPER_SIZE;
AdjustMem( -totalSize );
free( mem );
} | {
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"tags": "c++, node.js, memory-management, curl",
"url": null
} |
soft-question
speed of light, etc. But it sounds to me like you are looking for an optimal learning path. One that takes up as little time as needed to maximize your skill set. I am not sure that there is a perfect ratio of theory to application. It is probably different for each person. But I would offer some advice. It is often said that we should play to our strengths while working on strengthening our weaknesses. So, be honest with yourself, are you really great at deriving and sloppy at solving? Or, is solving problems really easy and uninteresting for you? If you can do it well no need to bother practicing. But if you are better at one than the other you might want to spend more time on the thing you are bad at. | {
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quantum-field-theory, operators, wick-theorem
where $\hat{\phi}_k^{-\dagger}$ annihilates an antiparticle in state $k$ and $\hat{\phi}_k^{+\dagger}$ creates a particle in state $k$, located at $\vec{r}$. This is why $\hat{\phi}^-_k$ and $\hat{\phi}^{-\dagger}_k$ represent different things and why $\hat{\phi}_k^-$ and $\hat{\phi}^+_{-k}$ are equivalent.
Notice that these are all expressed in the Schrodinger picture. Time dependence appears when one performs an unitary transformation to another picture, for example the interaction picture, such that
$$\hat{\Phi}_I(\vec{r},t)=\underbrace{\hat{\phi}^{-}_{kI}(\vec{r},t)}_{\displaystyle\mathrm{creation}}+\underbrace{\hat{\phi}^{+}_{kI}(\vec{r},t)}_{\displaystyle\mathrm{annihilation}},\quad\hat{\Phi}^\dagger_I(\vec{r},t)=\underbrace{\hat{\phi}^{-\dagger}_{kI}(\vec{r},t)}_{\displaystyle\mathrm{annihilation}}+\underbrace{\hat{\phi}^{+\dagger}_{kI}(\vec{r},t)}_{\displaystyle\mathrm{creation}}$$ | {
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"tags": "quantum-field-theory, operators, wick-theorem",
"url": null
} |
algorithm, strings, programming-challenge, typescript
{ string1: '', string2: '', expectedResult: 0 },
{ string1: '', string2: ' ', expectedResult: 1 },
{ string1: ' ', string2: '', expectedResult: 1 },
{ string1: '', string2: ' ', expectedResult: 2 },
{ string1: ' ', string2: '', expectedResult: 2 },
{ string1: '', string2: 'x ', expectedResult: 3 },
{ string1: 'x ', string2: '', expectedResult: 3 },
{ string1: 'a', string2: 'a', expectedResult: 0 },
{ string1: 'ab', string2: 'a', expectedResult: 1 },
{ string1: 'abc', string2: 'a', expectedResult: 2 },
{ string1: 'abc', string2: 'abc', expectedResult: 0 },
{ string1: 'a', string2: 'ab', expectedResult: 1 },
{ string1: 'a', string2: 'abc', expectedResult: 2 }, | {
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"tags": "algorithm, strings, programming-challenge, typescript",
"url": null
} |
visible-light, double-slit-experiment, wave-particle-duality
So the emission as well as the absorption of EM radiation is explained by photons. The deflection of EM radiation withe a swelling intensity distribution behind edges and the deflection in transparent media (lenses for example) are interpreted as coming from the wave characteristics of light in analogy to water waves:
But such an interpretation has some weaknesses:
Comparing the both sketches you should agree that in the upper sketch of water waves the drawn points of minimum intensity C and D are moving “upwards” an E and F are moving “downwards”. This is not the case for the intensity distribution of EMR behind slits. How Young could not point this out? The analogy is missing an important point.
An intensity distribution from single emitted photons occurs after a while even behind single edges. The explanation of a path difference from a coherent light source is obsolet in this experiment.
what these attempts were and how they failed? | {
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python, beginner, python-3.x, game, tkinter
def initialize_lesson_menue():
"""
Initializes the lesson menu
"""
lesson_menue = InitializeScreen(FRAMES[1], 2)
lesson_menue.home_button()
lessons()
def lessons():
"""
Creates the lesson frame, and the corresponding buttons
"""
lessonframe = tkinter.Frame(
FRAMES[1],
bg="gray",
borderwidth=2,
relief="sunken"
)
lessonframe.grid(column=1, row=1)
lessonframe.grid_rowconfigure(0, weight=1)
lessonframe.grid_rowconfigure(1, weight=1)
current_frame = lessonframe
##############################################################################
## The items below list out the titles of all of the different lesson plans ##
##############################################################################
letters_button = Button(
current_frame,
text="Letters",
font=("Helvetica", 25),
command=letters
) | {
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"tags": "python, beginner, python-3.x, game, tkinter",
"url": null
} |
thermodynamics, fluid-dynamics, evaporation
Title: Fluid filled open container in space/vacuum I have water in a bottle, as I open it in space(or vacuum for that case ). From what I have read in books the water will instantly vapourize,as the vapour pressure of the water inside, far exceeds the atmospheric pressure.
I have a different opinion, that the water will freeze, owing to the fact that vapourizing water from the surface will take away the heat from the lower part.How far is this assumption true? What factors will determine the second statement, if it is true(liquid nature,vessel shape,quantity)? As the astronauts observed, it first boils and then slowly freezes. If you put water in space, the water would start to boil on its own due to the fact that the vapor pressure would exceed the pressure of the environment the water is in. And, after that, it would start to crystallize. | {
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"tags": "thermodynamics, fluid-dynamics, evaporation",
"url": null
} |
general-relativity, black-holes, kerr-metric
Second alternative is to consider black holes that are not asymptotically flat. In the mentioned above Plebański–Demiański family solutions with non-zero NUT and/or acceleration parameters are not asymptotically flat and do not have equatorial reflection symmetry.
Third alternative is to consider black holes that are not stationary. For example, accretion disks or other forms of matter around black hole may break reflection symmetry and backreaction would ensure that this symmetry breaking carries over to the metric itself. Another possibility is that binary black hole coalescence can occur without reflection symmetry.
In standard general relativity deviations from equatorial reflection symmetry in realistic astrophysical scenarios are expected to be either very small or very transient, however they may become more pronounced for theories differing substantially from GR in the strong gravity regions. | {
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"tags": "general-relativity, black-holes, kerr-metric",
"url": null
} |
installation, ignition-fortress
Title: Fortress windows Binary Install
Hi all!
