Why am I getting an Infinite loop in javascript

Question

I'm trying to return the factorial of the provided integer. When doing it like this, e.g:

factorialize(num) {
    for (var i = 1;i <= num; i++){
        num*=i;
    }
        return num;
    }
factorialize(5);

I'm getting an infinite loop. While I understand that this shouldn't give me the correct answer because my understanding of it would go something like this:

n! = 5 * 1 * 2 * 3 * 4 * 5 = 600

when really it should go:

n! = 1 * 2 * 3 * 4 * 5 = 120

But still, I don't understand why I am getting an infinite loop here?

Answer 1

The variable that you use on the for loop it's the same variable that you use to store the multiplication.

This should work:

factorialize(num) {
    var len = num;
    for (var i = 1;i <= len; i++){
        num*=i;
    }
        return num;
    }
factorialize(5);

Answer 2

The end condition for i<=num is always true hence the infinite loop because the argument num keeps increasing with each iteration. A better approach is

function factorialize(num) {
  var res = 1;
  if(num ===0 || num === 1){
    return 1;
	}
   for (var i = 2;i <= num; i++){
      res*=i;
    }
      return res;
    }
factorialize(5);

Answer 3

Lets say that the value of num variable is 2.

• First cycle:

  for (var i = 1; i <= 2; i++) { //true
     num *= i; //2 = 2 * 1 => 2
  }
  

• Second cycle:

  for (var i = 2; i <= 2; i++) { //true
     num *= i; //2 = 2 * 2 => 4
  }
  

• Third cycle:

  for (var i = 3; i <= 4; i++) { //true
     num *= i; //4 = 4 * 3 => 12
  }
  

• Fourth cycle:

  for (var i = 4; i <= 12; i++) { //true
     num *= i; //12 = 12 * 4 => 48
  }
  

The num value increases exponentially, while the i value increases linearly.

i <= num condition will be always fulfilled, that's why you are getting an infinite loop.

Snippet including the chart:

var num = {
  x: [1, 2, 3, 4, 5],
  y: [2, 4, 12, 48, 240],
  type: 'scatter',
  name: 'num'
};
var i = {
  x: [1, 2, 3, 4, 5],
  y: [1, 2, 3, 4, 5],
  type: 'scatter',
  name: 'i'
};
var data = [num, i];
Plotly.newPlot('myDiv', data);

<script src="https://cdn.plot.ly/plotly-latest.min.js"></script>
<div id="myDiv" style="width: 480px; height: 400px;">

Answer 4

This is because you are dynamically updating the value of num. So, after each iteration, the number gets bigger and bigger, and that's why the value of i will never be more than the value of num (for values of num larger than 1).

Answer 5

here i will never catch num

i = 1 num = 5  1 <= 5  //(num = 5*1)
i = 2 num = 5  2 <= 10 //(num = 5*2) 
i = 3 num = 10 3 <= 30 //(num = 10*3)
... 

Answer 6

Taking your example:

Before run 1: num = 5; After run 1: num = 5; After run 2: num = 10; After run 3: num = 30;

As per your condition, loop would stop only when num is more than i, which would never be the case, as you're modifying it's value on every iteration by a huge difference.

Answer 7

Your loop is chasing a moving target.

It will end when i reaches num, but you keep making num bigger; actually i will never reach num.

As you pointed out, your algorithm is wrong anyway.

function factorialize(num) {
   var result = 1;
   for (var i = 2; i <= num; i++) {
      result *= i;
   }
   return result;
}

console.log(factorialize(0));  // 1
console.log(factorialize(1));  // 1
console.log(factorialize(2));  // 2
console.log(factorialize(3));  // 6
console.log(factorialize(4));  // 24
console.log(factorialize(5));  // 120

Answer 8

You are getting an infinite loop because you are always updating num. You are doing num = num * i. So it can never reach the end of the loop. i will never be greater then num.

Answer 9

Because you change the value of 'num' in every iteration. You should use a third variable to count the product.