Fn::Select of Alibaba Cloud ROS - Resource Orchestration Service

Fn::Select retrieves data elements from a list or dictionary by index or key.

Declaration

Query one or more data elements from a list or an array

Query a single data element based on an index

JSON

{
  "Fn::Select": [
    "index",
    [
      "value1",
      "value2",
      ...
    ],
    "default_value"
  ]
}

YAML

Syntax for the full function name:

Fn::Select:
  - index
  - - value1
    - value2
    - ...
  - default_value

Syntax for the short form:

!Select [index, [value1, value2, ...], default_value]

Query multiple data elements based on an index

JSON

{
  "Fn::Select": [
    "start:stop:step",
    [
      "value1",
      "value2",
      ...
    ]
  ]
}

YAML

Syntax for the full function name:

Fn::Select:
  - start:stop:step
  - - value1
    - value2
    - ...

Syntax for the short form:

!Select [start:stop:step, [value1, value2, ...]]

Query the value of a key from a dictionary or mapping table

JSON

{
  "Fn::Select": [
    "key",
    {
      "key1": "value1",
      "key2": "value2",
      ...
    },
    "default_value"
  ]
}

YAML

Syntax for the full function name:

Fn::Select:
  - key
  - key1: value1
    key2: value2
    ...
  - default_value

Syntax for the short form:

!Select [key, {key1:value1, key2:value2}, default_value]

Parameters

index: The index of the data element to query. Valid values: 0 to N - 1 or -N to -1, where N is the number of elements in the list. Negative values read from right to left. If the index is invalid, the default value is returned. If no default value is specified, an empty string is returned.
start, stop, and step: The start and stop positions define the range of elements to query from a list. The step value sets the interval between selected elements.
- start:stop: Valid values follow the same rules as the index parameter. Default value of start: 0. Default value of stop: N. The function returns elements from position start + 1 to position stop. If the values are invalid, an empty list is returned.
- start:stop:step: Default value of step: 1. If step is negative, the start index must be greater than the stop index. The function selects elements at each -(step - 1) interval from start to stop + 1 and returns a list.
key: A key in the dictionary. The function returns the corresponding value. If the key does not exist, the default value is returned. If no default value is specified, an empty string is returned.
default_value: The fallback value returned when the specified index or key is invalid. If omitted and the index or key is invalid, an empty string is returned.

Return value

One or more data elements.

Examples

Query one or more data elements from a list or an array

YAML

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

JSON

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

In this example, "grapes" is returned.

YAML

!Select ['1:3', [1,2,3,4,5]]

JSON

{
  "Fn::Select": [
    "1:3",
    [1,2,3,4,5]
  ]
}

In this example, [2, 3] is returned.

YAML

!Select ['::2', [1,2,3,4,5]]

JSON

{
  "Fn::Select": [
    "::2",
    [1,2,3,4,5]
  ]
}

In this example, [1, 3, 5] is returned.

YAML

!Select ['5:0:-2', [1,2,3,4,5]]

JSON

{
  "Fn::Select": [
    "5:0:-2",
    [1,2,3,4,5]
  ]
}

In this example, [5, 3] is returned.

Query the value of a key from a dictionary

YAML

!Select [key1, {key1: grapes, key2: mangoes}]

JSON

{
  "Fn:Select": [
    "key1",
    {
      "key1": "grapes",
      "key2": "mangoes"
    }
  ]
}

In this example, "grapes" is returned.

Query a data element from a comma-delimited list

Parameters:
  userParam:
    Type: CommaDelimitedList
    Default: 10.0.0.1, 10.0.0.2, 10.0.0.3
Resources:
  resourceID:
    Properties:
      CidrBlock:
        !Select:
          - '0'
          - !Ref userParam

In this example, 10.0.0.1 is returned.

Supported functions

The Fn::Select index value supports the Ref function.

The Fn::Select list of objects supports the following functions: