text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
|---|---|---|
of a second. | 5,148.89 | 1.71 |
But in terms of targeting a
missile traveling at mach 5, | 5,150.6 | 3.61 |
it resulted in a tracking
error of over 600 meters. | 5,154.21 | 4.5 |
It would be a fatal error
for the soldiers on what | 5,158.71 | 3.41 |
happened is a Scud launch was
detected by early Warning satellites | 5,162.12 | 6.82 |
and they knew that the Scud was
coming in their general direction. | 5,168.94 | 3.92 |
They didn't know where it was coming. | 5,172.86 | 2.46 |
>> -It was now up to the radar
component of the Patriot system | 5,175.32 | 2.93 |
defending Dhahran to locate and keep
track of the incoming enemy missile. | 5,178.25 | 4.94 |
>> -The radar was very smart. | 5,183.19 | 1.419 |
It would actually track
the position of the Scud, | 5,184.609 | 2.041 |
and then predict where it probably
would be the next time the radar sent | 5,186.65 | 3.7 |
a pulse out. | 5,190.35 | 1.07 |
That was called a range gate. | 5,191.42 | 1.69 |
>> -Then, once the Patriot
decides enough time has | 5,193.11 | 4.55 |
passed to go back and check the next
location for this detected object, | 5,197.66 | 4.79 |
it goes back. | 5,202.45 | 1.15 |
So when it went back to the wrong
place, it then sees no object. | 5,203.6 | 5.05 |
And it decides that there was no
object, it was a false detection, | 5,208.65 | 3.51 |
and drops the track. | 5,212.16 | 1.77 |
>> -The incoming Scud disappeared
from the radar screen. | 5,213.93 | 3.1 |
And seconds later, it
slammed into the barracks. | 5,217.03 | 3.23 |
The Scud killed 28, and was the last
one fired during the first Gulf War. | 5,220.26 | 5.89 |
>> Tragically, the updated software
arrived at Dhahran the following day. | 5,226.15 | 5.81 |
The software flaw had
been fixed, closing | 5,231.96 | 2.97 |
one chapter in the troubled
history of the Patriot missile. | 5,234.93 | 4.876 |
>> [VIDEO PLAYBACK] | 5,239.806 | 0.923 |
DAVID J. MALAN: So this is all to
say that these issues of overflow | 5,240.729 | 2.791 |
and imprecision are all too real. | 5,243.52 | 2.34 |
So how did we get here? | 5,245.86 | 1.06 |
We began with just talking about printf. | 5,246.92 | 1.975 |
Again, this function that
prints something to the screen, | 5,248.895 | 2.375 |
and we introduced thereafter
a few other functions | 5,251.27 | 2.18 |
from the so-called CS50's library. | 5,253.45 | 1.495 |
And we'll continue to
see these in due time. | 5,254.945 | 1.965 |
And we, particularly, used get string,
and get int, and now also get float, | 5,256.91 | 3.85 |
and yet others still will we encounter
and use ourselves before long. | 5,260.76 | 3.65 |
>> But on occasion, have
we already seen a need | 5,264.41 | 2.81 |
to store what those functions hand back? | 5,267.22 | 3.3 |
They hand us back a string,
or an int, or a float. | 5,270.52 | 2.4 |
And sometimes we need to put that
string, or int, or float, somewhere. | 5,272.92 | 3.15 |
>> And to store those things, recall just
like in Scratch, we have variables. | 5,276.07 | 4.03 |
But unlike in Scratch,
in C we have actual types | 5,280.1 | 3.16 |
of variables-- data
types, more generally-- | 5,283.26 | 2.27 |
among them, a string, an int, a
float, and these others still. | 5,285.53 | 3.11 |
>> And so when we declare variables in C,
we'll have to declare our data types. | 5,288.64 | 3.681 |
This is not something we'll
have to do later in the semester | 5,292.321 | 2.499 |
as we transition to other languages. | 5,294.82 | 1.99 |
But for now, we do need
to a priori in advance, | 5,296.81 | 2.8 |
explain to the computer what type
