text stringlengths 1 81 | start float64 0 10.1k | duration float64 0 24.9 |
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Now I'm, of course, going to say
printf positive, backslash n. | 5,442.37 | 4.66 |
And then else if-- I could do this. | 5,447.03 | 3.66 |
>> I could do if i equals 0. | 5,450.69 | 2.72 |
But I'd be making at
least one mistake already. | 5,453.41 | 2.43 |
Recall that the equal sign is
not equal, as we humans know it. | 5,455.84 | 3.64 |
>> But it's the assignment operator. | 5,459.48 | 1.53 |
And we don't want to take 0 on the
right and put it in i on the left. | 5,461.01 | 4.63 |
So to avoid this confusion, or
perhaps misuse of the equals sign, | 5,465.64 | 6.17 |
humans decided some years ago
that in many programming languages | 5,471.81 | 2.93 |
when you want to check for equality
between the left and the right, | 5,474.74 | 3.26 |
you actually use equals equals. | 5,478 | 1.635 |
So you hit the equals sign twice. | 5,479.635 | 1.375 |
When you want to assign from right
to left, you use a single equal sign. | 5,481.01 | 4.59 |
So we could do this-- else
if i equals equals zero. | 5,485.6 | 3.76 |
>> I could then go and
open my curly braces, | 5,489.36 | 2.35 |
and say, printf 0, backslash n, done. | 5,491.71 | 4.377 |
But remember how these
forks in the road can work. | 5,496.087 | 2.083 |
And, really, just think about the logic. | 5,498.17 | 1.666 |
i is a number. | 5,499.836 | 1.674 |
It's an integer, specifically. | 5,501.51 | 1.81 |
And that means it's going to be less
than 0, or greater than 0, or 0. | 5,503.32 | 5.28 |
So there is kind of this
implied default case. | 5,508.6 | 3 |
>> And so we could, just like
Scratch, dispense with the else if, | 5,511.6 | 3.32 |
and just say else. | 5,514.92 | 0.827 |
Logically, if you the
programmer know there's only | 5,515.747 | 2.083 |
three buckets into which a
scenario can fall-- the first, | 5,517.83 | 3.805 |
the second, or the third
in this case-- don't | 5,521.635 | 1.875 |
bother adding the additional precision
and the additional logic there. | 5,523.51 | 3.59 |
Just go ahead with the
default case here of else. | 5,527.1 | 2.59 |
>> Now, let's go ahead
after saving this, make | 5,529.69 | 2.26 |
conditions dot slash conditions--
not a great user interface, | 5,531.95 | 3.81 |
because I'm not prompting the
user, as I mentioned earlier. | 5,535.76 | 3.154 |
But that's fine. | 5,538.914 | 0.666 |
We'll keep it simple. | 5,539.58 | 0.874 |
Let's try the number 42. | 5,540.454 | 1.436 |
And that's positive. | 5,541.89 | 1.35 |
Let's try the number
negative 42, negative. | 5,543.24 | 2.88 |
>> Let's try the value 0. | 5,546.12 | 2.124 |
And, indeed, it works. | 5,548.244 | 0.916 |
Now, you'll see with problems before
long, testing things three times, | 5,549.16 | 4.74 |
probably not sufficient. | 5,553.9 | 1.08 |
You probably want to test some
bigger numbers, some smaller | 5,554.98 | 2.458 |
numbers, some corner cases, as
we'll come to describe them. | 5,557.438 | 3.082 |
>> But for now, this is a
pretty simple program. | 5,560.52 | 1.98 |
And I'm pretty sure, logically,
that it falls into three cases. | 5,562.5 | 2.66 |
And, indeed, even though we just
focused on the potential downsides | 5,565.16 | 4.2 |
of imprecision and overflow, in
reality where many of CS50's problems, | 5,569.36 | 4.12 |
we are not going to worry
about, all the time, | 5,573.48 | 2.52 |
those issues of overflow and
imprecision, because, in fact, in C, | 5,576 | 3.05 |
it's actually not all that
easy to avoid those things. | 5,579.05 | 2.839 |
If you want to count up
