sum-by-factors.js

// Given an array of positive or negative integers

// I= [i1,..,in]

// you have to produce a sorted array P of the form

// [ [p, sum of all ij of I for which p is a prime factor (p positive) of ij] ...]

// P will be sorted by increasing order of the prime numbers. The final result has to be given as a string in Java, C#, C, C++ and as an array of arrays in other languages.

// Example:
// I = [12, 15]; //result = [[2, 12], [3, 27], [5, 15]]
// [2, 3, 5] is the list of all prime factors of the elements of I, hence the result.

// Notes:

// It can happen that a sum is 0 if some numbers are negative!
// Example: I = [15, 30, -45] 5 divides 15, 30 and (-45) so 5 appears in the result, the sum of the numbers for which 5 is a factor is 0 so we have [5, 0] in the result amongst others.

function sumOfDivided(lst) {
  let pL;
  let primesObject = {};
  lst.forEach(n => {
    pL = formPrimes(n);
    pL.forEach(p => {
      if (!primesObject[p]) primesObject[p] = [];
      primesObject[p].push(n);
    })
  });

  const result = [];
  for (let l in primesObject) {
    result.push([parseInt(l), primesObject[l].reduce((a, b) => a + b, 0)])
  }

  return result;
}
 
// forming the list of distinct prime divisors of a number
function formPrimes(n) {
  n = Math.abs(n);
  let i = 2;
  let primesList = new Set();
  for (i; i <= n/i; i++){
    while (n % i === 0) {
      n=n/i;
      primesList.add(i);
    }
  }
  
  if (n > 1) primesList.add(n);
  return Array.from(primesList);
}