of variable we want it to give us. | 5,299.61 | 4.76 |
>> Now, meanwhile, to print
those kinds of data types, | 5,304.37 | 2.92 |
we have to tell printf what to expect. | 5,307.29 | 2.28 |
And we saw percent s for strings,
and percent i for integers, | 5,309.57 | 2.88 |
and a few others already. | 5,312.45 | 1.34 |
And those are simply requirements
for the visual presentation | 5,313.79 | 3.447 |
of that information. | 5,317.237 | 0.833 |
>> And each of these can actually be
parametrized or tweaked in some way, | 5,318.07 | 4.01 |
if you want to further control
the type of output that you get. | 5,322.08 | 3.29 |
And, in fact, it turns out that not only
is there backslash n for a new line. | 5,325.37 | 4.234 |
There's something else called backslash
r for a carriage return, which | 5,329.604 | 2.916 |
is more akin to an
old school typewriter, | 5,332.52 | 1.84 |
and also Windows used for many years. | 5,334.36 | 3.33 |
>> There's backslash t for tabs. | 5,337.69 | 2 |
Turns out, that if you want to
double quote inside of a string, | 5,339.69 | 3.48 |
recall that we've used
double quote double | 5,343.17 | 1.83 |
quote on the left and the right
ends of our strings thus far. | 5,345 | 2.9 |
That would seem to confuse things. | 5,347.9 | 1.52 |
>> If you want to put a double quote in
the middle of a string-- and, indeed, | 5,349.42 | 3.083 |
it is confusing to see. | 5,352.503 | 1.167 |
And so you have to escape, so to
speak, a double quote with something | 5,353.67 | 3.45 |
like, literally, backslash double quote. | 5,357.12 | 1.74 |
And there's a few other still. | 5,358.86 | 1.37 |
And we'll see more of those
in actual use before long. | 5,360.23 | 4.31 |
>> So let's now transition from
data, and representation, | 5,364.54 | 3.39 |
and arithmetic operators, all
of which gave us some building | 5,367.93 | 2.89 |
blocks with which to play. | 5,370.82 | 1.25 |
But now let's actually give
us the rest of the vocabulary | 5,372.07 | 2.411 |
that we already had
last week with Scratch | 5,374.481 | 1.749 |
by taking a look at some other
constructs in C-- not all of them. | 5,376.23 | 3.12 |
But the ideas we're
about to see really just | 5,379.35 | 2.33 |
to emphasize the translation from
one language, Scratch, to another, C. | 5,381.68 | 3.93 |
>> And over time, we'll pick up
more tools for our toolkit, | 5,385.61 | 2.86 |
so to speak, syntactically. | 5,388.47 | 1.35 |
And, indeed, you'll see that the ideas
are now rather familiar from last week. | 5,389.82 | 4.37 |
So let's do this. | 5,394.19 | 1.01 |
>> Let's go ahead and whip up a program
that actually uses some expressions, | 5,395.2 | 3.67 |
a Boolean expression. | 5,398.87 | 1.85 |
Let me go ahead here
and create a new file. | 5,400.72 | 2.09 |
I'll call this condition.c. | 5,402.81 | 3.28 |
>> Let me go ahead and
include the CS50 library. | 5,406.09 | 3.26 |
And let me go ahead and include
standard IO.h for our functions, | 5,409.35 | 3.29 |
and printf, and more respectively. | 5,412.64 | 2.05 |
Let me give myself that boilerplate of
int main void, whose explanation we'll | 5,414.69 | 4.21 |
come back to in the future. | 5,418.9 | 1.46 |
>> Now let me go ahead and give
myself an int via get int. | 5,420.36 | 3.46 |
Then let me go ahead and do this. | 5,423.82 | 2.15 |
I want to say if i is less-- let's
distinguish between positive, negative, | 5,425.97 | 4.18 |
or zero values. | 5,430.15 | 1.11 |
>> So if i is less than zero, let me
just have this program simply say, | 5,431.26 | 5.37 |
negative, backslash n, else
if i is greater than zero. | 5,436.63 | 5.74 |
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