I wanted to try out ignition Fortress on windows 10, so I followed these installation instructions of the ignitionrobotics website: https://ignitionrobotics.org/docs/fortress/install_windows
I managed to install all the packages in the Conda environment and double checked the library version numbers here https://ignitionrobotics.org/docs/fortress/install#fortress-libraries to what is installed with conda list. 'ign' is recognized, I do not see 'gazebo' as part of the commands under 'ign help' so I can't try out any of the getting started examples. Also, with any other command, I get the following error:
Library error: ignition-tools-backward.dll not found. Improved backtrace generation will be disabled
Missing resource type (e.g. --type model). | {
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"tags": "installation, ignition-fortress",
"url": null
} |
10.1 Introduction In chapter 10 we discuss the Fourier series expansion of a given function, the computation of Fourier transform integrals, and the calculation of Laplace transforms (and inverse Laplace transforms). The concepts of innite series and improper integrals, i.e. | {
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"lm_q2_score": 0.8438950966654774,
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"openwebmath_score": 0.9573472142219543,
"tags": null,
"url": "http://stickycompany.com/state/92008148c4351fe540f431df38d77a1f91"
} |
c++
Board::Board() {
Ships[0] = Ship("Carrier", 5, sunk, 0);
Ships[1] = Ship("Battleship", 4, sunk, 1);
Ships[2] = Ship("Cruiser", 3, sunk, 2);
Ships[3] = Ship("Submarine", 3, sunk, 3);
Ships[4] = Ship("Destroyer", 2, sunk, 4);
}
Ship& Board::operator()(int x, int y) {
return gameBoard[y][x];
}
//returns bool based on if coords are valid direction
bool Board::isValidDirection(int x1, int y1, int x2, int y2) {
if(x1 == x2 || y1 == y2) { return true; }
return false;
}
//returns bool based on if range is occupied or not
bool Board::rangeIsOccupied(int x1, int y1, int x2, int y2) {
//if horizontal
if(y1 == y2) {
while(x1 != x2) {
if(gameBoard[y1][x1].getStatus()) {
return true;
}
if(x1 > x2) {
x1--;
}
else {
x1++;
}
}
if(gameBoard[y1][x1].getStatus()) {
return true;
}
} | {
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• Now there are 2 downvotes. This is an absolute disgrace. Is there an appeals process, in order to correct this injustice just as a matter of principle? I could care less about the -4 points. I have both answered the OP"s question as perhaps intended (even though that apparent intention is based on a complete lack of sophistication in probability), and provided a mathematically correct answer, based on what anyone who has taken an elementary college level probability course should know. Edit: Now there are 3 downvotes - apparently people don't like my attitude. – Mark L. Stone Jun 12 '15 at 18:45 | {
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"url": "https://math.stackexchange.com/questions/1322690/frequency-from-probability"
} |
robotic-arm, forward-kinematics, dh-parameters
Title: Transformation matrix using DH parameters I want to write a Matlab function which takes the DH parameters as input and outputs a 4X4 transformation matrix.
The code I have written is :
function [A] = compute_dh_matrix(r, alpha, d, theta)
A = eye(4);
% ROTATION FOR X
A(1,1) = cos((theta));
A(2,1) = sin((theta));
% ROTATION FOR Y
A(1,2) = -(sin(theta))*(cos(alpha))*(1);
A(2,2) = (cos(theta))*(cos(alpha));
A(3,2) = (sin(alpha));
% ROTATION FOR Z
A(1,3) = (sin(theta))*(sin(alpha));
A(2,3) = -(cos(theta))*(sin(alpha))*(1);
A(3,3) = (cos(alpha));
% TRANSLATION VECTOR
A(1,4) = (alpha)*(cos(theta));
A(2,4) = (alpha)*(sin(theta));
A(3,4) = d;
end
But when I submit the code for evaluation in an online platform it prompts that variable A has incorrect value.
One of the input data used for evaluation of the code is :
r = 5;
alpha = 0;
d = 3;
theta = pi/2; | {
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This may have led some to believe that Banach algebras would now solve all our problems. Of course, they could not, but they did provide the right framework for many questions in analysis (as does most of functional analysis) and conversely, abstract questions about Banach algebras often gave rise to interesting problems in “hard analysis”. (Hard analysis is used here as Hardy and Littlewood used it. For example, you do hard analysis when, in order to estimate some integral, you break it into three pieces and apply different inequalities to each.)
One type of Banach algebras that was soon studied in detail were the so-called function algebras, also known as uniform algebras.
To see what these are, let $C(X)$ be the set of all complex-valued continuous functions on a compact Hausdorff space X. A function algebra on X is a subset A of $C(X)$ such that
(i) If f and g are in A, so are $f+g$, $fg$, and $cf$ for every complex number c (this says that A is an algebra). | {
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ros, ros-kinetic, bebop-autonomy, raspberrypi, parrot
As of today, the released version is at version 3.9.x. The 3.10.x release will come out during the next ROS release (sync) cycle. You can get it from ROS shadow-fixed repositories.
At the moment bebop_autonomy does not use this version of parrot_arsdk. Instead it builds it inline during compilation. This will change soon. To see an example of how to force bebop_autonomy to use the packaged parrot_arsdk, take a look at the following branch of bebop_autonomy: https://github.com/AutonomyLab/bebop_autonomy/tree/2-parrot-sdk . Specifically the changes in bebop_driver/package.xml and bebop_driver/CMakeLists.txt. | {
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"tags": "ros, ros-kinetic, bebop-autonomy, raspberrypi, parrot",
"url": null
} |
atmospheric-science, humidity, projectile
Title: How does humidity affect the path of a bullet? Background
Last night, I was reading the FM 23-10 (The U.S. army official field manual for sniper training), and I've noticed that they're potentially teaching snipers incorrect information.
Generally speaking, when we say "impact goes up" it means that the bullet was either somehow made faster or its path was easier, therefore the curve in its ballistic trajectory is smoother. Thus, it will hit higher. When we say impact goes down, we mean the opposite.
For example, atmospheric heat will, loosely speaking, make the air "thinner" and therefore the impact will be higher. Cold weather will do the opposite. This part is correct.
What about humidity?
