# Recursion: A Programming Concept

If you studied computer science or you're into programming, you most likely have come across the term *Recursion*. Quite a number of developer don't really use recursion, or perhaps it's the ones I know that don't use it. 🤔🤔

So, what is recursion?

Recursion is an iteration technique where a function calls itself. It's applicable when you need to call the same function repeatedly with different parameters from within a loop. 

Search for *Recursion* on google and you get this.
![image.png](https://cdn.hashnode.com/res/hashnode/image/upload/v1601636931241/c-Q5vsvpu.png)
Now, click against the suggested *recursion*. 

What happens? You're redirected to the same page. That's recursion with a sense of humour.

## Recursion vs Loop

Let's take a look at 2 scenarios

Scenario A- Loop

```
const pow = (x,n)=>{
  let result = 1;

  for (let i = 0; i < n; i++) {
    result *= x;
  }
  return result;
}

console.log( pow(2, 3) ); // 8
``` 


Scenario B- Recursion
```
const pow = (x,n)=>{
  if (n == 1) {
    return x;
  } else {
    return x * pow(x, n - 1);
  }
}
console.log( pow(2, 3) ); // 8
```

What do you note between these 2 scenarios? 
In scenario A, we *loop* over the variables i.e instructions are repeatedly executed while in scenario B, the function calls itself.

To calculate pow(2, 3) the recursive steps are as below:

pow(2, 3) = 2 * pow(2, 2)  => 2 * 4 = 8

pow(2, 2) = 2 * pow(2, 1)   => 2 * 2 = 4

pow(2, 1) = 2

Recursion reduces a function call to a simpler one until the result is gotten.

It's essential to define a breakpoint (```n==1``` as in the example above) else your function will run indefinitely.

## Differences between Recursion and Loops
1. **Time:** Recursion is more time consuming than loops.
2. **Usage:** Recursion is used when code size needs to be small, and time complexity is not an issue. A loop is used when time complexity needs to be measured against the code size.
3. **Termination:** Recursive function is terminated when there is no function call.	A loop is terminated when the condition for the iterator ceases to be satisfied.


## Application of Recursion
1. Fractal maths
2. Sorting
3. Traversing the nodes of complex or non-linear data structure
4. File trees

## Conclusion
Recursion is a powerful technique and is the most direct way to solve a complex problem. However, due to its shortcomings, we will need to be very careful about how and where we apply recursion.

## References
1. [Recursion in Functional JavaScript](https://www.sitepoint.com/recursion-functional-javascript/#:~:text=Recursion%20is%20a%20technique%20for,to%20looping%20is%20the%20default.)
2. [Understanding Recursion Using Real-World Examples](https://www.byte-by-byte.com/understanding-recursion/) 
3. [Difference between Recursion and Iteration](https://www.geeksforgeeks.org/difference-between-recursion-and-iteration/) 
4. [Recursion and stack](https://javascript.info/recursion) 

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> Ciao👋🏼