Running sum of the input array in Javascript-CodePudding

I want to build a function that

takes an array of numbers as an input.
calculates the running sum of the numbers in the input array.
returns a new array that contains the running sum.

For example, if I call this function with the input of [1,2,3,4], the output should be [1,3,6,10]. However, when I run my code, it returns an empty array.

The following is the code I got so far.

function sumOfArray(nums) {
  var output = [];        // tip: initialize like let output = [nums?.[0] ?? 0];
  for(let i = 1 ; i < nums.length ; i  ) {
    nums[i] = nums[i]   nums[i-1];
    output.push(nums[i]);
  }
  return output;
};

console.log(
  'calling sumOfArray([1,2,3,4])',
  sumOfArray([1,2,3,4])
);

CodePudding user response：

const input =  [1,2,3,4]

const theFunctionYouWannaWrite = (acc, v) => {
  acc.push(v   (acc[acc.length - 1] || 0))
  return acc;
}

const output = input.reduce(theFunctionYouWannaWrite, [])

console.log(output)

if you want to use it with for

const input =  [1,2,3,4]

const theFunctionYouWannaWrite = (acc, v) => {
  acc.push(v   (acc[acc.length - 1] || 0))
  return acc;
}

function sumOfArray(nums) {

  var output = [];

  for(let i = 0 ; i < nums.length ; i  ) {
    theFunctionYouWannaWrite(output, nums[i])
  }

  return output;
};

console.log(sumOfArray(input))

CodePudding user response：

Just add first item in array initialization and everything works fine.

function sumOfArray(nums) {
  var output = [nums[0]];        // tip: initialize like let output = [nums?.[0] ?? 0];
  for(let i = 1 ; i < nums.length ; i  ) {
    nums[i] = nums[i]   nums[i-1];
    output.push(nums[i]);
  }
  return output;
};

console.log(
  'calling sumOfArray([1,2,3,4])',
  sumOfArray([1,2,3,4])
);

CodePudding user response：

let 
  sum = 0,
  nums = [1, 2, 3, 4],
  res = [];

nums.forEach((item, index, arr) => {
  sum  = arr[index];
  res.push(sum);
});

console.log(res);

CodePudding user response：

You can do it using Array.prototype.reduce.

function sumOfArray(nums) {
  return nums.reduce((s, n) => (s.push((s.at(-1) ?? 0)   n), s), []);
}

console.log(sumOfArray([1, 2, 3, 4]));

And the problem with your solution is that you need start the loop from index 0 and check if the i-1th index is valid using either ?? or ||.

function sumOfArray(nums) {
  const output = [];
  for (let i = 0; i < nums.length; i  ) {
    nums[i] = nums[i]   (nums[i - 1] ?? 0);
    output.push(nums[i]);
  }
  return output;
}

console.log(sumOfArray([1, 2, 3, 4]));

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