Creating an array of cumulative sum in javascript

Question

This is an example of what I need to do:

var myarray = [5, 10, 3, 2];

var result1 = myarray[0];
var result2 = myarray[1] + myarray[0];
var result3 = myarray         


        

Answer 1

Use arrow function instead of function, comma operator instead of return, and currentIndex in reduce callback.

[5, 10, 3, 2].reduce((r, a, i) => (r.push((i && r[i - 1] || 0) + a), r), []); // [ 5, 15, 18, 20 ]

Answer 2

var nums= [5, 10, 3, 2];
var runningSum = function(nums) {
   
    nums.reduce((acc, _, i) => (nums[i] += acc));
    return nums;
};

Answer 3

/**
 * Turn an array of numbers to cumulative sum array
 * @param { Array } [1,2,3,4,5]
 * @return { Array } [1,3,6,10,15]
 */

const accumulate = (a, c) => a + c

const cusum = arr => arr.map((v, i, data) => {
    return data.slice(0, i + 1).reduce(accumulate)
})

Answer 4

My initial ES6 thought was similar to a few above answers by Taeho and others.

const cumulativeSum = ([head, ...tail]) =>
   tail.reduce((acc, x, index) => {
      acc.push(acc[index] + x);
      return acc
  }, [head])
console.log(cumulativeSum([-1,2,3])

The solution performs:

n lookups, n - 1 sums and 0 conditional evaluations

Most of what I saw above appeared to use:

n lookups, 2n sums, and n conditional evaluations:

You could do this with ie6 safe js as well. This is possibly more efficient since you don't have to create the tail spread array.

function cumulativeSum(a) {
    var result = [a[0]];
    var last = a[0];
    for (i = 1; i < a.length; i++) {
        last = last + a[i];
        result.push(last)
    }
    return result;
}
console.log(cumulativeSum([-1,2,3]))

Answer 5

Old school and simpler :

let myarray = [5, 10, 3, 2], result = [];

for (let i = 0, s = myarray[0]; i < myarray.length; i++, s += myarray[i]) result.push(s);

console.log(result); // [5, 15, 18, 20]

Answer 6

Another clean one line solution with reduce and concat

var result = myarray.reduce(function(a,b,i){ return i === 0 ?  [b]: a.concat(a[i-1]+b);},0);
//[5, 10, 3, 2] => [5, 15, 18, 20]