Trying to find factors of a number in JS

Question

I am just starting JS, and understand the concept of finding a factor. However, this snippet of code is what I have so far. I have the str variable that outputs nothing but the

Answer 1

function factorialize(num) {
 var result = '';
  if( num === 0){
    return 1;
  }else{
    var myNum = [];

  for(i = 1; i <= num; i++){
    myNum.push(i);
    result = myNum.reduce(function(pre,cur){
      return pre * cur;
    });
  }
     return result;
    }
}

factorialize(9);

Answer 2

I came looking for an algorithm for this for use in factoring quadratic equations, meaning I need to consider both positive and negative numbers and factors. The below function does that and returns a list of factor pairs. Fiddle.

function getFactors(n) {
  if (n === 0) {return "∞";} // Deal with 0
  if (n % 1 !== 0) {return "The input must be an integer.";} // Deal with non-integers

  // Check only up to the square root of the absolute value of n
  // All factors above that will pair with factors below that
  var absval_of_n = Math.abs(n),
      sqrt_of_n = Math.sqrt(absval_of_n),
      numbers_to_check = [];
  for (var i=1; i <= sqrt_of_n; i++) {
    numbers_to_check.push(i);
  }

  // Create an array of factor pairs
  var factors = [];
  for (var i=0; i <= numbers_to_check.length; i++) {
    if (absval_of_n % i === 0) {
      // Include both positive and negative factors
      if (n>0) {
        factors.push([i, absval_of_n/i]);
        factors.push([-i, -absval_of_n/i]);
      } else {
        factors.push([-i, absval_of_n/i]);
        factors.push([i, -absval_of_n/i]);
      }
    }
  }

  // Test for the console
  console.log("FACTORS OF "+n+":\n"+
              "There are "+factors.length+" factor pairs.");
  for (var i=0; i<factors.length; i++) {
    console.log(factors[i]);
  }

  return factors;
}

getFactors(-26);

Answer 3

function calculate(num){
    var str = "0"   // initializes a place holder for var str
      for(i=2;i<num;i++){     
        var num2 = num%i;
        if(num2 ==0){       
            str = str +i; // this line joins the factors to the var str
        }
    }
    str1 = str.substr(1) //This removes the initial --var str = "0" at line 2
    console.log(str1) 
}
calculate(232);

//Output 2482958116

Answer 4

UPDATED ES6 version:

As @gengns suggested in the comments a simpler way to generate the array would be to use the spread operator and the keys method:

const factors = number => [...Array(number + 1).keys()].filter(i=>number % i === 0);
console.log(factors(36));      //  [1, 2, 3, 4, 6, 9, 12, 18, 36]

ES6 version:

const factors = number => Array
    .from(Array(number + 1), (_, i) => i)
    .filter(i => number % i === 0)

console.log(factors(36));      //  [1, 2, 3, 4, 6, 9, 12, 18, 36]

https://jsfiddle.net/1bkpq17b/

• Array(number) creates an empty array of [number] places

• Array.from(arr, (_, i) => i) populates the empty array with values according to position [0,1,2,3,4,5,6,7,8,9]

• .filter(i => ...) filters the populated [0,1,2,3,4,5] array to the elements which satisfy the condition of number % i === 0 which leaves only the numbers that are the factors of the original number.

Note that you can go just until Math.floor(number/2) for efficiency purposes if you deal with big numbers (or small).

Answer 5

Here's an optimized solution using best practices, proper code style/readability, and returns the results in an ordered array.

function getFactors(num) {
    const maxFactorNum = Math.floor(Math.sqrt(num));
    const factorArr = [];
    let count = 0;  //count of factors found < maxFactorNum.

    for (let i = 1; i <= maxFactorNum; i++) {
        //inserting new elements in the middle using splice
        if (num % i === 0) {
            factorArr.splice(count, 0, i);
            let otherFactor = num / i; //the other factor
            if (i != otherFactor) {
                //insert these factors in the front of the array
                factorArr.splice(-count, 0, otherFactor);
            }
            count++;
        }
    }

    //swapping first and last elements
    let lastIndex = factorArr.length - 1;
    let temp = factorArr[lastIndex];
    factorArr[lastIndex] = factorArr[0];
    factorArr[0] = temp;

    return factorArr;
}

console.log(getFactors(100));
console.log(getFactors(240));
console.log(getFactors(600851475143)); //large number used in Project Euler.

I based my answer on the answer written by @Harman

Answer 6

function calculate(num) {
    var str = "0";
    for (var i = 1; i <= num; i++) {
        if (num % i == 0) {
            str += ',' + i;
        }
    }
    alert(str);
}

calculate(232);

http://jsfiddle.net/67qmt/