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?

make another variable to hold the result. Rather than changing the value of the parameter. — Ousmane D.
– Ousmane D., Commented Apr 13, 2017 at 22:16
@Lixus: Please answer in the answer section (clue's in the name) — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Apr 13, 2017 at 22:18

kind user · Accepted Answer · 2017-04-13 22:36:15Z

8

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;">

edited Apr 13, 2017 at 22:36

answered Apr 13, 2017 at 22:22

kind user

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1 Comment

Lightness Races in Orbit Over a year ago

num increases exponentially, not logarithmically.

Lightness Races in Orbit · Accepted Answer · 2017-04-13 22:19:47Z

2

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

answered Apr 13, 2017 at 22:19

Lightness Races in Orbit

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Comments

Matt D. Webb · Accepted Answer · 2017-04-13 23:09:44Z

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);

edited Apr 13, 2017 at 23:09

Matt D. Webb

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answered Apr 13, 2017 at 22:20

Andoni

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Comments

Nekomajin42 · Accepted Answer · 2017-04-13 22:18:40Z

0

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

answered Apr 13, 2017 at 22:18

Nekomajin42

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Comments

Michael Gibson · Accepted Answer · 2017-04-13 22:19:45Z

0

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.

answered Apr 13, 2017 at 22:19

Michael Gibson

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Comments

Mohit · Accepted Answer · 2017-04-13 22:20:09Z

0

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.

answered Apr 13, 2017 at 22:20

Mohit

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Comments

shivam Gupta · Accepted Answer · 2017-04-13 22:21:54Z

0

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)
...

answered Apr 13, 2017 at 22:21

shivam Gupta

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Comments

Omar AlEisa · Accepted Answer · 2017-04-13 22:28:54Z

0

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).

edited Apr 13, 2017 at 22:28

answered Apr 13, 2017 at 22:18

Omar AlEisa

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5 Comments

Lightness Races in Orbit Over a year ago

It will if num is <= 0 at the start of the function.

Omar AlEisa Over a year ago

@BoundaryImpositio Yes you are right. But for value larger than 1, this will always be an infinite loop.

Lightness Races in Orbit Over a year ago

You said "never" :)

Omar AlEisa Over a year ago

@BoundaryImposition Just made an edit. Thanks for noticing this! :)

Lightness Races in Orbit Over a year ago

No problem Omar!

Kayode Adeola · Accepted Answer · 2017-04-13 22:47:51Z

0

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);

edited Apr 13, 2017 at 22:47

answered Apr 13, 2017 at 22:38

Kayode Adeola

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