text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
|---|---|---|
This is going to be Z, so
I'll just put this over here. | 5,451.47 | 3.97 |
This is going to be C, so I'll put
this over here, and B here, and A. | 5,455.44 | 5.925 |
And we thought we might get chased
away by the folks on the loading dock, | 5,461.365 | 4.025 |
so we didn't bother getting D
through Y, So we'll just pretend | 5,465.39 | 2.98 |
that we have 26 such buckets here. | 5,468.37 | 2.01 |
And suppose that the goal
at hand is-- I don't know, | 5,470.38 | 3.482 |
it's like at the end
of an exam, so we've | 5,473.862 | 1.708 |
got our old blue books that a
class might use for students | 5,475.57 | 3.23 |
writing essays in some class. | 5,478.8 | 1.67 |
And it's time for the students
to come submit their blue books. | 5,480.47 | 3.16 |
Now, we could just collect them all
and make a big mess as would generally | 5,483.63 | 3.28 |
be the case, or we can be a little
more methodical to at least make | 5,486.91 | 3.24 |
our jobs easier. | 5,490.15 | 1.384 |
Now, at the end of the day, what's going
to be interesting about hash tables | 5,491.534 | 3.166 |
is that there's going
to be this distinction | 5,494.7 | 1.833 |
between actual benefits and
theoretical benefit, or lack thereof. | 5,496.533 | 3.317 |
So, we'll come to that in just a
moment, but here's A, B, C, D, and Z. | 5,499.85 | 4.181 |
And you know what? | 5,504.031 | 0.749 |
I just am going to ask the students
in this class-- there are so | 5,504.78 | 1.99 |
many people in the room
after an exam, I just | 5,506.77 | 2.27 |
want them to at least make
my life 1/26 as difficult | 5,509.04 | 4.96 |
by putting all the As over there,
all the Bs here, all the Cs here, | 5,514 | 3 |
all the Zs here, so that I don't have
a massive mountain of As through Zs | 5,517 | 4.66 |
that I have to sift
through individually. | 5,521.66 | 2.12 |
It would just be nice if
they do the first pass | 5,523.78 | 2.22 |
of bucketizing the values based on
the first letter in their last name. | 5,526 | 4.13 |
In other words, my hash
function, my algorithm, | 5,530.13 | 3.48 |
is going to be for each student
to consider his or her last name, | 5,533.61 | 4.09 |
look at the first letter they're
in, and put his or her exam | 5,537.7 | 3.22 |
in the appropriate bucket. | 5,540.92 | 1.38 |
So, here is, for instance,
someone with the letter | 5,542.3 | 3.34 |
C. I'm going to put
that blue book in here. | 5,545.64 | 2.63 |
Here's someone with the letter
A. That one's going to go here. | 5,548.27 | 2.62 |
Letter Z? | 5,550.89 | 0.746 |
This one's going to go over here. | 5,551.636 | 1.374 |
Letter B? | 5,553.01 | 1.11 |
This is going to go over here. | 5,554.12 | 1.25 |
C, and B, and F-- Z, I mean, and all of
[? the ?] [? letters ?] of the alphabet | 5,555.37 | 6.8 |
in between. | 5,562.17 | 0.78 |
So, hashing really has this
visual and conceptual equivalence | 5,562.95 | 3.774 |
of putting something in this bucket,
putting something in that bucket, | 5,566.724 | 2.916 |
putting something in this
other bucket, ultimately | 5,569.64 | 2.083 |
bucketizing all of your elements. | 5,571.723 | 2.147 |
And you can think of this,
frankly, as just an array, | 5,573.87 | 2.21 |
but it's not just an
array with one spot. | 5,576.08 | 1.72 |
It looks I can stack multiple
numbers or multiple blue books inside | 5,577.8 | 5.04 |
of that array. | 5,582.84 | 0.71 |
So, we're going to have to come
back to that, because this clearly | 5,583.55 | 1.55 |
can't be an array. | 5,585.1 | 0.75 |
