Introduction: recursion

Review: recursion
- Recursion:

Recursion is nothing special !!!

Note:

Although you may have heard that a recursive method/function calls ifself; I have to say that:

What I mean is this:

When a (recursive) function F(n) calls the (recursive) function F(n-1):

F(n) and F(n-1) shares nothing !!
I.e.:

If 2 methods do not share their state (= information in variables), then they are actually 2 different activations that operate independently from each other.

Analogy:

A pair of identical twins look alike.
But: we will never call the twins the same person because they do not share anything !!!

How to implement recursive methods/functions

Implementing recursive methods/functions is very simple:

Because a recursive method/function always contains a call/invocation of another method/function (that happens to have the same method/function name):

Therefore:

We cannot keep the parameters and local variables in registers
(Because the values of the parameters and local variables kept in registers of F(n) will be destroyed by F(n-1) !!!)

Conclussion:

Recursive methods/functions must use the program stack to store parameters and local variables !!!

Surprise:

If you use the program stack to store the parameters and local variables (a technique that we have discussed already !!!), the method/function will be recursive !!!

A how to guide to write recursive methods/functions:

We must store the parameters and local variables of recursive methods/functions on the program stack !!!

That means:

The caller (of the recursive method must push the parameters onto the program stack
The recursive method must allocate its local variables on the program stack

We have already studied how to pass parameters using the stack and how to allocate local variables on the stack

I will illustrate the how to guide by writing a few recursive methods in assembler code next.