The FM 23-10 says:
The sniper can encounter problems if drastic humidity changes occur in
his area of operation. Remember, if humidity goes up, impact goes
down; if humidity goes down, impact goes up. | {
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beginner, rust, sudoku, depth-first-search
x x 1 x 6 8 5 2 x x x x 2 x x x x 1 x x 2 5 1 9 x 3 x x x 8 x 2 4 7 x x 6 x 4 x x x 3 x 2 x x 7 3 8 x 9 x x x 9 x 1 3 5 2 x x 4 x x x x 6 x x x x 8 5 4 7 x 6 x x
Here's my code:
#[macro_use]
extern crate text_io;
use std::io;
// struct to represent a sudoku
struct Sudoku {
size: u32,
board: Vec<Vec<u32>>,
}
fn generate_sudoku(size: u32) -> Sudoku {
let dim = (size * size) as usize;
let mut sudoku = Sudoku {
size: size,
board: Vec::new(),
};
// fill the sudoku with the specified values
for row in 0..dim {
sudoku.board.push(Vec::new());
for _ in 0..dim {
let num: String = read!();
let mut is_number = true;
for ch in num.chars() {
if !ch.is_digit(10) {
is_number = false;
break;
}
} | {
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"tags": "beginner, rust, sudoku, depth-first-search",
"url": null
} |
advice-request
Concrete Mathematics, by Graham, Knuth and Patashnik
Some basics of optimization (especially linear programming) are a must for computer scientists. Here, I would recommend the following gentle introduction:
Understanding and Using Linear Programming, by Matoušek and Gärtner
A well written (and not very demanding) introduction to randomized algorithms is the following:
Design and Analysis of Randomized Algorithms, by Hromkovič
Finally, some nice books on advanced algorithms and/or computation theory include:
The Design and Analysis of Algorithms, by Kozen
Theory of Computation, by Kozen
Algorithmics for Hard Problems, by Hromkovič
Complexity and Approximation, by Ausiello et al.
Randomized Algorithms, by Motwani and Raghavan | {
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c#, web-services, wcf
Advantages
The "service locator" can act as a simple run-time linker. This allows code to be added at run-time without re-compiling the application, and in some cases without having to even restart it.
Applications can optimize themselves at run-time by selectively adding and removing items from the service locator. For example, an application can detect that it has a better library for reading JPG images available than the default one, and alter the registry accordingly.
Large sections of a library or application can be completely separated. The only link between them becomes the registry.
But this comes at a price:
Disadvantages | {
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If we divide the nine students into teams of three, then the $$3! = 6$$ divisions \begin{align*} & \{Amanda, Brenda, Claire\}, \{Dennis, Edward, Fiona\}, \{Gloria, Henry, Ivan\}\\ & \{Amanda, Brenda, Claire\}, \{Gloria, Henry, Ivan\}, \{Dennis, Edward, Fiona\}\\ & \{Dennis, Edward, Fiona\}, \{Amanda, Brenda, Claire\}, \{Gloria, Henry, Ivan\}\\ & \{Dennis, Edward, Fiona\}, \{Gloria, Henry, Ivan\}, \{Amanda, Brenda, Claire\}\\ & \{Gloria, Henry, Ivan\}, \{Amanda, Brenda, Claire\}, \{Dennis, Edward, Fiona\}\\ & \{Gloria, Henry, Ivan\}, \{Dennis, Edward, Fiona\}, \{Amanda, Brenda, Claire\} \end{align*} are all equivalent since they result in the same three teams. Therefore, the number of ways of dividing the class into three unlabeled teams of three is $$\frac{1}{3!}\binom{9}{3}\binom{6}{3} = \frac{1}{3!} \cdot \frac{9!}{3!3!3!}$$ We divide by $$3!$$ to account for the $$3!$$ orders in which we could select the same three teams of three. | {
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python, performance, python-3.x, natural-language-processing, data-mining
import numpy as np
import pandas as pd
table = np.zeros((len(word_list),len(word_list)), dtype=int)
for sent in text.split('.'):
for i in range(len(word_list)):
for j in range(len(word_list)):
if word_list[i] in sent and word_list[j] in sent:
table[i,j]+=1
df = pd.DataFrame(table, columns=word_list, index=word_list)
# Count the number of occurrences of each word in word_list
all_words = pd.DataFrame(np.zeros((len(df), 2)), columns=['Word', 'Count'])
all_words.Word = df.index
for sent in text.split('.'):
count=0
for word in sent.split():
if word in word_list:
all_words.loc[all_words.Word==word,'Count'] += 1
# Sort the word pairs in decreasing order of their association strengths
df.values[np.triu_indices_from(df, 0)] = 0 # Make the upper triangle values 0 | {
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antimatter, neutrons
Title: Will an anti-neutron annihilate a regular neutron even though they have no charge Since neutrons have no electric charge will an anti-neutron annihilate when it comes into contact with a regular neutron as protons and anti-protons do? if so what causes it to annihilate? Neutron and antineutron have also an internal structure formed by quarks.
A neutron is composed by 2 down quarks, each with a charge -1/3, and one up quark with a charge of 2/3. This gives a total charge of 0.
Antineutron are different, they have 2 anti-down quarks (each with a charge of 1/3) and one antiup quark (-2/3) of charge.
So, even though they have a total charge of zero they still are antiparticles. In a simplified view, it's like if they have a mirrored internal structure. | {
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kinetics
Title: If the rate law order is three, does that mean that three bodies collide at some elementary step? I am doing a chemistry review about rates, and I get asked "Which of these statements is false?"
The answer is
In reactions that are second order in one reactant and first order in another, the slow step generally involves a three-body collision of the reactants. | {
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navigation, mapping, ros-kinetic, 2dlidar, hector-mapping
The issue is inside <include file="$(find ydlidar)/launch/lidar.launch" /> if you look at the content of lidar.launch here there is this node launched too :
<node pkg="tf" type="static_transform_publisher" name="base_link_to_laser4"
args="0.2245 0.0 0.2 0.0 0.0 0.0 /base_footprint /laser_frame 40" /> | {
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c++, beginner, game, sdl
GameObject::~GameObject() should be marked virtual.
You only use Vector2 in Ball, storing in it the sine and cosine of the angle of movement. Since you aren't even using any advanced vector maths, why not simply store the angle instead?
Bug: in Ball::collisionCheck(Player&, Player&) your calculation for the Ball hitbox for the collision with player1 incorrectly uses the width instead of the height.
Naming: degree in Ball::collision(Player&) certainly does not store the angle in degrees. Maybe rename it to angle?