Normally, the array would be filled
the moment you put one value in it. | 5,585.85 | 3.2 |
But this hashing is
the interesting part. | 5,589.05 | 2.93 |
The juicy ingredient today is if I take
into account as input what it is I'm | 5,591.98 | 4.87 |
trying to store, use some piece of that
information to decide where to put it, | 5,596.85 | 4.66 |
that's an algorithm, because
I can repeat that process, | 5,601.51 | 2.85 |
so long as it's not random. | 5,604.36 | 1.27 |
You go over here, you go over here. | 5,605.63 | 1.51 |
That's amazing. | 5,607.14 | 1.04 |
Wow, OK, pushing my luck. | 5,608.18 | 2.84 |
OK, so I'm not just randomly
putting things here. | 5,611.02 | 2.644 |
I'm actually giving some thought
as to where I'm putting things, | 5,613.664 | 2.666 |
and that makes the algorithm
deterministic, repeatable, predictable | 5,616.33 | 4.54 |
so that if you insert something
now, you can absolutely | 5,620.87 | 3.38 |
find it if present later. | 5,624.25 | 2.37 |
Unfortunately, if our
hash table does look | 5,626.62 | 2.03 |
like this, just a simple array from
bracket 0 to bracket n minus 1 dot, | 5,628.65 | 4.12 |
dot, dot in between,
and it's just an array | 5,632.77 | 2.33 |
for integers or an array for strings or
whatever, once you put something here, | 5,635.1 | 5.59 |
or here, or here, that's it. | 5,640.69 | 3.14 |
There is no more room to
put another element there | 5,643.83 | 2.64 |
wide as I might have drawn this table. | 5,646.47 | 1.94 |
If there's an int there, that's it. | 5,648.41 | 1.98 |
So, what could you do? | 5,650.39 | 2.12 |
Suppose that you do have an
array structure like this, | 5,652.51 | 4.06 |
and that is unacceptable. | 5,656.57 | 3.47 |
You have a whole bunch of elements
here and this table looks like this, | 5,660.04 | 6.96 |
and you consider this table like this. | 5,667 | 3.62 |
And maybe it's just where you're
supposed to take attendance or put | 5,670.62 | 3.04 |
people's names. | 5,673.66 | 0.914 |
So, if you say, oh, Alice is here today. | 5,674.574 | 1.666 |
Let me go ahead and hash on Alice's
name and put her where the As should go. | 5,676.24 | 4.225 |
Oh, Zoe is here, Z-O-E, so
we'll put her down there. | 5,680.465 | 5.685 |
And then who else? | 5,686.15 | 1.83 |
Alex is here. | 5,687.98 | 0.77 |
Dammit, Alex, no room for
you in our hash table, | 5,688.75 | 3.34 |
because Alice is already there. | 5,692.09 | 1.38 |
This is stupid. | 5,693.47 | 0.78 |
If we have data we want to
insert into this data structure, | 5,694.25 | 2.47 |
it would seem that I have 24 available
spots into which I could put Alex | 5,696.72 | 6.22 |
and yet I'm just stubbornly trying
to put him where only the As belong. | 5,702.94 | 3.58 |
So, why don't I, in this kind of
scenario, I need to put Alex in here. | 5,706.52 | 3.85 |
I clearly have space. | 5,710.37 | 0.997 |
You know what? | 5,711.367 | 0.583 |
Let me just probe the array looking
for the first available spot. | 5,711.95 | 2.91 |
OK, Alex, you're just going to go here,
and if someone else like Erin appears, | 5,714.86 | 5.43 |
fine. | 5,720.29 | 0.5 |
You just are going to go over here. | 5,720.79 | 2.29 |
So, you try to put the letter
As where you want them to go, | 5,723.08 | 3.77 |
but if there's already
someone there, just | 5,726.85 | 1.77 |
probe deeper into the data structure
looking for the first available slot. | 5,728.62 | 4.07 |
So, this is a general technique in
programming called linear probing | 5,732.69 | 3.35 |
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