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I need to calculate the sum of all k-sized sub-arrays in an array using sliding window algorithm. Is that a valid sliding window algorithm? If not, why?

var sumOfSubArrays = function(arr, k) {
    let currentSubArray = 0;
    for(let i=0; i<k; i++) {
        currentSubArray += arr[i];
    }

    let sum = currentSubArray;
    for(let i=0; i<arr.length-k; i++) {
        current = currentSubArray - arr[i] + arr[i+k];
        sum += current;
        currentSubArray = current;
    }

    return sum;
};

let arr = [1,2,3,4,5]
let k = 3;

console.log(sumOfSubArrays(arr, k));

Additionally, what could I improve in this code?

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2 Answers 2

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Generally, LGTM. That said,

  • It unclear what should happen if k > arr,length. In any case, better test it before entering the first loop.

  • If the sliding window is a requirement, I don't see how it could be improved. Otherwise, if k is much smaller than arr.length there is a marginally faster approach. Most of the elements, namely from k to n - k, contribute exactly k times. Compute their sum, multiply it by n - 2*k, and deal with the lead-in and lead-out separately.

  • Give your operator some breathing space. i < arr.length - k is more readable than i<arr.length-k.

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  • 1
    \$\begingroup\$ For those who are wondering, like me, what "LGTM" means: "Looks Good To Me". \$\endgroup\$ Sep 23, 2023 at 8:44
  • \$\begingroup\$ With most elements contributing exactly k times, why multiply with n - 2*k? That's their number, the weight should be k. \$\endgroup\$
    – greybeard
    Sep 29, 2023 at 17:00
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A short review;

  • You use var for the function, I would use const, actually I would just use function ;)
  • I would use a n inline function that can process a window, otherwise it's hard to tell that you implemented a sliding window
  • I would avoid the word array in variable names, unless they are an array
  • You never declare current, making it a global

Counter-proposal;

function sumSubArrays(list, k){

  const sumWindow = (list) => list.reduce((a, b) => a + b, 0);

  let sum = 0;
  for(let i = 0; i <= list.length-k; i++){
    sum += sumWindow( list.slice(i, i + k) );  
  } 
  return sum;
}



let arr = [1,2,3,4,5]
let k = 3;

console.log(27, 27 === sumSubArrays(arr, k));

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