bigger, and bigger, and bigger, | 5,581.889 | 2.291 |
it turns out there are techniques you
can use, often involving things called | 5,584.18 | 3.33 |
libraries, collections of code, that
other people wrote that you can use, | 5,587.51 | 3.73 |
and other languages like
Java and others, actually | 5,591.24 | 2.67 |
make it a lot easier
to count even higher. | 5,593.91 | 1.89 |
So it really is some of these dangers
a function of the language you use. | 5,595.8 | 4.01 |
And in the coming weeks, we'll
see how dangerous C really | 5,599.81 | 2.9 |
can be if you don't use it properly. | 5,602.71 | 2.24 |
But from there, and with
Python, and JavaScript, will | 5,604.95 | 2.66 |
we layer on some additional protections,
and run fewer of those risks. | 5,607.61 | 5.01 |
>> So let's make a little more
interesting logic in our program. | 5,612.62 | 3.2 |
So let me go ahead and create
a program called Logical | 5,615.82 | 3.29 |
just so I can play with some
actual logic, logical.c. | 5,619.11 | 4.694 |
I'll just copy and paste some
code from earlier so I get back | 5,623.804 | 3.066 |
to this nice starting point. | 5,626.87 | 3.08 |
>> Let me this time do char C. I'm
going to give it a name of C | 5,629.95 | 4.03 |
just because it's conventional,
get a character from the user. | 5,633.98 | 4.53 |
And let's pretend like
I'm implementing part | 5,638.51 | 2.22 |
of that Rm program, the remove
program before that prompted the user | 5,640.73 | 3.4 |
to remove a file. | 5,644.13 | 1.27 |
How could we do this? | 5,645.4 | 1.35 |
>> I want to say, if C equals
equals, quote unquote, | 5,646.75 | 4.34 |
y, then I'm going to assume
that the user has chosen yes. | 5,651.09 | 5.214 |
I'm just going to print yes. | 5,656.304 | 1.166 |
If it were actually writing
the removal program, | 5,657.47 | 1.97 |
we could remove the file
with more lines of code. | 5,659.44 | 1.98 |
But we'll keep it simple. | 5,661.42 | 1.041 |
>> Else if c equals equals n--
and now here, I'm going to say, | 5,665.95 | 5.3 |
the user must have meant no. | 5,671.25 | 1.73 |
And then else, you know what? | 5,672.98 | 1.38 |
I don't know what else
the user is going to type. | 5,674.36 | 1.84 |
So I'm just going to say that
that is an error, whatever | 5,676.2 | 2.333 |
he or she actually typed. | 5,678.533 | 1.537 |
>> So what's going on here? | 5,680.07 | 1.11 |
There is a fundamental difference
versus what I've done in the past. | 5,681.18 | 3.35 |
Double quotes, double quotes, double
quotes, and, yet, single quotes, | 5,684.53 | 4.77 |
single quotes. | 5,689.3 | 0.87 |
It turns out in C, that when
you want to write a string, | 5,690.17 | 2.69 |
you do use double quotes, just as we've
been using all this time with printf. | 5,692.86 | 3.82 |
>> But if you want to deal with just a
single character, a so-called char, | 5,696.68 | 5.35 |
then you actually use single quotes. | 5,702.03 | 1.75 |
Those of you who've programmed
before, you might not have | 5,703.78 | 1.67 |
had to worry about this
distinction in certain languages. | 5,705.45 | 2.4 |
In C, it does matter. | 5,707.85 | 1.6 |
And so when I get a char and I want
to compare that char using equals | 5,709.45 | 3.11 |
equals to some letter like y or n, I do,
indeed, need to have the single quotes. | 5,712.56 | 5.79 |
>> Now, let's go ahead and do this. | 5,718.35 | 1.42 |
Let's go ahead and do make
logical dot slash logical. | 5,719.77 | 6.41 |
And now I'm being prompted. | 5,726.18 | 1.125 |
So, presumably, a better user experience
would actually tell me what to do here. | 5,727.305 | 3.333 |
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