Thread by @meetjain74

What is recursion?

A thread🧵

“In order to understand recursion, one must first understand recursion.”

📌Fun Activity
Google 'recursion' and you will find that Google will ask 'Did you mean: recursion'. Despite clicking on recursion from spell checker – Did you mean, Google will keep on displaying it. Though the spelling is right, Google keeps on telling it again and again.

Well, that is what exactly recursion is.

📌What is recursion?
Recursion is the process that comes into existence when a function calls a copy of itself to work on a smaller problem. Any function which calls itself is called a recursive function.

📌Example
Consider you need to find a way to your home. So you could define the operation "find your way home" as-
1. If you are at home, stop moving.
2. Take one step toward home.
3. "find your way home".

In the above example -
First, we don't go home if we are already home. Secondly, we do a very simple action that makes our situation simpler to solve. Finally, we redo the entire algorithm.

This is where the very last statement is calling the recursive algorithm.

→ RECURSION

📌Infinite recursion
Infinite Recursion occurs when the recursion carries on infinitely.
Example:

A recursive function always has to say when to stop repeating itself. Hence there should always be two parts to a recursive function: the recursive case and the base case.

📌Base case and the recursive case
The recursive case is when the function calls itself. The base case is when the function stops calling itself. This prevents infinite loops.
Example:

📌Factorial recursion example
You can code the factorial function easily using the recursion approach by using the formula
factorial(x)=x*factorial(x-1)

📌The call stack
Recursive functions use something called “the call stack.” When a program calls a function, that function goes on top of the call stack.

I will show you the call stack in action with the factorial function.

📌Illustration
Let’s see what happens if you call factorial(3). The illustration below shows how the stack changes, line by line.
The topmost box in the stack tells you what call to factorial you’re currently on.

Notice how each call to factorial 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.

I hope you might have understood what recursion is. Thanks for getting to the end of the thread.

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