DAVID J. MALAN: Let's write a program in which we try to swap the values of two variables, x and y. In advance, I've written much of this program already. First, I declare a variable, x, and store in it the value, 1. Then I declare variable y and store in it the value, 2. 

Then I proceed with printf to display the values of those two variables. Then I claim with another printf to be swapping those variables. Then I call a function called swap. And then I claim that the variables have been swapped. And then I print out that what I claim are the new values of x and y. 

Now what is this function swap? It's not something that comes with C. Indeed, if we look at the top of my file, you'll notice that I've declared a prototype for this function swap specifying that it takes two ints, arbitrarily called a and b, and this function doesn't return anything. So presumably, it does what it says in swaps a's and b's values. Let's now implement swap. 

First, I'm going to declare a temporary variable and assign to it the value in a, though I could just as easily stored in it b. I'm then going to change the value of a to be equal to that in b. And then lastly, I'm going to change the value of b to be what a was but is now in temp. 

Now I claim what I've just done is logically correct. I've stored a in a temporary variable, changed a's value to be b's, and then changed b's value to be what a's was. But when I compile and run this program, I worry that's not going what I see. 

Make, no swap. dot slash, no swap. And unfortunately, according to printf, the value of x and y, even after we claim to be swapping them, is still 1 and 2. 

Now why is that? Well, it turns out that in main, when we call this function swap, passing in x and y as arguments, it's actually copies of x and y that are passed into swap. Namely, their values 1 and 2 are passed into swap as a and b. And it's a and b that we're ultimately swapping in main. 

That is, 1 becomes 2, 2 becomes 1. But because swap has no return value and because it received copies of x and y, it's behavior has no effect on x and y. Indeed, a and b are scoped, so to speak, to swap, whereas x and y remain scoped to main. 

So we have a problem. But how to fix it?