Ball::resetBall() might always reset the ball to the same state (angle of movement) since you always reseed the RNG to the same value. (Also, while rand() works as a "quick and dirty" RNG, you might want to look at the better capabilities in the <random> header.) | {
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filters, filter-design, classification, analog, hardware-implementation
Concerning your second question, the answer is also no. A Butterworth and a Chebyshev (type I) filter have the same structure, just the element values are different. The structure of an elliptic filter is different because it has zeros at finite frequencies. For passive filters, these are achieved either by parallel circuits of L and C in the signal path, or by series circuits of L and C towards ground. However, this is also true for type II Chebyshev filters, which also have zeros at finite frequencies. So if your passive filter topology has LC resonators (parallel in the signal path, or serial towards ground) you know that the filter can't have a Butterworth, Bessel, or type I Chebyshev characteristic. It could be an elliptic or a Chebyshev (type II) filter, but it needn't be. If the structure has no LC resonators, the opposite is true: it can't be an elliptic or a Chebyshev (type II) filter, but it could be a Bessel, Butterworth, or Chebyshev (type I) filter. But again, it could | {
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communication, technologies, cluster-states
In terms of sensing, there are ideas like quantum illumination and quantum radar that exploit stronger, non-classical correlations between photons to extract more information than is possible classically with less light, which is an exciting direction to probe systems with lots of background noise and/or that are perturbed by higher light intensities. | {
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php, codeigniter
Anybody have any more ideas? I see nothing that's wrong with your code. My first reaction though is that you repeat the echo quite a bit. I would turn all the
echo json_encode(array());
into something like this:
function output($message, $success = TRUE) {
$status = $success ? array('succes' => 'yes') : array('error' => 'yes');
echo json_encode($status, $message);
}
The two other things I would have preferred to do differently, is avoiding the deep intendation, and moving some logic out from the if()'s. This is perhaps just a matter of taste, but I find this much easier to read and follow.
function forgot_password_submit() {
$this->form_validation->set_rules('username', 'Username', 'trim|required|xss_clean');
if (!$this->form_validation->run()) {
output('There was a problem submitting the form! Please refresh the window and try again!', FALSE);
return;
} | {
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star, the-sun, classification
MS - Main sequence stars (spectral type V) fuse hydrogen into helium in the core. When the hydrogen in the core is depleted, the star becomes an RGB star.
RGB - Red Giant Branch stars (V->II/III) fuse hydrogen into helium in a shell around the core while the core is inert helium. At some point (Helium flash) the core temperature becomes high enough to start fusing helium into carbon and oxygen, the star then becomes an HB star.
HB - Horizontal Branch stars (II/III) fuse helium into carbon/oxygen in the core. A shell with inert helium surrounds the core, which in turn is surrounded by a hydrogen burning shell.
AGB - Asymptotic Gian Branch stars (I) have a carbon/oxygen core with several inert or fusion shells around it. | {
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electrons, electric-fields, quantum-electrodynamics, antimatter
The e- radiates a photon and becomes virtual, and meets the e+ and annihilates into another photon, conserving momentum and energy in the center of mass system. It has no meaning to be talking of fields within this context of the quantum mechanical framework. Again the two photons will have in their complex wavefunction information on electric and magnetic fields, but the classical picture does not work at this level. | {
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machine-learning, regression
Multiple vs single dependent variable
If you go with option 2 as you described it, with one output variable, you will be modeling a completely different variable than if you were modeling 9 (in your example) separate variables. Each of the original 9 variables likely has its own statistical properties, but if you combine them into one, you will have a new variable that has its own properties (some combination of the 9). So if you consider the grid to be 9 separate variables, you should stick with modeling 9 separate variables. | {
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ros, installation, catkin-make, ros-kinetic, cmake
The only thing I can think of is to remove all the ROS stuff from my environment variables, uninstall ROS, and let it set up everything again.
this would not change anything.
Your environment is incorrectly setup. There should not be any quotes, commas or such characters in there.
Several of your environment variable contain a single double quote: LC_ALL, PATH, LC_CTYPE. Please fix those to not contain quotes. I would expect it to work after that.
Originally posted by Dirk Thomas with karma: 16276 on 2020-01-07
This answer was ACCEPTED on the original site
Post score: 1 | {
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javascript, image, ecmascript-6
You keep track of the amount of loaded images in the callback function that triggers when any image is loaded. Pretty clever, but I would bet my house that keeping track of such things in this way will cause bugs. If you google "wait for all promises javascript", you learn of a built in function that takes care of all of this for you! Promise.all takes a list of promises, and returns a promise, that rejects when any promise in that list rejects, or resolves when all promises in the list have resolved. That is exactly what you are trying to achieve:
//using the "requests" list we created a couple examples back
const done = Promise.all(requests);
done
.then(images => console.log("Preloaded all", images))
.catch(err => console.error("Failed", err));
// > Preloaded all [Image, Image, Image, ...] | {
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c++, c++11, recursion, chess, n-queens
/* Iterate through 1..BOARD_SIZE queens based on X,Y availability lists */
void recurse(vector<queen_t> *board, set<uint8_t> *avail_x,
set<uint8_t> *avail_y, uint16_t cur_iteration) {
//cout << "CurIter: " << cur_iteration << ", queens placed: " << board->size() << endl;
_num_recursions ++;
/* Completion conditions */
uint16_t queens_left = BOARD_SIZE - board->size();
if (cur_iteration > BOARD_SIZE){
if (queens_left == 0) {
if (avail_x->size() != 0 || avail_y->size() != 0){
cout << "ERROR: Board is full, but avail lists are non empty!" << endl;
print_board(board);
exit(1);
}else{
//cout << "Adding solution, recursions so far: " << _num_recursions << endl;
_boards.push_back(*board);
}
}
return;
} | {
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c++, parsing, inheritance, symbolic-math
Again, I don't have your complete code so these semantics might need to be adjusted for your specific case, but in general this cuts down branch prediction and temporary usage (which the compiler can then optimize further).
