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reduce: sliding window

Obnoxious Ocelot answered on September 30, 2023 Popularity 9/10 Helpfulness 1/10

answer reduce: sliding window

reduce: sliding window

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/* inefficient: double counts intermediate results! */
const numbers = [2, 3, 4, 1, 5, 7, 6, 8];
const windowSize = 3; // Size of the sliding window

const maxSum = numbers.reduce((max, _, index, arr) => {
  index++; // increment to account for last window iteration
  if (index >= windowSize) {
    // Calculate the sum of the current window
    const cArr = arr.slice(index - windowSize, index);
    const currentSum = cArr.reduce((acc, val) => acc + val, 0);

    // Update the maximum sum & max sum sequence if the current window's sum is greater
    if(currentSum > max.value){
      max.value = currentSum;
      max.sequence = cArr;
    }
    return max;
  }
  return max;
}, {value: -Infinity, sequence: [] as number[]}); // Start with a very small initial value (-Infinity)

console.log(maxSum); // Output: {"value": 21,"sequence": [7,6,8]} 

/* Incorrect: Doesn't account for last window! */
const numbers = [2, 3, 4, 1, 5, 7, 6, 8];
const windowSize = 3; // Size of the sliding window

const maxSum = numbers.reduce((max, _, index, arr) => {
  if (index >= windowSize) {
    // Calculate the sum of the current window
    const currentSum = arr.slice(index - windowSize, index).reduce((acc, val) => acc + val, 0);

    // Update the maximum sum if the current window's sum is greater
    return Math.max(max, currentSum);
  }
  return max;
}, -Infinity); // Start with a very small initial value (-Infinity)

console.log(maxSum); // Output: 18 (sum of the subarray [7, 6, 8]) <<incorrect!

/* more Efficient algo */
const numbers = [2, 3, 4, 1, 5, 7, 6, 8];
const windowSize = 3;

const max = { value: -Infinity, sequence:[] as number[]};
let currentSum = 0;

// Calculate the initial sum for the current window
for (let i = 0; i < windowSize; i++) {
  currentSum += numbers[i];
}

max.sequence = numbers.slice(0, windowSize);
// currentSum = max.sequence.reduce((a,ce)=>a+ce, 0);
max.value = currentSum;

// Iterate through the array to find the maximum sum
for (let i = windowSize; i < numbers.length; i++) {
  const exitedElmtIdx = i - windowSize;
  const exitedElmt = numbers[ exitedElmtIdx ]; // check against last window 1st
  const enteringElmt = numbers[i];
  currentSum = currentSum - exitedElmt + enteringElmt; // 7,6 not dbl counted
  if(currentSum > max.value){
    max.value = currentSum;
    max.sequence = numbers.slice( exitedElmtIdx+1, i+1 ) // +1 => exclusive to inclusive.
  }
}

console.log(max); // Output: {"value": 21,"sequence": [7,6,8]}

Popularity 9/10 Helpfulness 1/10 Language javascript

Source: chat.openai.com

Tags: javascript max reduce sliding-window sub-array sum

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Contributed on Sep 30 2023

Obnoxious Ocelot

0 Answers Avg Quality 2/10

reduce: sliding window

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