73 lines
1.9 KiB
Markdown
73 lines
1.9 KiB
Markdown
---
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layout: "functions"
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page_title: "list function"
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sidebar_current: "docs-funcs-collection-matchkeys"
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description: |-
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The matchkeys function takes a subset of elements from one list by matching
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corresponding indexes in another list.
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---
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# `matchkeys` Function
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`matchkeys` constructs a new list by taking a subset of elements from one
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list whose indexes match the corresponding indexes of values in another
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list.
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```hcl
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matchkeys(valueslist, keyslist, searchset)
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```
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`matchkeys` identifies the indexes in `keyslist` that are equal to elements of
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`searchset`, and then constructs a new list by taking those same indexes from
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`valueslist`. Both `valueslist` and `keyslist` must be the same length.
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The ordering of the values in `valueslist` is preserved in the result.
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## Examples
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```
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> matchkeys(["i-123", "i-abc", "i-def"], ["us-west", "us-east", "us-east"], ["us-east"])
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[
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"i-abc",
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"i-def",
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]
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```
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If the result ordering is not significant, you can achieve a similar result
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using a `for` expression with a map:
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```
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> [for i, z in {"i-123"="us-west","i-abc"="us-east","i-def"="us-east"}: i if z == "us-east"]
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[
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"i-def",
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"i-abc",
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]
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```
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If the keys and values of interest are attributes of objects in a list of
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objects then you can also achieve a similar result using a `for` expression
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with that list:
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```
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> [for x in [{id="i-123",zone="us-west"},{id="i-abc",zone="us-east"}]: x.id if x.zone == "us-east"]
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[
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"i-abc",
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]
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```
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For example, the previous form can be used with the list of resource instances
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produced by a `resource` block with the `count` meta-attribute set, to filter
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the instances by matching one of the resource attributes:
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```
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> [for x in aws_instance.example: x.id if x.availability_zone == "us-east-1a"]
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[
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"i-abc123",
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"i-def456",
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]
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```
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Since the signature of `matchkeys` is complicated and not immediately clear to
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the reader when used in configuration, prefer to use `for` expressions where
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possible to maximize readability.
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