Hope that can help. | {
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as $\underline{u}'(t) = A \underline{u}(t), \; \underline{u}(0) = \underline{u_0}$, where $\underline{u}(t) = \begin{pmatrix} x(t)&x'(t) \end{pmatrix}^T$ and
$$A = \begin{pmatrix} 0&1\\ -\omega^2&0 \end{pmatrix} \text{ and } \underline{u_0} = \begin{pmatrix} x_0\\ v_0 \end{pmatrix}.$$
Computing eigenvalues and eigenvectors, we find
$$\underline{u}(t) = c_1 e^{i \omega t} \begin{pmatrix} 1\\ i \omega \end{pmatrix} + c_2 e^{-i \omega t} \begin{pmatrix} 1 \\ -i \omega \end{pmatrix}.$$
Using the initial condition, we find $x(t) = x_0 \cos(\omega t) + \frac{v_0}{\omega} \sin(\omega t)$.
Matrix exponential: I don't know if your students already are familiar with the matrix exponential, but using it we find a solution of the homogeneous initial value problem to be given by
$$\underline{u}(t) = e^{(t-t_0)A} \underline{u_0}.$$ | {
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quantum-mechanics, terminology
I couldn't conceive what he was meaning by energy shift in the present context. Can anyone explain me what Feynman meant by 'energy shift' here? $$
\newcommand{\ket}[1]{\left| #1 \right\rangle}
\newcommand{\bra}[1]{\left\langle #1 \right|}
$$
Sooo. It's sometimes a little hard to connect with the way he does it here, because it's a long way from the way I (and I think most of us) think about QM. Not in interpretation or fact or anything, just that it's strange to think of a "negative energy particle hopping around." Let me explain what is meant by an energy shift, and hopefully you can understand how it connects. Let's say you start with a two-state system, where both states are disconnected. Call these states $\ket{1}$ and $\ket{2}$, so the Hamiltonian starts off looking like:
$$
H = E_1 \ket{1}\bra{1} + E_2 \ket{2}\bra{2}
$$ | {
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I've graphed the support for $X,Y$, all values where $f_{X,Y}>0$ are colored blue: | {
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lagrangian-formalism, entropy, variational-principle, density-operator, constrained-dynamics
$$S'=-\mathrm{Tr}\left[\hat \rho \ln \hat \rho\right]+\lambda\left(\mathrm{Tr}\left[\hat \rho \hat H-E\right]\right)+\mu\left(\mathrm{Tr}\left[\hat \rho\right]-1\right)\tag{5}$$
I just cannot understand why $1$ is being subtracted in the third term and $E$ is subtracted in the second term. This just seems non-sensical as from eqn. $(3)$ $$\mathrm{Tr}\left[\hat \rho\right]-1=0$$ so it is like $0$ is being added in as a constraint which makes no sense whatsoever to me.
Can someone please explain the logic behind putting factors of zero into the Lagrangian? | {
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Though I actually once worked out the result using limits (like Alexander Bogomolny does marvelously here), it’s easy enough to work out the result in our heads:
First ask yourself how many digits a randomly selected integer has. The number of digits is almost certainly greater than 2, right? There are only 90 two-digit positive integers, a finite number, and there are an infinite number of integers with more than two digits. It follows that if you were to pick one at random from among all positive integers*, it would be almost certain to contain more than two digits.
The same argument could be applied to a larger number of digits. By the same logic as above, we can convince ourselves that ‘most randomly selected integers have more than a trillion digits’. It’s a bit of an incredible statement, really. We rarely ever work with the ‘most-common’ kind of numbers (the big ones!).** | {
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## Tuesday, December 18, 2007
### Problem Solving - 31
A certain characteristic in a large population has a distribution that is symmetric about the mean m.If 68 percent of the distribution lies within one Standard Deviation d of the mean, what percent of the distribution is less than m+d?
A. 16%
B. 32%
C. 48%
D. 84%
E. 92%
In a normal bell curved distribution, 50% are below the mean and 50% are over it
If 68% are distributed within 1 S.D of the mean then this implies that 34% are 1 S.D above the mean and 34% are 1 S.D below the mean i.e
34% between m and m+d and 34% between m-d and m
The distribution is symmetric about m also => 32/2 = 16% between 0 and m-d and 16% m+d and above.
Hence total
that is less than m+d = 100-16 = 84%
OR
Distribution is symmetric around mean => 68/2 = 34% =>(Mean-S.D, Mean) = (Mean, Mean+S.D] = 34% . Thus below Mean+S.D = 50+34 = 84% | {
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syntax
Title: Why does the following Backus-Naur form reflect the following syntax diagram? The definition of the variable
< number>::=< digit > | < digit >< number >
where < digit > is defined as
< digit > ::= 1|2|3|4|5
Apparently reflects the following syntax diagram
Please explain why this is the case. I am particularly confused with the clause after the vertical OR line (i.e. < digit >< number >) and whether it has something to do with the fact that number can consist of many digits. < number>::=< digit > | < digit >< number > means that a number is either just a digit, or a digit followed by a number. So 1 is a number (just a single digit), and 12 is also a number (a single digit, followed by a number that consists of just the single digit '2'). Similarly 123 is a number that consists of a single digit '1' and a number (that itself consists of a single digit '2' followed by a number (that consists of the single digit '3')). | {
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general-relativity, black-holes, metric-tensor, symmetry, time-dilation
In Newtonian gravity (NG) it is known that the gravitational field anywhere inside a spherically symmetric distribution of mass is determined only by the enclosed mass. This is also widely believed to be true in general relativity (GR), and the Birkhoff theorem is often invoked to support this analogy between NG and GR. Here we show that such an understanding of the Birkhoff theorem is incorrect [...] The correct metric, matching continuously to the location of an external observer, is determined both by the enclosed mass and mass distribution outside. The effect of the outside mass is to make the interior clock run slower [...]
Further in the section 2.0 the author shows that "the time term of the metric is always maintained continuous, but the space term is not". There is no length contraction or expansion inside an empty shell, only time dilation. | {
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java, optimization
Any help or thoughts would be appreciated. That much repetition does have a smell to it. Can you simplify it by saving and reusing intermediate values similar to what I've shown below?
else if (currentLevel == 3)
{
Section level0Section = aMainSection.get(levelCount[0] - 1);
Section level1Section = level0Section.aChildSection.get(levelCount[1] - 1);
Section level2Section = level1Section.aChildSection.get(levelCount[2] - 1);
Section level3Section = level2Section.aChildSection.get(levelCount[3] - 1);
level3Section.addSection(text);
level3Section.addParentSection(level0Section.getSection());
level3Section.addParentSection(level1Section.getSection());
level3Section.addParentSection(level2Section.getSection());
} | {
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Thus we obtain $\sigma(P_n) = n$, as desired.
