$reverseArray (aggregation)
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Definition
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$reverseArray
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New in version 3.4.
Accepts an array expression as an argument and returns an array with the elements in reverse order.
$reverseArray
has the following operator expression syntax:The argument can be any valid expression as long as it resolves to an array.
Behavior
If the argument resolves to a value of null
or refers to a missing field, $reverseArray
returns null
.
If the argument does not resolve to an array or null
nor refers to a missing field, $reverseArray
returns an error.
$reverseArray
returns an empty array when the argument is an empty array.
If the argument contains subarrays, $reverseArray
only operates on the top level array elements and will not reverse the contents of subarrays.
Example [1] | Results |
---|---|
[ 3, 2, 1 ] |
|
[ "baz", "bar" ] |
|
null |
|
[ ] |
|
[ [ 4, 5, 6 ], [ 1, 2, 3 ] ] |
[1] | The examples in the table take a literal argument. To avoid parsing ambiguity if the literal argument is an array, you must wrap the literal array in a $literal expression or keep the outer array that designates the argument list (e.g. [ [ 1, 2, 3 ] ] ) to pass in the literal array [1, 2, 3] . |