text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
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So for instance, if the
address of a is say, | 2,892.08 | 3.78 |
33 Oxford Street,
Cambridge, Massachusetts. | 2,895.86 | 2.739 |
That happens to be the address
of the CS building at Harvard. | 2,898.599 | 2.541 |
Star a means go to 33 Oxford
Street in Cambridge, Massachusetts. | 2,901.14 | 5.06 |
The star just means go to
that particular address. | 2,906.2 | 4.12 |
So what does that mean, then, down
here, when I say star a gets star b? | 2,910.32 | 4.17 |
That means go to the address
in b and get its value, | 2,914.49 | 3.56 |
and store it at whenever
a is pointing at to. | 2,918.05 | 3.45 |
So go to a and wait for me for a value. | 2,921.5 | 2.97 |
Go to b, get a value, and put
that value at the location in a. | 2,924.47 | 4.61 |
And then lastly, this just
means go to the location in b, | 2,929.08 | 3.645 |
go to whatever building
that is, so to speak, | 2,932.725 | 2.295 |
and put the value that is in
temp inside of that building. | 2,935.02 | 4.88 |
So a pointer is just an address. | 2,939.9 | 2.96 |
These stars just mean
pointers are involved. | 2,942.86 | 3.75 |
Give me the address of an int,
give me the address of an int. | 2,946.61 | 3.04 |
And again, confusing,
admittedly, the star | 2,949.65 | 2.58 |
in this context where we don't have
the word int in front of it again, | 2,952.23 | 3.73 |
on the side of the equal sign,
just means go to that address. | 2,955.96 | 2.83 |
Go to that building. | 2,958.79 | 1.29 |
Go to that other building
and put something there. | 2,960.08 | 3.55 |
So we can now fix our
swap program correctly. | 2,963.63 | 3.98 |
We can now open up, as I will here,
swap dot c, which I wrote in advance. | 2,967.61 | 5.1 |
That looks almost the same, except
that I've changed the swap function | 2,972.71 | 3.94 |
as follows. a is now
int*, b is now int*, | 2,976.65 | 3.5 |
and I also borrowed the stars
inside of the function, as well. | 2,980.15 | 3.62 |
But something's gotta change. | 2,983.77 | 1.88 |
There's one more line of code I need
to change for all of this to work. | 2,985.65 | 4.46 |
What is that? | 2,990.11 | 0.67 |
What line needs to change? | 2,994.161 | 1.649 |
Well who cares about swap? | 2,995.81 | 1.73 |
It's main that was calling
this thing in the first place, | 2,997.54 | 3.07 |
so let's go back to the original story. | 3,000.61 | 2.29 |
Main, here, declares x and y as 1 and
2, does some printfs here, as before. | 3,002.9 | 5.73 |
But notice this line has to change. | 3,008.63 | 1.64 |
So one more piece of syntax today. | 3,010.27 | 2.739 |
And we're running out of new symbols. | 3,013.009 | 1.541 |
We've seen most of C already. | 3,014.55 | 2.11 |
&x and &y means get me the address
of x, and get me the address of y, | 3,016.66 | 7 |
and pass those in instead. | 3,023.66 | 1.89 |
So x,y would just mean pass in
a copy of x and a copy of y, | 3,025.55 | 4.11 |
or the values thereof. | 3,029.66 | 2.23 |
&x &y means give me a little, you know,
map with the address of x and a little | 3,031.89 | 5.36 |
map with the address of y, so that
swap-- who's receiving those maps-- | 3,037.25 | 4.71 |
can go there. | 3,041.96 | 1.98 |
So what does this mean
in pictorial form? | 3,043.94 | 2.81 |
If we now go back to the
beginning of this story, | 3,046.75 | 2.26 |
where we were looking at my computer's
