Fn::Select - AWS CloudFormation (original) (raw)

The intrinsic function Fn::Select returns a single object from a list of objects by index.

Important

Fn::Select doesn't check for null values or if the index is out of bounds of the array. Both conditions will result in a stack error, so you should be certain that the index you choose is valid, and that the list contains non-null values.

Declaration

JSON

{ "Fn::Select" : [ index, listOfObjects ] }

YAML

Syntax for the full function name:

Fn::Select: [ index, listOfObjects ]

Syntax for the short form:

!Select [ index, listOfObjects ]

Parameters

index

The index of the object to retrieve. This must be a value from zero to N-1, where N represents the number of elements in the array.

listOfObjects

The list of objects to select from. This list must not be null, nor can it have null entries.

Return value

The selected object.

Examples

Basic example

The following example returns: "grapes".

JSON

{ "Fn::Select" : [ "1", [ "apples", "grapes", "oranges", "mangoes" ] ] }

YAML

!Select [ "1", [ "apples", "grapes", "oranges", "mangoes" ] ]

Comma-delimited list parameter type

You can use Fn::Select to select an object from aCommaDelimitedList parameter. You might use aCommaDelimitedList parameter to combine the values of related parameters, which reduces the total number of parameters in your template. For example, the following parameter specifies a comma-delimited list of three CIDR blocks:

JSON

"Parameters" : {
  "DbSubnetIpBlocks": {
    "Description": "Comma-delimited list of three CIDR blocks",
    "Type": "CommaDelimitedList",
      "Default": "10.0.48.0/24, 10.0.112.0/24, 10.0.176.0/24"
  }
}

YAML

Parameters: 
  DbSubnetIpBlocks: 
    Description: "Comma-delimited list of three CIDR blocks"
    Type: CommaDelimitedList
    Default: "10.0.48.0/24, 10.0.112.0/24, 10.0.176.0/24"

To specify one of the three CIDR blocks, use Fn::Select in the Resources section of the same template, as shown in the following sample snippet:

JSON

"Subnet0": {
  "Type": "AWS::EC2::Subnet",
    "Properties": {
      "VpcId": { "Ref": "VPC" },
      "CidrBlock": { "Fn::Select" : [ "0", {"Ref": "DbSubnetIpBlocks"} ] }
    }
}

YAML

Subnet0: 
  Type: "AWS::EC2::Subnet"
  Properties: 
    VpcId: !Ref VPC
    CidrBlock: !Select [ 0, !Ref DbSubnetIpBlocks ]

Nested functions with short form YAML

The following examples show valid patterns for using nested intrinsic functions with the !Select short form. You can't nest short form functions consecutively, so a pattern like !GetAZs !Ref isn't valid.

YAML

AvailabilityZone: !Select 
  - 0
  - !GetAZs 
    Ref: 'AWS::Region'

YAML

AvailabilityZone: !Select 
  - 0
  - Fn::GetAZs: !Ref 'AWS::Region'

Supported functions

For the Fn::Select index value, you can use the Ref andFn::FindInMap functions.

For the Fn::Select list of objects, you can use the following functions: