Question

Data Structures

Draw memory call stack to show how recursion works in memory.

Answer 1

Recursive functions use something called “the call stack.” When a program calls a function, that function goes on top of the call stack. This similar to a stack of books. You add things one at a time. Then, when you are ready to take something off, you always take off the top item.

I will show you the call stack in action with the factorial function. factorial(5) is written as 5! and it is defined like this: 5! = 5 * 4 * 3 * 2 * 1. Here is a recursive function to calculate the factorial of a number:

function fact(x) {
  if (x == 1) {  
    return 1;  
  } else {      
    return x * fact(x-1);
  }
}

Now let’s see what happens if you call fact(3) The illustration bellow shows how the stack changes, line by line. The topmost box in the stack tells you what call to fact you’re currently on.

Notice how each call to fact has its own copy of x. This is very important to making recursion work. You can’t access a different function’s copy of x.