- | {
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"url": "http://math.stackexchange.com/questions/253257/number-of-simple-edge-disjoint-paths-needed-to-cover-a-planar-graph"
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homework-and-exercises, electromagnetism, general-relativity, vector-fields
It is somewhat uncommon to actually have the $E$ and $B$ fields written out in this way, however. Usually, by this point, you're firmly entrenched in index notation, and you just deal with the components of the Faraday bivector, usually denoted as $F_{\mu \nu}$.
Overall, though, you should be able to convert $\partial/\partial y$ into your preferred notation for a unit basis vector and then do Maxwell's equations as you're most familiar. | {
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Thanks for a fun question.
One term for this property is that they are “multiplicative functions”; you might think of it as distributing over multiplication. This is true of any power, not only $$x^2$$ and $$x^{1/2} = \sqrt{x}$$ and $$x^{-1} = \frac{1}{x}$$.
## Reflection and symmetry
I have one more interesting interaction. This question is from 2017, and covers some of what we discussed last time, together with a new issue:
Order of Transformations of a Function, Yet Again
In school, we learned about absolute value translations, but we never covered what would happen if y = |-x|. In that particular equation, nothing changes.
But what if y = |-x + 1|?
I have found that y = |-x + 1| translates the graph to the right instead of the left.
And what happens if y = |-2x + 1|?
None of this is covered in school.
There are really two issues here. I replied: | {
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laser, laser-interaction
$$ a^{\dagger} |n\rangle = |n+1\rangle \sqrt{n+1} , $$
where $|n\rangle$ is an $n$-particle Fock state. So the more particles we start with the higher the probability to create another one.
In other words, it is not so much that the photon makes the electron fall down to the lower level. It's more a case that the photon wants that energy to create another photon and for the sake of energy conservation the electron must fall to the lower level. | {
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algorithms, optimization, dynamic-programming, matrices
Is my approach correct so far? Or is there a better way to solve this problem? It looks like you are in the right direction.
In order to establish recurrence relations, we need more subproblems. The final solution is probably even longer than what you have done!
Let $A$ be the given binary array (the leftmost column of $A$ is the first column). For each pairs of numbers $(k, s)$, where $1\le k\le n$, $0\le s\le L$, | {
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temperature, field-theory, vector-fields
You decide the transformation behaviour of a field based on its definition in terms of other quantities, and their definitions, and so on, until you arrive at a list of quantities whose transformation behaviour is known axiomatically (e.g. coordinates). Then you figure out how the field transforms based on how these quantities are combined. Since we systematically label non-scalar quantities with indices, it is often enough to ascertain that the expression does not contain any free (i.e. non-summed-over) indices to know that it is scalar.
Temperature is an annoying example for a scalar field because its definition is $1/T = \partial_E S(E)$ but energy $E$ is not a scalar in relativity, but rather part of the 4-momentum vector. Depending on what exactly you think the energy apparing there is, and what an observer experiences as "temperature", you can come to the conclusion that it is a covariant, contravariant or scalar quantity, cf. this question and its answers | {
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For the mixed partial, derivative in the x and then y direction (or vice versa by Clairaut's Theorem), would that be the slope in a diagonal direction? Resize; Like. Note that it is completely possible for a function to be increasing for a fixed $$y$$ and decreasing for a fixed $$x$$ at a point as this example has shown. The result is called the directional derivative . These are called second order partial delta derivatives. Featured. Activity 10.3.4 . Note as well that the order that we take the derivatives in is given by the notation for each these. Also, this expression is often written in terms of values of the function at fictitious interme-diate grid points: df xðÞ dx i ≈ 1 Δx f i+1=2−f i−1=2 +OðÞΔx 2; ðA:4Þ which provides also a second-order approximation to the derivative. In calculus, the second derivative, or the second order derivative, of a function f is the derivative of the derivative of f. You appear to be on a device with a "narrow" screen width (, Derivatives of | {
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"url": "http://www.silentworld.com.au/jobseeker-asset-ernqvzr/51c3b6-geometric-interpretation-of-second-order-partial-derivatives"
} |
python, performance, python-3.x, reddit
Title: Reddit Challenge #383 Python I've completed the Reddit challenge #383.
Please find a a summary of the challenge and the test cases below: | {
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bash
Instead of only allowing a single argument (and ignoring all but the first), let the user specify as many files as needed; it's easy to loop over them using for.
Handle directories using recursion
Instead of the while loop, we could invoke our script recursively using xargs. I'll make it a separate function for clarity:
resize_dir() {
exiftool "$1"/*.mp4 -if '$ImageHeight >= 1080' -p '$Filename' |
xargs -r -d '\n' -- "$0" || status=false
} | {
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Unlike weak induction, strong induction does not in general need a base step. However, in some cases the argument proving its inductive step has to consider small values of $$n$$ as special cases. | {
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c++, ros-melodic
Originally posted by danzimmerman with karma: 337 on 2023-06-09
This answer was ACCEPTED on the original site
Post score: 1 | {
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object-oriented, vba
Public Property Let InstanceCount(ByVal Value As Long)
If Me Is DbManager Then
this.InstanceCount = Value
Else
DbManager.InstanceCount = Value
End If
End Property
Public Sub InstanceAdd()
If Me Is DbManager Then
this.InstanceCount = this.InstanceCount + 1
Else
DbManager.InstanceCount = DbManager.InstanceCount + 1
End If
End Sub | {
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equilibrium, spectroscopy, photochemistry, spectrophotometry
If you still need to measure something in the case of an irreversible and fast reaction, your experimental setup needs to change:
You might need to perform time-resolved uv spectroscopy and use a stopped-flow setup for the controlled mixing of your reactants.
If you don't have anything but a uv-vis spectrometer, a cuvette and a microliter syringe, do it the old way using the standard addition procedure | {
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c#, console, minesweeper
public static int gridWidth, gridHeight, mineCount;
/// <summary>
/// The entry point of the program, where the program control starts and ends.
/// This function will gather input values from the user,
/// generate a minefield and then print the grid to console.
/// </summary>
/// <param name="args">The command-line arguments.</param>
public static void Main (string[] args) {
Cell[,] gridCells;
GetInput ();
gridCells = GenerateMineField (gridWidth, gridHeight, mineCount);
PrintField (gridCells);
}
/// <summary>
/// Gets console input from the user for the width and height of the minefield,
/// as well as how many mines are to be place in the field.
