This article is based on Free Code Camp Basic Algorithm Scripting “Factorialize a Number”
In mathematics, the factorial of a non-negative integer n can be a tricky algorithm. In this article, I’m going to explain three approaches, first with the recursive function, second using a while loop and third using a for loop.
We have already seen a recursion approach on a String in the previous article, How to Reverse a String in JavaScript in 3 Different Ways ? This time we will apply the same concept on a number.
Algorithm Challenge
Return the factorial of the provided integer.
If the integer is represented with the letter n, a factorial is the product of all positive integers less than or equal to n.
Factorials are often represented with the shorthand notation n!
For example: 5! = 1 * 2 * 3 * 4 * 5 = 120
function factorialize(num) {
return num;
}
factorialize(5);Provided test cases
- factorialize(0) should return 1
- factorialize(5) should return 120
- factorialize(10) should return 3628800
- factorialize(20) should return 2432902008176640000
What is factorializing a number all about?
When you factorialize a number, you are multiplying that number by each consecutive number minus one.
If your number is 5, you would have:
5! = 5 * 4 * 3 * 2 * 1The pattern would be:
0! = 1
1! = 1
2! = 2 * 1
3! = 3 * 2 * 1
4! = 4 * 3 * 2 * 1
5! = 5 * 4 * 3 * 2 * 11. Factorialize a Number With Recursion
function factorialize(num) {
// If the number is less than 0, reject it.
if (num < 0)
return -1;
// If the number is 0, its factorial is 1.
else if (num == 0)
return 1;
// Otherwise, call the recursive procedure again
else {
return (num * factorialize(num - 1));
/*
First Part of the recursion method
You need to remember that you won’t have just one call, you’ll have several nested calls
Each call: num === "?" num * factorialize(num - 1)
1st call – factorialize(5) will return 5 * factorialize(5 - 1) // factorialize(4)
2nd call – factorialize(4) will return 4 * factorialize(4 - 1) // factorialize(3)
3rd call – factorialize(3) will return 3 * factorialize(3 - 1) // factorialize(2)
4th call – factorialize(2) will return 2 * factorialize(2 - 1) // factorialize(1)
5th call – factorialize(1) will return 1 * factorialize(1 - 1) // factorialize(0)
Second part of the recursion method
The method hits the if condition, it returns 1 which num will multiply itself with
The function will exit with the total value
5th call will return (5 * (5 - 1)) // num = 5 * 4
4th call will return (20 * (4 - 1)) // num = 20 * 3
3rd call will return (60 * (3 - 1)) // num = 60 * 2
2nd call will return (120 * (2 - 1)) // num = 120 * 1
1st call will return (120) // num = 120
If we sum up all the calls in one line, we have
(5 * (5 - 1) * (4 - 1) * (3 - 1) * (2 - 1)) = 5 * 4 * 3 * 2 * 1 = 120
*/
}
}
factorialize(5);Without comments:
function factorialize(num) {
if (num < 0)
return -1;
else if (num == 0)
return 1;
else {
return (num * factorialize(num - 1));
}
}
factorialize(5);2. Factorialize a Number with a WHILE loop
function factorialize(num) {
// Step 1. Create a variable result to store num
var result = num;
// If num = 0 OR num = 1, the factorial will return 1
if (num === 0 || num === 1)
return 1;
// Step 2. Create the WHILE loop
while (num > 1) {
num--; // decrementation by 1 at each iteration
result = result * num; // or result *= num;
/*
num num-- var result result *= num
1st iteration: 5 4 5 20 = 5 * 4
2nd iteration: 4 3 20 60 = 20 * 3
3rd iteration: 3 2 60 120 = 60 * 2
4th iteration: 2 1 120 120 = 120 * 1
5th iteration: 1 0 120
End of the WHILE loop
*/
}
// Step 3. Return the factorial of the provided integer
return result; // 120
}
factorialize(5);Without comments:
function factorialize(num) {
var result = num;
if (num === 0 || num === 1)
return 1;
while (num > 1) {
num--;
result *= num;
}
return result;
}
factorialize(5);3. Factorialize a Number with a FOR loop
function factorialize(num) {
// If num = 0 OR num = 1, the factorial will return 1
if (num === 0 || num === 1)
return 1;
// We start the FOR loop with i = 4
// We decrement i after each iteration
for (var i = num - 1; i >= 1; i--) {
// We store the value of num at each iteration
num = num * i; // or num *= i;
/*
num var i = num - 1 num *= i i-- i >= 1?
1st iteration: 5 4 = 5 - 1 20 = 5 * 4 3 yes
2nd iteration: 20 3 = 4 - 1 60 = 20 * 3 2 yes
3rd iteration: 60 2 = 3 - 1 120 = 60 * 2 1 yes
4th iteration: 120 1 = 2 - 1 120 = 120 * 1 0 no
5th iteration: 120 0 120
End of the FOR loop
*/
}
return num; //120
}
factorialize(5);Without comments:
function factorialize(num) {
if (num === 0 || num === 1)
return 1;
for (var i = num - 1; i >= 1; i--) {
num *= i;
}
return num;
}
factorialize(5);I hope you found this helpful. This is part of my “How to Solve FCC Algorithms” series of articles on the Free Code Camp Algorithm Challenges, where I propose several solutions and explain step-by-step what happens under the hood.
If you have your own solution or any suggestions, share them below in the comments.
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