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$zip (aggregation)

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Definition

$zip

New in version 3.4.

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:

copy 
{
    $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:

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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:

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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:

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{ "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:

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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:

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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:

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

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