Factorialize a number using Javascript arrays

Question

I tried making a function that takes a parameter, and puts every number less then the parameter up to zero and pushes it into an array(descending order). and then used a for loop to multiply each index of the array by the next index, but my return comes back null. please help.

function factorialize(num) {
     var arrayOfFactorial = [];
     var factorial = 1;
     for(var i = num;i > 0;i--){
        arrayOfFactorial.push([i]);
        factorial = factorial * arrayOfFactorial[i];
     } 

     return factorial;
}

factorialize(10);

`i` starts at 10, but you don't have 10 items in your array. Try `arrayOfFactorial[num - i]` — Marco de Zeeuw, Sep 11 '17 at 20:49
There is no point to use the array as you don't even return it. — Loïc Faure-Lacroix, Sep 11 '17 at 20:51
You also might want to take a look at https://stackoverflow.com/questions/3959211/fast-factorial-function-in-javascript — Marco de Zeeuw, Sep 11 '17 at 21:52

score 2 · Accepted Answer · answered Sep 11 '17 at 20:48

in the first iteration, arrayOfFactorial will be of length 1, and i will be equal to num which will be more than 1, you are trying to access arrayOfFactorial[num] which will be undefined and that is why you get NaN. you can just multiply factorial with i. Also you don't really need another array for this.

function factorialize(num) {
   var arrayOfFactorial = [];
   var factorial = 1;
   for(var i = num;i > 0;i--){
     arrayOfFactorial.push([i]);  //can be removed.
     factorial = factorial * i;
   } 

   return factorial;
}

console.log(factorialize(10));

baao · Answer 2 · 2017-09-11T22:10:45.747

I think using a while loop makes it easier, and you don't need to create an array.

function factorialize(num) {
    let res = 1;
    while (num > 0) res *= num--;
    return res;
};

console.log(factorialize(10));

If you insist on using an array, a version using es6 spread operator could be

function factorialize(num) {
  return [...Array(num).keys()]
    .reduce((a, b) => a * (b + 1), 1)
}

console.log(factorialize(10));

score 0 · Answer 3 · edited Sep 11 '17 at 21:45

0

function factorialize(num) {
    var factorial = 1;
    for(var i = num;i > 0;i--){
        factorial = factorial * i;
    }

    console.log( factorial);
    return factorial;
}

factorialize(10);

i think this is what you are trying to do..i changed the [i] to i.and arrayOffactorial[i] to i

edited Sep 11 '17 at 21:45

Marco de Zeeuw

496
3
10

answered Sep 11 '17 at 20:48

Debabrata

488
4
13

You're storing the index in the array and returning nothing. – Loïc Faure-Lacroix Sep 11 '17 at 20:52
I know that..I just showed the way it should be done..who asked the question already have that return statement.So i know that he is obviously aware of tat fact.so i just showed what was wrong in his code – Debabrata Sep 11 '17 at 20:54

score 0 · Answer 4 · answered Sep 11 '17 at 20:50

1-You are pushing an array into arrayOfFactorial, and then trying to multiply a number with an array. While it is legit in maths, it is not in javascript. You should use arrayOfFactorial.push(i);

2-You don't need an array. You can just use factorial = factorial * i

3-According to your explanation, You should use 2 loops. The second action, should be in a different loop.

4-You return something, but you do not print it.

See the Dij's answer if you can not fix the problem.

score 0 · Answer 5 · answered Sep 11 '17 at 21:02

The array in you case isn't used for the calculation. What you were trying to do is probably this:

function factorialize(num) {
   var arrayOfFactorial = [];

   // Push all values in the array (order not really important here)
   for(var i = num;i > 0; i--){
     arrayOfFactorial.push(i);
   }

   var factorial = 1;
   for(var i = 0; i<arrayOfFactorial.length; i++) { 
     factorial = arrayOfFactorial[i] * factorial 
   }

   return factorial;
}

console.log(factorialize(10));

That said, there is no real point to do that as your memory usage will grow with the size of the factorial you're trying to compute and you don't really need to keep reference of everything. The best way would be to compute it by multiplying the factorial directly in the for loop. Like this:

function factorialize(num) {
   var factorial = 1

   for(var i = num;i > 0; i--){
     factorial = factorial * i
   }

   return factorial;
}

Factorialize a number using Javascript arrays

5 Answers5