What is recursion

April 11, 2020

What is recursion?

Recursion is a process of calling itself. A function that calls itself is called a recursive function.

The syntax for recursive function is:

function recurse() {
    // function code
    recurse();
    // function code
}

recurse();

πŸ‘‰ The recurse() function is a recursive function. It is calling itself inside the function.

A recursive function must have a condition to stop calling itself. Otherwise, the function is called indefinitely.

Once the condition is met, the function stops calling itself. This is called a base condition.

To prevent infinite recursion, you can use if...else statement (or similar approach) where one branch makes the recursive call, and the other doesn't.

function recurse() {
    if(condition) {
        recurse();
    }
    else {
        // stop calling recurse()
    }
}

recurse();

Print Numbers

// program to count down numbers to 1
function countDown(number) {

    // display the number
    console.log(number);

    // decrease the number value
    const newNumber = number - 1;

    // base case
    if (newNumber > 0) {
        countDown(newNumber);
    }
}

countDown(4);

πŸ‘‰ the user passes a number as an argument when calling a function.

In each iteration, the number value is decreased by 1 and function countDown() is called until the number is positive. Here, newNumber > 0is the base condition.

This recursive call can be explained in the following steps:

countDown(4) prints 4 and calls countDown(3)
countDown(3) prints 3 and calls countDown(2)
countDown(2) prints 2 and calls countDown(1)
countDown(1) prints 1 and calls countDown(0)

When the number reaches 0, the base condition is met, and the function is not called anymore.

Find Factorial

// program to find the factorial of a number
function factorial(x) {

    // if number is 0
    if (x === 0) {
        return 1;
    }

    // if number is positive
    else {
        return x * factorial(x - 1);
    }
}

const num = 3;

// calling factorial() if num is non-negative
if (num > 0) {
    let result = factorial(num);
    console.log(`The factorial of ${num} is ${result}`);
}


// The factorial of 3 is 6

When you call function factorial() with a positive integer, it will recursively call itself by decreasing the number.

This process continues until the number becomes 1. Then when the number reaches 0, 1 is returned.

factorial(3) returns 3 * factorial(2)
factorial(2) returns 3 * 2 * factorial(1)
factorial(1) returns 3 * 2 * 1 * factorial(0)
factorial(0) returns 3 * 2 * 1 * 1

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