Navigation

$indexOfArray (aggregation)

On this page

Definition

$indexOfArray

New in version 3.4.

Searches an array for an occurrence of a specified value and returns the array index (zero-based) of the first occurrence. If the value is not found, returns -1.

$indexOfArray has the following operator expression syntax:

{ $indexOfArray: [ <array expression>, <search expression>, <start>, <end> ] }
Field Type Description
<array> string

Can be any valid expression as long as it resolves to an array. For more information on expressions, see Expressions.

If the array expression resolves to a value of null or refers to a field that is missing, $indexOfArray returns null.

If the array expression does not resolve to an array or null nor refers to a missing field, $indexOfArray returns an error.

<search value> string Can be any valid expression. For more information on expressions, see Expressions.
<start> integer

Optional. An integer, or a number that can be represented as integers (such as 2.0), that specifies the starting index position for the search. Can be any valid expression that resolves to a non-negative integral number.

If unspecified, the starting index position for the search is the beginning of the string.

<end> integer

Optional. An integer, or a number that can be represented as integers (such as 2.0), that specifies the ending index position for the search. Can be any valid expression that resolves to a non-negative integral number. If you specify a <end> index value, you should also specify a <start> index value; otherwise, $indexOfArray uses the <end> value as the <start> index value instead of the <end> value.

If unspecified, the ending index position for the search is the end of the string.

Behavior

If the <search expression> is found multiple times within the <array expression>, then $indexOfArray returns the index of the first <search expression> from the starting index position.

$indexOfArray returns null:

  • If <array expression> is null, or
  • If <array expression> refers to a non-existing field in the input document.

$indexOfArray returns an error:

  • If <array expression> is not an array and not null, or
  • If <start> or <end> is a negative integer (or a value that can be represented as a negative integer, like -5.0).

$indexOfArray returns -1:

  • If the <search expression> is not found in the array, or
  • If <start> is a number greater than <end>, or
  • If <start> is a number greater than the length of the array.
Example Results
{ $indexOfArray: [ [ "a", "abc" ], "a" ] } 0
{ $indexOfArray: [ [ "a", "abc", "de", ["de"] ], ["de"] ] } 3
{ $indexOfArray: [ [ 1, 2 ], 5 ] } -1
{ $indexOfArray: [ [ 1, 2, 3 ], [1, 2] ] } -1
{ $indexOfArray: [ [ 10, 9, 9, 8, 9 ], 9, 3 ] } 4
{ $indexOfArray: [ [ "a", "abc", "b" ], "b", 0, 1 ] } -1
{ $indexOfArray: [ [ "a", "abc", "b" ], "b", 1, 0 ] } -1
{ $indexOfArray: [ [ "a", "abc", "b" ], "b", 20 ] } -1
{ $indexOfArray: [ [ null, null, null ], null ] } 0
{ $indexOfArray: [ null, "foo" ] } null
{ $indexOfArray: [ "foo", "foo" ] } Error

Example

The example uses this inventory collection:

db.inventory.insertMany( [
   { _id: 0, items: [ "one", "two", "three" ] },
   { _id: 1, items: [ 1, 2, 3 ] },
   { _id: 2, items: [ 1, 2, 3, 2 ] },
   { _id: 3, items: [ null, null, 2 ] },
   { _id: 4, items: [ 2, null, null, 2 ] },
   { _id: 5, items: null },
   { _id: 6, amount: 3 }
] )

The following example uses $indexOfArray to find 2 in the items array:

db.inventory.aggregate( [ {
   $project: {
      index: { $indexOfArray: [ "$items", 2 ] }
   }
} ] )

The example returns:

  • The first array index for the value 2 in each items array, if found. Array indexes start at 0.
  • -1 for the index if 2 is not in the items array.
  • null for the index if items is not an array or items does not exist.

Example output:

[
   { _id: 0, index: -1 },
   { _id: 1, index: 1 },
   { _id: 2, index: 1 },
   { _id: 3, index: 2 },
   { _id: 4, index: 0 },
   { _id: 5, index: null },
   { _id: 6, index: null }
]

See also

$indexOfBytes, $indexOfCP, and $in