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>> DAVID J. MALAN: Let's write a program in
which we try to swap the values of

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two variables, x and y.

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In advance, I've written much
of this program already.

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First, I declare a variable, x,
and store in it the value, 1.

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Then I declare variable y and
store in it the value, 2.

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>> Then I proceed with printf to display
the values of those two variables.

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Then I claim with another printf
to be swapping those variables.

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Then I call a function called swap.

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And then I claim that the variables
have been swapped.

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And then I print out that what I claim
are the new values of x and y.

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>> Now what is this function swap?

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It's not something that comes with C.
Indeed, if we look at the top of my

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file, you'll notice that I've declared
a prototype for this function swap

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specifying that it takes two ints,
arbitrarily called a and b, and this

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function doesn't return anything.

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So presumably, it does what it says
in swaps a's and b's values.

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Let's now implement swap.

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>> First, I'm going to declare a temporary
variable and assign to it

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the value in a, though I could just
as easily stored in it b.

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I'm then going to change the value
of a to be equal to that in b.

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And then lastly, I'm going to change the
value of b to be what a was but is

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now in temp.

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>> Now I claim what I've just done
is logically correct.

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I've stored a in a temporary variable,
changed a's value to be b's, and then

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changed b's value to be what a's was.

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But when I compile and run this
program, I worry that's not

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going what I see.

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>> Make, no swap.

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dot slash, no swap.

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And unfortunately, according to printf,
the value of x and y, even

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after we claim to be swapping
them, is still 1 and 2.

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>> Now why is that?

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Well, it turns out that in main, when we
call this function swap, passing in

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x and y as arguments, it's actually
copies of x and y that

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are passed into swap.

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Namely, their values 1 and 2 are
passed into swap as a and b.

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And it's a and b that we're ultimately
swapping in main.

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>> That is, 1 becomes 2, 2 becomes 1.

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But because swap has no return value
and because it received copies of x

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and y, it's behavior has
no effect on x and y.

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Indeed, a and b are scoped, so to speak,
to swap, whereas x and y remain

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scoped to main.

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>> So we have a problem.

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But how to fix it?

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