text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
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So now let's re-compile this
program, do dot slash floats. | 3,241.14 | 3.56 |
>> And for the third time, let's
see if it meets my expectations. | 3,244.7 | 3.25 |
1, 10, enter, yes, OK, 1.000,
divided by 10.000, is 0.100000. | 3,247.95 | 13.53 |
And it turns out we can control how many
numbers are after those decimal points. | 3,261.48 | 3.472 |
We actually will. | 3,264.952 | 0.708 |
We'll come back to that. | 3,265.66 | 1.13 |
>> But now, in fact, the math is correct. | 3,266.79 | 1.65 |
So, again, what's the takeaway here? | 3,268.44 | 1.65 |
It turns out that in C, there are
not only just strings-- and, in fact, | 3,270.09 | 2.96 |
there aren't really, because we
add those with the CS50 library. | 3,273.05 | 3.07 |
But there aren't just ints. | 3,276.12 | 1.59 |
>> There are also floats. | 3,277.71 | 1.28 |
And it turns out a bunch of other data
types too, that we'll use before long. | 3,278.99 | 3.82 |
Turns out if you want a single
character, not a string of characters, | 3,282.81 | 3.46 |
you can use just a char. | 3,286.27 | 1.34 |
>> Turns out that if you want a bool,
a Boolean value, true or false only, | 3,287.61 | 4.74 |
thanks to the CS50 library, we've
added to C the bool data type as well. | 3,292.35 | 4.49 |
But it's also present in
many other languages as well. | 3,296.84 | 2.34 |
And it turns out that sometimes you
need bigger numbers then come by default | 3,299.18 | 4.95 |
with ints and floats. | 3,304.13 | 1.08 |
>> And, in fact, a double is a number
that uses not 32 bits, but 64 bits. | 3,305.21 | 5.38 |
And a long long is a number that
uses not 32, bits but 64 bits, | 3,310.59 | 4.4 |
respectively, for floating point
values and integers, respectively. | 3,314.99 | 4.2 |
So let's actually now
see this in action. | 3,319.19 | 3.59 |
>> I'm going to go ahead here
and whip up one other program. | 3,322.78 | 3.37 |
Here, I'm going to go ahead
and do include CS50.h. | 3,326.15 | 5.87 |
And let me go, include standard IO.h. | 3,332.02 | 2.89 |
>> And you'll notice something
funky is happening here. | 3,334.91 | 2.41 |
It's not color coding things in
the same way as it did before. | 3,337.32 | 3.272 |
And it turns out, that's because I
haven't given the thing a file name. | 3,340.592 | 2.958 |
>> I'm going to call this one
sizeof.c, and hit Save. | 3,343.55 | 3.72 |
And notice what happens to my very
white code against that black backdrop. | 3,347.27 | 3.769 |
Now, at least there's
some purple in there. | 3,351.039 | 1.791 |
And it is syntax highlighted. | 3,352.83 | 1.66 |
>> That's because, quite simply, I've
told the IDE what type of file | 3,354.49 | 3.21 |
it is by giving it a name, and
specifically a file extension. | 3,357.7 | 3.36 |
Now, let's go ahead and do this. | 3,361.06 | 2.56 |
I'm going to go ahead and very
simply print out the following-- bool | 3,363.62 | 5.29 |
is percent LU. | 3,368.91 | 2.17 |
>> We'll come back to
that in just a moment. | 3,371.08 | 1.87 |
And then I'm going to
print size of bool. | 3,372.95 | 2.89 |
And now, just to save
myself some time, I'm | 3,375.84 | 2.33 |
going to do a whole
bunch of these at once. | 3,378.17 | 2.11 |
And, specifically, I'm going to
change this to a char and char. | 3,380.28 | 4.34 |
This one, I'm going to change
to a double and a double. | 3,384.62 | 3.14 |
>> This one, I'm going to change
to a float and a float. | 3,387.76 | 3.68 |
This one, I'm going to
change to an int and an int. | 3,391.44 | 4.23 |
And this one, I'm going
to change to a long long. | 3,395.67 | 2.99 |
