text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
|---|---|---|
On this line here, if m is
less than or equal to 0, | 4,798.88 | 4.06 |
it's really just a sanity check. | 4,802.94 | 1.55 |
If the human was messing
with me and somehow | 4,804.49 | 3.15 |
was able to inject a negative value
or a 0 value into this function, | 4,807.64 | 4.609 |
I don't want to get stuck in
some random, like infinite loop. | 4,812.249 | 2.541 |
I want to catch that. | 4,814.79 | 1.02 |
So I'm just going to say less than or
equal to 0 just to be super-defensive | 4,815.81 | 3.124 |
when programming return 0. | 4,818.934 | 1.926 |
It's not correct to say
that, but I just want-- | 4,820.86 | 2.31 |
and I'll document this in
my comments, for instance. | 4,823.17 | 2.65 |
Return 0 if the user does not cooperate. | 4,825.82 | 1.96 |
I need that base case so
that stuff stops eventually. | 4,827.78 | 3.94 |
And then, here's kind
of the amazing thing. | 4,831.72 | 2.12 |
In my "else" condition,
is one line of code | 4,833.84 | 2.84 |
that implements the
entirety of this function. | 4,836.68 | 3.7 |
The result of asking me the
question, what is sigma of m, | 4,840.38 | 3.39 |
is really the same thing as
saying m plus sigma of m minus 1. | 4,843.77 | 8.07 |
In other words, if the goal is to count
up all of the numbers between 1 and m, | 4,851.84 | 4.28 |
logically that's the same as counting
up all the numbers from m to 1. | 4,856.12 | 4.12 |
And so if I view it that
way, if you give me m, | 4,860.24 | 4.48 |
I am going to sort of on my cheat
sheet write down the number m, | 4,864.72 | 3.35 |
and then say plus whatever the
summation is from m minus 1 | 4,868.07 | 4.05 |
all the way down to 1. | 4,872.12 | 1.11 |
And you know what? | 4,873.23 | 0.75 |
I have a function, a piece of code, that
gives me exactly that answer if I just | 4,873.98 | 3.88 |
pass-in that slightly smaller value. | 4,877.86 | 2.7 |
So on first glance, it might
look like sigma calls sigma. | 4,880.56 | 3.81 |
That can't possibly be a good thing. | 4,884.37 | 2.19 |
But it doesn't do it forever. | 4,886.56 | 1.48 |
Sigma might call sigma might
call sigma might call sigma. | 4,888.04 | 2.54 |
But at what point in the story
does sigma stop calling sigma? | 4,890.58 | 4.67 |
AUDIENCE: [INAUDIBLE]. | 4,895.25 | 1.5 |
SPEAKER 1: When it's counted. | 4,896.75 | 1.32 |
When m, the input, is what? | 4,898.07 | 3.01 |
Negative. | 4,901.08 | 0.53 |
Or we shouldn't hit negative ever. | 4,901.61 | 1.46 |
We should first hit? | 4,903.07 | 1.231 |
AUDIENCE: 0. | 4,904.301 | 0.499 |
SPEAKER 1: 0. | 4,904.8 | 1.01 |
So once m minus 1 gets so small, 1 minus
1, that it equals 0, this line of code | 4,905.81 | 6.507 |
is going to kick in. | 4,912.317 | 0.833 |
And it's going to say return 0. | 4,913.15 | 1.92 |
Thereby, short circuiting
this seemingly infinite loop, | 4,915.07 | 3.82 |
which does, in fact, terminate
eventually once the input | 4,918.89 | 3.74 |
gets small enough. | 4,922.63 | 1.41 |
Now, probably you shouldn't bother
implementing code like this. | 4,924.04 | 3.195 |
If anything, we'll talk
about this in future weeks. | 4,927.235 | 2.125 |
Calling function after function after
function after function after actually | 4,929.36 | 3.66 |
can take up a lot of memory. | 4,933.02 | 1.352 |
It can actually cause your
computer to crash in some cases. | 4,934.372 | 2.458 |
And we'll talk about when and why. | 4,936.83 | 1.82 |
So this isn't necessarily
the best technical decision, | 4,938.65 | 2.95 |
but it's illustrative of the capability
we have in programming to map intuition | 4,941.6 | 4.48 |
from our binary search or our
merge sort into actual code. | 4,946.08 | 5.301 |
And indeed, there's going to
be data structures that we soon | 4,951.381 | 2.499 |
see far fancier than arrays. | 4,953.88 | 2.11 |
We're going to see things
like link lists, and trees, | 4,955.99 | 2.39 |
and tries, and hash tables. | 4,958.38 | 1.46 |
And other data structures
still that are going | 4,959.84 | 2.71 |
to allow us opportunities to implement
algorithms on top of those data | 4,962.55 | 5.22 |
structures so to speak,
to answer questions | 4,967.77 | 2.45 |
using an ingredient like this one here. | 4,970.22 | 2.96 |
But of course, we're left then
fortunately with a better algorithm | 4,973.18 | 3.51 |
than something like bubble sort. | 4,976.69 | 1.5 |
And indeed, this is fairly
well-known now it would seem. | 4,978.19 | 2.75 |
And I thought we'd conclude with
a look at this interview that | 4,980.94 | 3.13 |
was conducted with a certain senator
some years ago who was running | 4,984.07 | 3.71 |
for the presidency and was asked
by Google's own Eric Schmidt, | 4,987.78 | 4.03 |
the CEO at the time, a set
of interview questions. | 4,991.81 | 3.06 |
One of which we thought was
appropriate note to end on here. | 4,994.87 | 4.45 |
[AUDIO PLAYBACK] | 4,999.32 | 3.666 |
[APPLAUSE] | 5,002.986 | 2.334 |
-Now, Senator. | 5,005.32 | 1.86 |
You're here at Google. | 5,007.18 | 1.01 |
And I like to think of the
presidency as a job interview. | 5,008.19 | 5.71 |
Now, it's hard to get
a job as president. | 5,013.9 | 3.142 |
You're going [INAUDIBLE] now. | 5,017.042 | 1.208 |
It's also hard to get a job at Google. | 5,018.25 | 2.17 |
We have questions. | 5,020.42 | 2.67 |
And we ask our candidates questions. | 5,023.09 | 2.58 |
And this one is from Larry Schwimmer. | 5,025.67 | 3.676 |
[LAUGHTER] | 5,029.346 | 1.969 |
You guys think I'm kidding? | 5,031.315 | 1.125 |
It's right here. | 5,032.44 | 2.46 |
What is the most efficient way to
sort a million 32-bit integers? | 5,034.9 | 4.148 |
[LAUGHTER] | 5,039.048 | 2.922 |
-Well-- | 5,041.97 | 2.4 |
-I'm sorry. | 5,044.37 | 0.591 |
Maybe-- | 5,044.961 | 0.499 |
-No, no, no, no. | 5,045.46 | 0.75 |
I think the bubble sort
would be the wrong way to go. | 5,046.21 | 4.98 |
-Come on. | 5,054.19 | 0.745 |
Who told him this? | 5,054.935 | 1.565 |
OK. | 5,056.5 | 1.84 |
I didn't see computer
science in your background. | 5,058.34 | 2.352 |
-We've got our spies in there. | 5,060.692 | 2.878 |
[END PLAYBACK] | 5,063.57 | 1.32 |
SPEAKER 1: All right,
so that's it for week 3. | 5,064.89 | 2.23 |
We will see you next time. | 5,067.12 | 1.6 |
SPEAKER 2: So you want
to know the full story? | 5,076.94 | 2.352 |
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