Question 373769 in Javascript Question posted in
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Best way to remove repeated data in JavaScript array

Suppose you have a list like this:

[1, 1, 2, 2, 1, 2, 2, 2]

What’s the most efficient way to remove repeated data (not duplicates) so no element appears after itself? For this example, the result should be:

[1, 2, 1, 2]

For example, this could be solved with filter or a for loop remembering the last element. But is there something more elegant? Or which option is most efficient?

for loop:

const arr = [1, 1, 2, 2, 1, 2, 2, 2];
const result = [arr[0]];
let lastElement = arr[0];
for (let i = 1, n = arr.length; i < n; ++i) {
  if (arr[i] !== lastElement) {
    lastElement = arr[i];
    result.push(lastElement);
  }
}

filter:

const result = [1, 1, 2, 2, 1, 2, 2, 2].filter((element, index, arr) => element !== arr[index - 1]);
Tags:

3

Answers


  1. 0

    Just filter with comparing with the previous item:

    const result = [1, 1, 2, 2, 1, 2, 2, 2].filter((item, idx, arr) => !idx || item !== arr[idx - 1]);
console.log(result);
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  2. 0

    Reduce and filter are normally slower for large arrays than for loops

    JSPerf on mine, Alexander’s and your code: https://jsperf.app/popose/2

    Your for loop is faster than this (but not by much)

    enter image description here

    const removeRepeated = (data) => {
  const result = [];
  for (let i = 0, len = data.length; i < len; i++) {
    if (i === 0 || data[i] !== data[i - 1]) {
      result.push(data[i]);
    }
  }
  return result;
};

// Example usage
const data = [1, 1, 2, 2, 1, 2, 2, 2];
const result = removeRepeated(data);
console.log(result); // Expected output: [1, 2, 1, 2]
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  3. 0

    Your code seems quite optimal. Just one remark: it will fail for an empty array as input, so I would suggest adding a guard for that.

    If it is acceptable to perform the action inplace, mutating the input array instead of creating a new one, you can still get some time gain:

    function removeRepetitionsInplace(arr) {
    if (arr.length < 2) return;
    let k = 0;
    let lastElement = arr[0];
    for (let i = 1; i < arr.length; i++) {
        if (arr[i] !== lastElement) {
            lastElement = arr[++k] = arr[i];
        }
    }
    arr.length = k + 1;
}

    See performance tests at https://jsperf.app/popose/3 (added test case to mplungjan’s)

    enter image description here

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