Finding the factorial using a loop in javascript

Question

I need to use a loop to find the factorial of a given number. Obviously what I have written below will not work because when i = inputNumber the equation will equal 0.

How can I stop i reaching inputNumber?

var inputNumber = prompt('Please enter an integer');
var total = 1;

for (i = 0; i <= inputNumber; i++){
    total = total * (inputNumber - i);
}

console.log(inputNumber + '! = ' + total);

Possible duplicate of [factorial of a number](http://stackoverflow.com/questions/4438131/factorial-of-a-number) — Harald Gliebe, Apr 30 '17 at 16:13
Possible duplicate of [Fast factorial function in JavaScript](https://stackoverflow.com/questions/3959211/fast-factorial-function-in-javascript) — Krisztián Balla, Sep 15 '18 at 13:02

score 2 · Accepted Answer · answered Apr 30 '17 at 16:15

2

here is an error i <= inputNumber

should be i < inputNumber

var inputNumber = prompt('Please enter an integer');
var total = 1;

for (i = 0; i < inputNumber; i++){
    total = total * (inputNumber - i);
}

console.log(inputNumber + '! = ' + total);

answered Apr 30 '17 at 16:15

Zhenya Telegin

589
2
9

or we can start the for loop from 1 for(i =1; i<=inputnumber; i++) and total = total * i; – DivyaMaheswaran Feb 27 '20 at 03:23

score 2 · Answer 2 · answered Oct 19 '17 at 10:41

2

you can keep this: i <= inputNumber

and just do this change: total = total * i;

then the code snippet would look like this:

var inputNumber = prompt('Please enter an integer');
var total = 1;

for (i = 1; i <= inputNumber; ++i){
total = total * i;
}

console.log(inputNumber + '! = ' + total);

answered Oct 19 '17 at 10:41

Islam Sayed

58
7

for(var i=1; i< = inputnumber; i++) too works isn't? – DivyaMaheswaran Feb 27 '20 at 03:30

score 1 · Answer 3 · answered Apr 30 '17 at 16:15

1

var inputNumber = prompt('Please enter an integer');
var total = 1;

for (i = 0; i < inputNumber; i++){
    total = total * (inputNumber - i);
}

alert(inputNumber + '! = ' + total);

answered Apr 30 '17 at 16:15

bboy

36
7

You should explain what changes you've made. It's not very helpful to just dump corrected code without explaining it. – Carcigenicate Apr 30 '17 at 16:27
Added explanation here: Should replace the "i<=inputNumber" condition with "i – bboy Apr 30 '17 at 16:33

score 1 · Answer 4 · answered Apr 30 '17 at 16:28

You could use the input value and a while statement with a prefix decrement operator --.

var inputNumber = +prompt('Please enter an integer'),
    value = inputNumber,
    total = inputNumber;

while (--value) {                           // use value for decrement and checking
    total *= value;                         // multiply with value and assign to value
}

console.log(inputNumber + '! = ' + total);

score 1 · Answer 5 · answered Feb 16 '18 at 20:26

Using total *= i; will set up all of your factorial math without the need of extra code. Also, for proper factorial, you'd want to count down from your input number instead of increasing. This would work nicely:

var inputNum = prompt("please enter and integer");
var total = 1;
for(i = inputNum; i > 1; i--){
 total *= i;
}
console.log(total);

score 0 · Answer 6 · answered Apr 26 '18 at 03:09

0

function factorialize(num) {
  
  var result = num;
  if(num ===0 || num===1){
    
    return 1;
  }
  
  while(num > 1){
    num--;
    result =num*result;
  }
  return result;
}

factorialize(5);

answered Apr 26 '18 at 03:09

Ogundeji Yusuff

1

1

Perhaps explain your solution for readers to understand. Thanks. – Lece Apr 26 '18 at 03:25

Finding the factorial using a loop in javascript

6 Answers6