Database Manual / Reference / Query Language / Expressions

$arrayElemAt (expression operator)

Definition

$arrayElemAt

Returns the element at the specified array index.

Compatibility

You can use $arrayElemAt for deployments hosted in the following environments:

  • MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Syntax

$arrayElemAt has the following syntax:

{ $arrayElemAt: [ <array>, <idx> ] }

The <array> expression can be any valid expression that resolves to an array.

The <idx> expression can be any valid expression that resolves to an integer.

For more information on expressions, see Expressions.

Behavior

  • If the <idx> expression resolves to zero or a positive integer, $arrayElemAt returns the element at the idx position, counting from the start of the array.

  • If the <idx> expression resolves to a negative integer, $arrayElemAt returns the element at the idx position, counting from the end of the array.

  • If idx exceeds the array bounds, $arrayElemAt does not return a result.
  • If the <array> expression resolves to an undefined array, $arrayElemAt returns null.
ExampleResults

{ $arrayElemAt: [ [ 1, 2, 3 ], 0 ] }

1

{ $arrayElemAt: [ [ 1, 2, 3 ], -2 ] }

2

{ $arrayElemAt: [ [ 1, 2, 3 ], 15 ] }

{ $arrayElemAt: [ "$undefinedField", 0 ] }

null

Example

A collection named users contains the following documents:

db.users.insertMany( [
   { _id: 1, name: "dave123", favorites: [ "chocolate", "cake", "butter", "apples" ] },
   { _id: 2, name: "li", favorites: [ "apples", "pudding", "pie" ] },
   { _id: 3, name: "ahn", favorites: [ "pears", "pecans", "chocolate", "cherries" ] },
   { _id: 4, name: "ty", favorites: [ "ice cream" ] }
] )

The following example returns the first and last element in the favorites array:

db.users.aggregate([
   {
     $project:
      {
         name: 1,
         first: { $arrayElemAt: [ "$favorites", 0 ] },
         last: { $arrayElemAt: [ "$favorites", -1 ] }
      }
   }
])

The operation returns the following results:

[
   { _id: 1, name: "dave123", first: "chocolate", "last" : "apples" },
   { _id: 2, name: "li", first: "apples", "last" : "pie" },
   { _id: 3, name: "ahn", first: "pears", "last" : "cherries" },
   { _id: 4, name: "ty", first: "ice cream", "last" : "ice cream" }
]

See Also