Question 192841 in Javascript Question posted in
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Javascript for all the length-3 combinations of an array of arrays?

I have an array of 16 arrays like this:

[['one', 'blue'], ['one', 'red'], ['two', 'blue'], ['two', 'red'], ...]

and I’m looking for Javascript to return every combination (not permutation, because I don’t want repeats in different orders) of THREE of them like this:

[[['one', 'blue'], ['one', 'red'], ['two', 'blue']],
 [['one', 'blue'], ['one', 'red'], ['two', 'red']],
 [['one', 'blue'], ['two', 'blue'], ['two', 'red']],
 [['one', 'red'], ['two', 'blue'], ['two', 'red']], ...]

I can do this in Python using itertools, but I don’t know how to in Javascript.

Thanks!

Tags:

2

Answers


  1. 0

    You could use three nested for loops.

    const arrays = [['one', 'blue'], ['one', 'red'], ['two', 'blue'], ['two', 'red']];

const result = [];
for (let i = 0; i < arrays.length; ++i) {
  for (let j = i + 1; j < arrays.length; ++j) {
    for (let k = j + 1; k < arrays.length; ++k) {
      result.push([arrays[i], arrays[j], arrays[k]]);
    }
  }
}

console.log(result);

    Or, if you need a shallow copy instead of references:

    const arrays = [['one', 'blue'], ['one', 'red'], ['two', 'blue'], ['two', 'red']];

const result = [];
for (let i = 0; i < arrays.length; ++i) {
  for (let j = i + 1; j < arrays.length; ++j) {
    for (let k = j + 1; k < arrays.length; ++k) {
      result.push([[...arrays[i]], [...arrays[j]], [...arrays[k]]]);
    }
  }
}

console.log(result);
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  2. 0

    You want to generate a list of permutations of a fixed length.

    There are many algorithms out there for recursively building a list of permutations.

    I modified one that was easy to find, with proper searching. I called stucturedClone to grab a deep copy of each result item.

    const arr = [
  ['one', 'blue'],
  ['one', 'red'],
  ['two', 'blue'],
  ['two', 'red']
];

// Based on: https://stackoverflow.com/a/59370795/1762224
const getPermutations = (list, maxLen) => {
  const permute = (list, maxLen) => {
    if (maxLen === 0) return [[]];
    const result = [];
    for (let i = 0; i < list.length; i++) {
      let
        copy = [...list],
        head = copy.splice(i, 1),
        rest = permute(copy, maxLen - 1);
      for (let j = 0; j < rest.length; j++) {
        result.push(structuredClone([...head, ...rest[j]]));
      }
    }
    return result;
  }
  return permute(list, maxLen);
}

const permutations = getPermutations(arr, 3);

console.log(permutations);
    .as-console-wrapper { top: 0; max-height: 100% !important; }

    Results:

    [
  [ ["one", "blue"] , ["one", "red"]  , ["two", "blue"] ]
  [ ["one", "blue"] , ["one", "red"]  , ["two", "red"]  ]
  [ ["one", "blue"] , ["two", "blue"] , ["one", "red"]  ]
  [ ["one", "blue"] , ["two", "blue"] , ["two", "red"]  ]
  [ ["one", "blue"] , ["two", "red"]  , ["one", "red"]  ]
  [ ["one", "blue"] , ["two", "red"]  , ["two", "blue"] ]
  [ ["one", "red"]  , ["one", "blue"] , ["two", "blue"] ]
  [ ["one", "red"]  , ["one", "blue"] , ["two", "red"]  ]
  [ ["one", "red"]  , ["two", "blue"] , ["one", "blue"] ]
  [ ["one", "red"]  , ["two", "blue"] , ["two", "red"]  ]
  [ ["one", "red"]  , ["two", "red"]  , ["one", "blue"] ]
  [ ["one", "red"]  , ["two", "red"]  , ["two", "blue"] ]
  [ ["two", "blue"] , ["one", "blue"] , ["one", "red"]  ]
  [ ["two", "blue"] , ["one", "blue"] , ["two", "red"]  ]
  [ ["two", "blue"] , ["one", "red"]  , ["one", "blue"] ]
  [ ["two", "blue"] , ["one", "red"]  , ["two", "red"]  ]
  [ ["two", "blue"] , ["two", "red"]  , ["one", "blue"] ]
  [ ["two", "blue"] , ["two", "red"]  , ["one", "red"]  ]
  [ ["two", "red"]  , ["one", "blue"] , ["one", "red"]  ]
  [ ["two", "red"]  , ["one", "blue"] , ["two", "blue"] ]
  [ ["two", "red"]  , ["one", "red"]  , ["one", "blue"] ]
  [ ["two", "red"]  , ["one", "red"]  , ["two", "blue"] ]
  [ ["two", "red"]  , ["two", "blue"] , ["one", "blue"] ]
  [ ["two", "red"]  , ["two", "blue"] , ["one", "red"]  ]
]
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