github.com/muratcelep/terraform@v1.1.0-beta2-not-internal-4/website/docs/language/functions/matchkeys.html.md (about)

     1  ---
     2  layout: "language"
     3  page_title: "matchkeys - Functions - Configuration Language"
     4  sidebar_current: "docs-funcs-collection-matchkeys"
     5  description: |-
     6    The matchkeys function takes a subset of elements from one list by matching
     7    corresponding indexes in another list.
     8  ---
     9  
    10  # `matchkeys` Function
    11  
    12  `matchkeys` constructs a new list by taking a subset of elements from one
    13  list whose indexes match the corresponding indexes of values in another
    14  list.
    15  
    16  ```hcl
    17  matchkeys(valueslist, keyslist, searchset)
    18  ```
    19  
    20  `matchkeys` identifies the indexes in `keyslist` that are equal to elements of
    21  `searchset`, and then constructs a new list by taking those same indexes from
    22  `valueslist`. Both `valueslist` and `keyslist` must be the same length.
    23  
    24  The ordering of the values in `valueslist` is preserved in the result.
    25  
    26  ## Examples
    27  
    28  ```
    29  > matchkeys(["i-123", "i-abc", "i-def"], ["us-west", "us-east", "us-east"], ["us-east"])
    30  [
    31    "i-abc",
    32    "i-def",
    33  ]
    34  ```
    35  
    36  If the result ordering is not significant, you can achieve a similar result
    37  using a `for` expression with a map:
    38  
    39  ```
    40  > [for i, z in {"i-123"="us-west","i-abc"="us-east","i-def"="us-east"}: i if z == "us-east"]
    41  [
    42    "i-def",
    43    "i-abc",
    44  ]
    45  ```
    46  
    47  If the keys and values of interest are attributes of objects in a list of
    48  objects then you can also achieve a similar result using a `for` expression
    49  with that list:
    50  
    51  ```
    52  > [for x in [{id="i-123",zone="us-west"},{id="i-abc",zone="us-east"}]: x.id if x.zone == "us-east"]
    53  [
    54    "i-abc",
    55  ]
    56  ```
    57  
    58  For example, the previous form can be used with the list of resource instances
    59  produced by a `resource` block with the `count` meta-attribute set, to filter
    60  the instances by matching one of the resource attributes:
    61  
    62  ```
    63  > [for x in aws_instance.example: x.id if x.availability_zone == "us-east-1a"]
    64  [
    65    "i-abc123",
    66    "i-def456",
    67  ]
    68  ```
    69  
    70  Since the signature of `matchkeys` is complicated and not immediately clear to
    71  the reader when used in configuration, prefer to use `for` expressions where
    72  possible to maximize readability.