This document provides an introduction to recursion in computer science. It explains that recursion is when a function calls itself, and gives an example of writing a recursive function to calculate the factorial of a number. The factorial function takes a number as a parameter, has a base case where it returns 1 if the number is 1, and recursively calls itself with the number-1 parameter while multiplying the return values to calculate the factorial.

1. Intro to
Recursion

2. What is
recursion?
Throughout your computer science journey so
far, you’ve most likely heard of iteration and
multiple iteration, which is repeating a
particular block of code for a certain amount of
times until you stop
These are represented as FOR loops and
WHILE loops
But iteration may not be the only way to solve
certain problems, and there may even be a
better way
An alternative way for programming problem
solving is called recursion

3. What is recursion?
Recursion happens when a function calls
ITSELF. This can be a strange concept for you
to wrap your head around. But it CAN be done
We will be building a recursive function to return
the factorial of a number. A factorial is the
product of a number and all the positive integers
that are smaller than it
(For all intents and purposes, this will be in
pseudocode so you can use any language you
want for your examples)
We will call this function factorial (int
number)
Recursion
Recursion
Recursion
Recursion
Recursion
Recursion
Recursion
Recursion

4. What is recursion?
The factorial function takes in ONE
parameter and that’s the number we are
trying to find the factorial of. Let’s assume
that no-one will input a number less than 1
There are 2 parts to a recursive function:
The first is the base case, which is critical for
getting our recursive function to EXIT
if(number == 1){
return 1;
}
We already know that the factorial of 1 is 1,
so we can say:

5. What is recursion?
We can write the recursive step as:
return number * factorial(number – 1).
Our completed function is:
int factorial (int number) {
if number == 1
return 1
return number * factorial(number – 1);
}
The next part is the recursive
step. This is where we get the
function to call itself. What do we
know about factorials? That the
factorial of a number CAN be
equal to the number itself
[multiplied by] the factorial of the
previous number

6. What is recursion?
Now let’s put a number into this function
as an example and see how we do, and
see how recursive functions operate
Factorial (4) 4 · Factorial (3)
=
3 · Factorial (2)
2 · Factorial (1)
Let’s try 4:
Every time we call the recursive function,
it gets pushed to the top of the stack until
we hit a base case (number = 1). Then
that value gets returned and all the
recursive calls get popped off the stack
one by one. 4 * 3 * 2 * 1 = 24, which IS 4!
This was a small intro to recursion!

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