Intro to
Recursio
n
What is
Throughout your computer science
journey so far, you’ve most likely heard
of iteration and multiple iteration, which
recursion
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
What is recursion?
Recursion happens when a function calls We will call this function factorial (int
ITSELF. This can be a strange concept for number)
you to wrap your head around. But it
CAN be done
Recursion
We will be building a recursive function Recursion
to return the factorial of a number. A Recursion
factorial is the product of a number and
Recursion
all the positive integers that are smaller
Recursion
than it
Recursion
(For all intents and purposes, this will be Recursion
in pseudocode so you can use any Recursion
language you want for your examples)
What is recursion?
The factorial function takes in ONE We already know that the factorial of
parameter and that’s the number we 1 is 1, so we can say:
are trying to find the factorial of. Let’s
assume that no-one will input a
number less than 1
if(number ==
There are 2 parts to a recursive 1){
function: return 1;
}
The first is the base case, which is
critical for getting our recursive
function to EXIT
What is recursion?
The next part is the We can write the recursive step as:
recursive step. This is where
we get the function to call
return number *
itself. What do we know
about factorials? That the
factorial(number – 1).
factorial of a number CAN be Our completed function is:
equal to the number itself
[multiplied by] the factorial int factorial (int number) {
of the previous number
if number == 1
return 1
return number *
factorial(number – 1);
}
What is recursion?
Now let’s put a number into this Every time we call the recursive
function as an example and see function, it gets pushed to the top
how we do, and see how recursive of the stack until we hit a base case
functions operate (number = 1). Then that value gets
Let’s try 4: returned and all the recursive calls
get popped off the stack one by
Factorial 4 · Factorial one. 4 * 3 * 2 * 1 = 24, which IS 4!
=
(4) (3)
3 · Factorial This was a small intro to recursion!
(2)
2 · Factorial
(1)
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s
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