memory as this big rectangular region | 3,049.01 | 4.53 |
like this. | 3,053.54 | 1.26 |
With main's chunk of
memory at the bottom here. | 3,054.8 | 4.61 |
And inside of main was two variables,
like x, and another variable y. | 3,059.41 | 6.16 |
And inside of those were
the numbers 1 and 2. | 3,065.57 | 3.4 |
And then I called swap. | 3,068.97 | 1.92 |
And so swap gets its own frame
on the stack, so to speak. | 3,070.89 | 3.57 |
This is swap's frame. | 3,074.46 | 1.77 |
It, too, had a variable called
a and a variable called b. | 3,076.23 | 6.472 |
But what goes in there now? | 3,082.702 | 1.638 |
It's not 1 and 2. | 3,084.34 | 1.49 |
We need to know a little something
more about my computer's memory. | 3,085.83 | 2.8 |
And I don't know where
everything's laid out, | 3,088.63 | 2.22 |
but let me just arbitrarily
assume that, you know, | 3,090.85 | 3.42 |
it's inside of my computer's memory. | 3,094.27 | 1.92 |
Maybe this is byte number 90. | 3,096.19 | 2.48 |
This is going to be 91. | 3,098.67 | 1.97 |
This here is going to be 92,
93, 94, 95, and so forth. | 3,100.64 | 3.91 |
I just need to know that there's
some kind of numbering scheme there. | 3,104.55 | 3.33 |
So what goes inside of a is 91. | 3,107.88 | 3.51 |
What goes inside of b is 92. | 3,111.39 | 3.94 |
And not the values 1 and 2, but rather
the addresses of those values 1 and 2. | 3,115.33 | 7.22 |
Because now my code for the swap
function, consider what it does. | 3,122.55 | 4.23 |
It says, upon receiving
the address of an integer, | 3,126.78 | 3.44 |
called a, upon receiving the address
of another integer, called b, | 3,130.22 | 4.531 |
go there and store that value in temp. | 3,134.751 | 3.469 |
Go to the address in b and store
that value at the address in a. | 3,138.22 | 4.46 |
Store the value in temp
at the address in b. | 3,142.68 | 3.6 |
So let's see what happens then. | 3,146.28 | 2.26 |
So first of all, I need another
variable here, called temp. | 3,148.54 | 4.51 |
Temp, meanwhile, is not a pointer. | 3,153.05 | 3.56 |
It's just an integer,
but what does it store? | 3,156.61 | 3.54 |
Well, according to my
code, temp gets the value | 3,160.15 | 4.55 |
of going to a, going
to the address in a. | 3,164.7 | 3.25 |
So what is a? | 3,167.95 | 1.583 |
a is 99. | 3,169.533 | 1.777 |
That's like a treasure map leading
to, OK, this chunk of memory | 3,171.31 | 3.4 |
down here in my computer. | 3,174.71 | 1.61 |
And what value is there
once I've gone there? | 3,176.32 | 2.11 |
Once I've gone to the CS building
inside of it, I see the number 1, | 3,178.43 | 3.8 |
and so I put the number 1 in temp. | 3,182.23 | 3.12 |
Meanwhile, my second line of
code says go to the address in b | 3,185.35 | 4.795 |
and grab its value, and put
it at the address that's in a. | 3,190.145 | 5.015 |
So what does that mean? | 3,195.16 | 1.64 |
Well, star b means
start here and go to 92. | 3,196.8 | 4.767 |
So it's like an arrow-- kind
of like chutes and ladders, | 3,201.567 | 2.333 |
if you know the game--
like go to address 92. | 3,203.9 | 2.64 |
What value is there? | 3,206.54 | 0.99 |
The number 2. | 3,207.53 | 0.98 |
And the other half of the
equation, on the left, | 3,208.51 | 2 |
said, go to the address
in a, which is here, | 3,210.51 | 2.79 |
and copy the 2 into that location. | 3,213.3 | 4.89 |
And then the last line--
only one more line-- | 3,218.19 | 2.61 |
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