And it's still taking
a long time, long long. | 3,398.66 | 2.18 |
>> And then, lastly, I gave
myself one too many, string. | 3,400.84 | 3.732 |
It turns out that in C, there's
the special operator called | 3,404.572 | 2.458 |
size of that's literally
going to, when run, | 3,407.03 | 3.23 |
tell us the size of
each of these variables. | 3,410.26 | 1.839 |
And this is a way, now,
we can connect back | 3,412.099 | 1.791 |
to last week's discussion
of data and representation. | 3,413.89 | 3.25 |
>> Let me go ahead and compile
size of dot slash size of. | 3,417.14 | 3.19 |
And let's see. | 3,420.33 | 0.88 |
It turns out that in C,
specifically on CS50 IDE, | 3,421.21 | 4 |
specifically on the
operating system Ubuntu, | 3,425.21 | 2.96 |
which is a 64-bit operating
system in this case, | 3,428.17 | 2.93 |
a bool is going to
use one byte of space. | 3,431.1 | 3.089 |
That's how size is measured,
not in bits, but in bytes. | 3,434.189 | 2.291 |
And recall that one byte is eight bits. | 3,436.48 | 2.21 |
So a bool, even though you
technically only need a 0 or 1, | 3,438.69 | 3.34 |
it's a little wasteful
how we've implemented it. | 3,442.03 | 2.062 |
It's actually going to use a whole
byte-- so all zeros, are maybe | 3,444.092 | 2.708 |
all ones, or something like that,
or just one 1 among eight bits. | 3,446.8 | 4.25 |
>> A char, meanwhile, used for a character
like an Ascii character per last week, | 3,451.05 | 3.912 |
is going to be one character. | 3,454.962 | 1.208 |
And that synchs up with our notion of
it being no more than 256 bits-- rather, | 3,456.17 | 6.17 |
synchs up with it being no
longer than 8 bits, which | 3,462.34 | 3.02 |
gives us as many as 256 values. | 3,465.36 | 2.09 |
A double is going to
be 8 bytes or 64 bits. | 3,467.45 | 2.23 |
>> A float is 4. | 3,469.68 | 0.83 |
An int is 4. | 3,470.51 | 1.18 |
A long, long is 8. | 3,471.69 | 1.29 |
And a string is 8. | 3,472.98 | 1.736 |
But don't worry about that. | 3,474.716 | 1.124 |
We're going to peel back that layer. | 3,475.84 | 1.5 |
It turns out, strings can
be longer than 8 bytes. | 3,477.34 | 2.6 |
>> And, indeed, we've written
strings already, hello world, | 3,479.94 | 2.37 |
longer than 8 bytes. | 3,482.31 | 1.39 |
But we'll come back to
that in just a moment. | 3,483.7 | 2.57 |
But the take away here is the following. | 3,486.27 | 3.42 |
>> Any computer only has a finite
amount of memory and space. | 3,489.69 | 5.63 |
You can only store so many
files on your Mac or PC. | 3,495.32 | 2.54 |
You can only store so many programs in
RAM running at once, necessarily, even | 3,497.86 | 5.17 |
with virtual memory, because
you have a finite amount of RAM. | 3,503.03 | 3.33 |
>> And just to picture-- if
you've never opened up a laptop | 3,506.36 | 2.63 |
or ordered extra memory
for a computer, you | 3,508.99 | 2.31 |
might not know that
inside of your computer | 3,511.3 | 2.37 |
is something that looks
a little like this. | 3,513.67 | 2.92 |
So this is just a common company named
Crucial that makes RAM for computers. | 3,516.59 | 3.95 |
And RAM is where programs
live while they're running. | 3,520.54 | 3.08 |
>> So on every Mac or PC, when you double
click a program, and it opens up, | 3,523.62 | 3.01 |
and it opens some Word document
or something like that, | 3,526.63 | 2.291 |
it stores it temporarily in
RAM, because RAM is faster | 3,528.921 | 2.843 |
than your hard disk, or
your solid state disk. | 3,531.764 | 1.916 |
So it's just where programs go
to live when they're running, | 3,533.68 | 2.92 |
or when files are being used. | 3,536.6 | 1.46 |
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