text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
|---|---|---|
Find a number behind these doors. | 305.94 | 2.3 |
Now, unfortunately, you
don't know in advance | 308.24 | 2.76 |
what numbers are behind those
doors and where they are. | 311 | 2.9 |
Indeed, that's why the doors are closed. | 313.9 | 1.74 |
But you know that, say, the number
50 is somewhere behind those doors. | 315.64 | 4.39 |
So what do we do to actually find this? | 320.03 | 2.43 |
Well, let's go ahead and do this. | 322.46 | 1.41 |
Let me go ahead and summon up
a brave volunteer if you would. | 323.87 | 5.482 |
There's a CS50 stress
ball in it for you. | 329.352 | 1.708 |
OK, come on up. | 331.06 | 1.26 |
You want to come up on up over here? | 332.32 | 2.77 |
So we have not 8, but
rather 7 doors here. | 335.09 | 4.1 |
Even easier. | 339.19 | 1.56 |
So 7/8 the size. | 340.75 | 0.751 |
And what's your name? | 341.501 | 0.874 |
AUDIENCE: Derek [INAUDIBLE]. | 342.375 | 0.905 |
SPEAKER 1: Derek? | 343.28 | 0.45 |
OK. | 343.73 | 0.61 |
David, come on up. | 344.34 | 2.04 |
Nice to meet you. | 346.38 | 0.89 |
All right. | 347.27 | 0.82 |
So here we have 7 doors. | 348.09 | 1.55 |
And the goal at hand for you is
to simply find the number 50. | 349.64 | 5.07 |
How do we find it? | 354.71 | 0.83 |
What are you going to do? | 355.54 | 1.715 |
AUDIENCE: [INAUDIBLE]. | 357.255 | 2.325 |
SPEAKER 1: It's behind one
of these doors, I think. | 359.58 | 2.735 |
AUDIENCE: All right. | 362.315 | 1.94 |
I'll just try a door? | 364.255 | 1.295 |
SPEAKER 1: Try a door. | 365.55 | 1.73 |
Not the door. | 367.28 | 0.7 |
So 16 turns up. | 367.98 | 1.27 |
OK. | 369.25 | 1.19 |
Stress ball gets one step away. | 370.44 | 3.4 |
Try another. | 373.84 | 1.27 |
42. | 375.11 | 1.52 |
Answer to life, the universe, and
everything, but not this question. | 376.63 | 5.2 |
50. | 381.83 | 0.65 |
OK, very well done. | 382.48 | 1.33 |
So a round of applause if
we could for Derek here. | 383.81 | 2.6 |
Very nicely done. | 386.41 | 1.3 |
OK. | 387.71 | 0.5 |
And let's pause for just a moment. | 388.21 | 1.25 |
What was the algorithm? | 389.46 | 0.958 |
What was the set of instructions
that you used to find the number 50? | 390.418 | 3.102 |
AUDIENCE: Just try and see if I
could find any sort of arrangement. | 393.52 | 5.57 |
But otherwise, try random doors. | 399.09 | 1.787 |
SPEAKER 1: OK. | 400.877 | 0.583 |
So try random doors. | 401.46 | 0.97 |
And indeed, given no
additional information from me, | 402.43 | 2.675 |
that's the best you can do. | 405.105 | 1.125 |
Just try random doors. | 406.23 | 1.13 |
Or, if it's a little easier, just
go left to right or right to left. | 407.36 | 3.05 |
But random is perfectly equivalent
to all of those algorithms | 410.41 | 2.75 |
because you don't know
anything necessarily a priori. | 413.16 | 3.16 |
So how about for two stress balls? | 416.32 | 4.63 |
Find the number 50 this time,
but the additional ingredient | 420.95 | 2.702 |
is that we have taken
the time in advance | 423.652 | 1.708 |
to sort the numbers behind these doors. | 425.36 | 2.04 |
Does that change your
thinking in any way? | 427.4 | 1.841 |
AUDIENCE: Well, I don't
know whether you're | 429.241 | 1.791 |
sorting increasing left or right. | 431.032 | 1.688 |
SPEAKER 1: Good. | 432.72 | 0.32 |
So good challenge. | 433.04 | 0.75 |
He doesn't know if we've sorted
from left to right or right to left. | 433.79 | 3.25 |
Left to right. | 437.04 | 0.59 |
So generally, a computer scientist
when he or she says "sorted" | 437.63 | 2.625 |
would say left to right. | 440.255 | 1.015 |
But reasonable question. | 441.27 | 1.38 |
AUDIENCE: Left to right
increasing or decreasing? | 442.65 | 1.2 |
SPEAKER 1: Left to right increasing. | 443.85 | 1.499 |
Yes. | 445.349 | 0.581 |
It would be left to right in any way. | 445.93 | 2.48 |
16. | 448.41 | 0.86 |
Not 50. | 449.27 | 0.5 |
So what are you going to do? | 449.77 | 1.166 |
AUDIENCE: Well, let's see. | 450.936 | 1.217 |
I know these have to be
less, so I can remove them. | 452.153 | 2.357 |
SPEAKER 1: Good. | 454.51 | 0.666 |
AUDIENCE: [INAUDIBLE]. | 455.176 | 2.284 |
SPEAKER 1: 42 is still not 50. | 457.46 | 2.43 |
And to the right. | 459.89 | 0.82 |
Very well done for Derek. | 460.71 | 1.9 |
OK, come on. | 462.61 | 1.11 |
OK. | 463.72 | 0.575 |
They're less impressed that time. | 464.295 | 1.375 |
So what did you do, though, that second
time that was actually more effective? | 465.67 | 4.76 |
I mean, frankly, at first glance it
looks like this approach was just as | 470.43 | 3.279 |
fast or just as slow
as your first approach | 473.709 | 1.791 |
because it still took you through steps. | 475.5 | 1.666 |
AUDIENCE: So what I did was since
you told me that it was sorted left | 477.166 | 2.914 |
to right increasing, I decided to
look at the middle number first | 480.08 | 3.42 |
and see whether it was
greater than or less than-- | 483.5 | 2.475 |
SPEAKER 1: Good | 485.975 | 0.625 |
AUDIENCE: --the target number. | 486.6 | 1.8 |
SPEAKER 1: Good. | 488.4 | 0.7 |
AUDIENCE: And since it was less than,
I knew that all of these three values-- | 489.1 | 2.88 |
SPEAKER 1: Wonderful. | 491.98 | 0.874 |
And indeed, are smaller by design. | 492.854 | 1.775 |
And so you can throw half of that
problem away, much like in week 0 | 494.629 | 2.791 |
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