$zip (aggregation)

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Definition

Behavior
Example

Definition

$zip

Transposes an array of input arrays so that the first element of the output array would be an array containing, the first element of the first input array, the first element of the second input array, etc.

For example, $zip would transform [ [ 1, 2, 3 ], [ "a", "b", "c" ] ] into [ [ 1, "a" ], [ 2, "b" ], [ 3, "c" ] ].

$zip has the following syntax:

{
    $zip: {
        inputs: [ <array expression1>,  ... ],
        useLongestLength: <boolean>,
        defaults:  <array expression>
    }
}

Operand	Description
`inputs`	An array of expressions that resolve to arrays. The elements of these input arrays combine to form the arrays of the output array. If any of the `inputs` arrays resolves to a value of `null` or refers to a missing field, `$zip` returns `null`. If any of the `inputs` arrays does not resolve to an array or `null` nor refers to a missing field, `$zip` returns an error.
`useLongestLength`	A boolean which specifies whether the length of the longest array determines the number of arrays in the output array. The default value is `false`: the shortest array length determines the number of arrays in the output array.
`defaults`	An array of default element values to use if the input arrays have different lengths. You must specify `useLongestLength: true` along with this field, or else `$zip` will return an error. If `useLongestLength: true` but `defaults` is empty or not specified, `$zip` uses `null` as the default value. If specifying a non-empty `defaults`, you must specify a default for each input array or else `$zip` will return an error.

Behavior

The input arrays do not need to be of the same length. By default, the output array has the length of the shortest input array, but the useLongestLength option instructs $zip to output an array as long as the longest input array.

Example

Results

{ $zip: { inputs: [ [ "a" ], [ "b" ], [ "c" ] ] }

[ [ "a", "b", "c" ] ]

{ $zip: { inputs: [ [ "a" ], [ "b", "c" ] ] } }

[ [ "a", "b" ] ]

{
  $zip: {
     inputs: [ [ 1 ], [ 2, 3 ] ],
     useLongestLength: true
  }
}

[ [ 1, 2 ], [ null, 3 ] ]

{
  $zip: {
     inputs: [ [ 1 ], [ 2, 3 ], [ 4 ] ],
     useLongestLength: true,
     defaults: [ "a", "b", "c" ]
  }
}

Because useLongestLength: true, $zip will pad the shorter input arrays with the corresponding defaults elements.

This yields [ [ 1, 2, 4 ], [ "a", 3, "c" ] ].

Example

Matrix Transposition

A collection called matrices contains the following documents:

db.matrices.insertMany([
  { matrix: [[1, 2], [2, 3], [3, 4]] },
  { matrix: [[8, 7], [7, 6], [5, 4]] },
])

To compute the transpose of each 3x2 matrix in this collection, you can use the following aggregation operation:

db.matrices.aggregate([{
  $project: {
    _id: false,
    transposed: {
      $zip: {
        inputs: [
          { $arrayElemAt: [ "$matrix", 0 ] },
          { $arrayElemAt: [ "$matrix", 1 ] },
          { $arrayElemAt: [ "$matrix", 2 ] },
        ]
      }
    }
  }
}])

This will return the following 2x3 matrices:

{ "transposed" : [ [ 1, 2, 3 ], [ 2, 3, 4 ] ] }
{ "transposed" : [ [ 8, 7, 5 ], [ 7, 6, 4 ] ] }

Filtering and Preserving Indexes

You can use $zip with $filter to obtain a subset of elements in an array, saving the original index alongside each retained element.

A collection called pages contains the following document:

db.pages.save( {
  "category": "unix",
  "pages": [
    { "title": "awk for beginners", reviews: 5 },
    { "title": "sed for newbies", reviews: 0 },
    { "title": "grep made simple", reviews: 2 },
] } )

The following aggregation pipeline will first zip the elements of the pages array together with their index, and then filter out only the pages with at least one review:

db.pages.aggregate([{
  $project: {
    _id: false,
    pages: {
      $filter: {
        input: {
          $zip: {
            inputs: [ "$pages", { $range: [0, { $size: "$pages" }] } ]
          }
        },
        as: "pageWithIndex",
        cond: {
          $let: {
            vars: {
              page: { $arrayElemAt: [ "$$pageWithIndex", 0 ] }
            },
            in: { $gte: [ "$$page.reviews", 1 ] }
          }
        }
      }
    }
  }
}])

This will return the following document:

{
  "pages" : [
    [ { "title" : "awk for beginners", "reviews" : 5 }, 0 ],
    [ { "title" : "grep made simple", "reviews" : 2 }, 2 ] ]
}

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