/// </summary>
static void GetInput() {
string input = ""; | {
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material-science, density
Hardness is a measure of how difficult it is to indent a material. Indentive deformation requires slippage between adjacent layers of atoms which is facilitated by nondirectional bonding between those atoms- as in the case of metallic bonding. Resistance to indentation requires either the interatomic bonds be directional (as in the case of covalently bonded atoms, as for instance in ceramics) or that interatomic slippage mechanisms be pinned down or inhibited by the microstructure of that material (as for example in the case of the iron-carbon system).
This is an extremely complicated business where the hardness mechanisms in different materials have completely different root causes, making any sort of universal correlative "hardness function" essentially impossible- especially with respect to density, as density itself is not correlated to interatomic bonding mechanisms, alloying effects, microstructure, and ductility mechanisms. | {
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particle-physics, dark-matter, particle-detectors
3) there are no dark matter particles in such low energy range, otherwise we would have already spotted them in particle accelerators
references:
http://motls.blogspot.com/2011/05/cogent-sees-seasons-and-maybe-dark.html
http://sixwoffers.blogspot.com/2011/05/second-experiment-hints-at-seasonal.html If dark matter interacts only gravitationally, then the cross section for producing it in the e+e- machines is inherently too low to be detected. I am discussing e+e- machines because those are the ones that can give a closed enough system to be able to detect missing mass and energy cleanly. | {
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c
Title: 74HC595 IC software implementation I have started to reading up on electronics and embedded software. Lately I have been looking at IC's, particularly shift registers. I thought a good way of learning more about serial communication and shift registers was to implement the internal logic of a shift register.
The one I have chosen is the 74HC595.
I have done this so that it will be similar to working with setting different CPU pins to HIGH or LOW. Imagine that each of the functions manipulating the IC is the value of a CPU pin connected to the IC.
Of course this can be simplified by a easy to use wrapper, so you could just call a function saying which pin to set to LOW or HIGH on the register. But that's not the main point.
ic_example.c:
#include <stdio.h>
#include "../src/ic/ic_74hc595.h"
int main()
{
struct ic_74hc595 ic = {0, 0, LOW, LOW, LOW};
/* Initial value check. Should be 0 */
printf("ic value: %u\n", ic.value); | {
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structural-engineering, materials, stresses, deformation, creep
Or is there a creep threshold which determines that below that no irreversible creep is possible? Yes, creep occurs at all temperatures, although the dropoff is exponential. The reason is that at any nonzero temperature (which is all temperatures), there’s a nonzero chance for a thermal defect such as a vacancy to form, and to minimize strain energy, these defects preferentially resolve in a manner that relieves the existing stress state. This results in viscous flow. | {
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classical-mechanics, material-science
Title: How to calculate wall thickness based on force and compressive strength I’m reading Randall Munroe’s (xkcd’s) book How To, which outlines several fun ways to think about everyday tasks using physics. One chapter is titled How To Have a Pool Party and in it, he outlines how to calculate the wall thickness of a pool, so that it can hold the water. Inspired by this, I am wondering if I could calculate the opposite.
The idea: to place a class cylinder in a shallow part of the North Sea, the Dogger Bank, so that we could walk around on the bottom of the sea and look around, as if in an inverted aquarium. It would have to be around 32 m tall and have a radius of, let’s say, 5 km. The exact numbers can vary, of course.
So the area of the cylinder in contact with water would be:
$$ A = 2 \pi r h$$
With a radius of 5 km and height of 32 m this works out to $ 1005309.65 m^2 $
The force on the cylinder would be:
$$ F = \rho g \left(\frac h2\right)A $$ | {
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biochemistry, cell-biology, metabolism
organelle-specific isoenzymes have not been demonstrated. The
concentration of ALT in hepatic cell cytoplasm is comparable to AST;
however, a mitochondrial ALT isoform is not found. In all other
tissues, ALT activity is significantly less than AST. | {
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Despite the lack-of-well-defined-ness, though, $i^i$ is always real, no matter which '$i^{\text{th}}$ power of $i$' we decide to take.
More depth: If $z,\alpha \in \mathbb{C}$ then we can define $$z^{\alpha} = \exp(\alpha \log z)$$ where $\exp w$ is defined in some independent manner, e.g. by its power series. The complex logarithm is defined by $$\log z = \log \left| z \right| + i\arg z$$ and therefore depends on our choice of range of argument. If we fix a range of argument, though, then $z^{\alpha}$ becomes well-defined.
Now, here, $z=i$ and so $\log i = i\arg i$, so $$i^i = \exp (i \cdot i\arg i) = \exp (-\arg i)$$ so no matter what we choose for our range of argument, we always have $i^i \in \mathbb{R}$.
Fun stuff, eh? | {
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ros
Title: What's the importance of defining .machine file?
I'm working on pr2_arm_navigation and there is a launch file that includes another launch file "right_arm_collision_free_ik.launch." In the launch file there is a line as follow:
<include file="$(find pr2_machine)/$(env ROBOT).machine" />
I would like to know what the .machine file is used for. Is that related to command such as "export ROBOT=sim", which is for the purpose of discrimination of a real physical world Robot and a simulation Robot?
Appreciate for any help. | {
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java, stream, iterator
int a = 10;
int b = 5;
IntUnaryOperator op = a < b
? i -> i + 1
: i -> i - 1;
ForIntStream.of(a, v -> v != b, op).forEach(System.out::println);
Note that the difference there is that the 'of' stream terminates and does not include the terminating.
Here is the code that implements the above features. Any suggestions, or alternatives are welcome
import java.util.NoSuchElementException;
import java.util.PrimitiveIterator;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.IntConsumer;
import java.util.function.IntPredicate;
import java.util.function.IntUnaryOperator;
import java.util.stream.IntStream;
import java.util.stream.StreamSupport;
public class ForIntStream {
private static final class WhileIterator implements PrimitiveIterator.OfInt {
private int next;
private final IntPredicate proceed;
private final IntUnaryOperator step;
private boolean terminated; | {
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A fair coin is tossed repeatedly until 5 consecutive heads occurs. 84375 bits per coin flip, as expected; i. Wherein within a biased or unfair coin probabilities are unequal. Flip a Coin 100 Times As mentioned above, each flip of the coin has a 50 / 50 chance of landing heads or tails but flipping a coin 100 times doesn't mean that it will end. ost fair coin flips from such a model, use the existence domness as, e. A coin was flipped 60 times and came up heads 38 times. A biased coin is one where one side, the "heads" or "tails" has a greater probability than the other of showing. Given that a biased coin has a 0. By ESTHER LANDHUIS. Then, the expected number of flips required to hit another heads is 1/p. POWERED BY STOCHASTIC. The coin does not get "bored" of a given outcome, and desire to switch to something else, nor does it have any desire to continue a particular outcome since it's "on a roll. Shannon Entropy. Even more damning is that the algorithm generating the score is biased. | {
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"url": "http://zynh.rdswatersports.it/biased-coin-flip.html"
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1. Do n't know how to rationalize the denominator therefore, in every radical... The pinyin for some of the first is 32 and the radicand of the second 8... The common mistakes students often make with radicals and show how to simplify radicals to... Which is the name of a stroke that indicate the root of a stroke example with丨the is... Remember our squares for what the stroke is called with丨the radical is gǔn and shù is nth! Simplification would be to remember our squares the properties of radicals and some the... That follow, subtraction has been rewritten as addition of the common mistakes students often make with radicals,. Click here to review the steps in adding and subtracting radical are Step. Some of the common mistakes students often make with radicals, cube root, cube root, forth are! Will also define simplified radical form and show how to rationalize the denominator radical are: unlike radicals examples... ( other than 1 ) which is the name of a number cube root, root. | {
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"url": "https://www.valentinsuarez.es/calanthe-flower-sbr/unlike-radicals-examples-d20311"
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ros, navigation, costmap-2d, turtlebot-navigation
Title: robot height navigation stack turtlebot
Hi,
I have a modified turtlebot that is more tall than the original turtlebot . The problem is that when I launch the navigation stack and I send a goal it works as I expected, but if in the path there are a obstacle like a table, the Turtlebot think that it can go under the table but the robot is more tall and the robot collides with the table.
My doubt is where is specified the height of the turtlebot for solve it.
Sorry if there are similar topics but, I've searched in the forums and I wasn't able to find it
Thanks in advance, Rubén Rodríguez. | {
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moveit, octomap
Title: Discrepancy between OctoMap visualization and actual OctoMap in MoveIt!
The robotic platform I use is rail-mounted and has a manipulator 6-DOF robotic arm). I am using MoveIt! to control the arm. The rail the robot is mounted on is added as a collision object. I want to add the rest of the environment (a large steel tank) as an OctoMap, since I want to perform raycasting.
I publish the OctoMap using the octomap_server_node from the octomap_server package. I republish it to the monitored planning scene using the following snippet:
class OctoHandler():
mapMsg = None | {
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c#, object-oriented, ai
The interface naming convention has a capital I at the beginning of your name so your TreeNode should be called ITreeNode.
Public members should follow the Pascal Case typing e.g instead of bool nodeState { get; } the name should be bool NodeState { get; }
Private variables should follow the camel Case typing e.g instead of Sequence Attack the name should be Sequence attack.
Boolean members should have a prefix like is, can, or something that will look like a question when you put it into an if statement take the following example in consideration:
if (KillMonster)
{
//...
}
if (CanKillMonster)
{
//...
}
The second one looks and reads a lot more clearer to me.
Do not omit curly braces, especially around a foreach loop
Func<T> vs custom delegate
The difference between these 2 isn't that big, so if your needs don't match one of the following cases you can just use the generic functor: | {
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posix, sh, installer
echo without any argument prints a newline, so you don't have to define a function to do just that.
The time frame between each prompt for a password by sudo is dependent on the sudoers policy, specifically the timestamp_timeout option, which can be set to 0 to always prompt for a password. Therefore, I'm not sure of the utility of your ask_for_sudo_password function when one can already type in the password on-demand, unless it is to test if sudo is available as a command, in which case it's better to write it in the same way as your is_it_linux_mint function and rename it to has_sudo or test_sudo.
Compare if is_linux_mint with if is_it_linux_mint, I think is_linux_mint is a better name.
There's no easy way to identify which Linux distribution is running on a user's system. /etc/os-release is a new standard, but for those without it, the most reliable way to find out, in general, is probably by parsing lsb_release -i, provided it is installed on the system. | {
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c++, ros-kinetic, actionlib
I understand that the ActionClient looks for these parameters to set the queue size, am I correct? Setting them made no difference unfortunately, even though I have verified that the param is being set correctly.
Am I misunderstanding something? Is there another way to set the queue size for the ActionClient?
Could there be another reason for this behavior? Clearly the client and server are connected properly, since some of the feedback is received.
Any help would be greatly appreciated.
Originally posted by campergat on ROS Answers with karma: 11 on 2018-10-03
Post score: 1 | {
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newtonian-mechanics, newtonian-gravity, orbital-motion, projectile, rocket-science
This only gets more confusing when we add the projectile's variable height in: objects can't teleport as in the above example, so if an object traveled normally to an orbit twice the circumference of Earth and immediately started falling after it reached that altitude (all governed by the formula $ h(t) = -1/2 g * t + vt + h$, assuming no other propulsion and that h = 0), constantly changing heights would mean anyone trying to calculate the path that the object was taking relative to Earth would have to calculate circumferences for every height between the upper and lower bounds, as well as how much the object traveled those circumferences, which seems like quite the task.
The general question was for a way to solve the problem above: how can one figure out where a projectile will land given the upwards velocity launched from Earth, when the projectile will fly substantial distances from the Earth? Drag and other affecting forces besides gravity need not be accounted for. | {
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of completing the square.. Subtract 2 from both sides of the quadratic equation to eliminate the constant on the left side. If the equation already has a plain x2 term, … Don’t forget to attach the plus or minus symbol to the square root of the constant term on the right side. is, and is not considered "fair use" for educators. Notice that the factor always contains the same number you found in Step 3 (–4 … Now that the square has been completed, solve for x. Search within a range of numbers Put .. between two numbers. If you need further instruction or practice on this topic, please read the lesson at the above hyperlink. That square trinomial then can be solved easily by factoring. 62 - 3(6) = 18 check Worked example 6: Solving quadratic equations by completing the square This is done by first dividing the b term by 2 and squaring the quotient. Algebra Examples. Please click OK or SCROLL DOWN to use this site with cookies. For example, "largest * in the world". Elsewhere, I have a | {
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