71 lines
2.3 KiB
Markdown
71 lines
2.3 KiB
Markdown
---
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layout: "language"
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page_title: "Conditional Expressions - Configuration Language"
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---
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# Conditional Expressions
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A _conditional expression_ uses the value of a boolean expression to select one of
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two values.
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> **Hands-on:** Try the [Create Dynamic Expressions](https://learn.hashicorp.com/tutorials/terraform/expressions?in=terraform/configuration-language&utm_source=WEBSITE&utm_medium=WEB_IO&utm_offer=ARTICLE_PAGE&utm_content=DOCS) tutorial on HashiCorp Learn.
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## Syntax
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The syntax of a conditional expression is as follows:
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```hcl
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condition ? true_val : false_val
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```
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If `condition` is `true` then the result is `true_val`. If `condition` is
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`false` then the result is `false_val`.
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A common use of conditional expressions is to define defaults to replace
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invalid values:
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```
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var.a != "" ? var.a : "default-a"
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```
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If `var.a` is an empty string then the result is `"default-a"`, but otherwise
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it is the actual value of `var.a`.
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## Conditions
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The condition can be any expression that resolves to a boolean value. This will
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usually be an expression that uses the equality, comparison, or logical
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operators.
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## Result Types
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The two result values may be of any type, but they must both
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be of the _same_ type so that Terraform can determine what type the whole
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conditional expression will return without knowing the condition value.
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If the two result expressions don't produce the same type then Terraform will
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attempt to find a type that they can both convert to, and make those
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conversions automatically if so.
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For example, the following expression is valid and will always return a string,
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because in Terraform all numbers can convert automatically to a string using
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decimal digits:
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```hcl
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var.example ? 12 : "hello"
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```
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Relying on this automatic conversion behavior can be confusing for those who
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are not familiar with Terraform's conversion rules though, so we recommend
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being explicit using type conversion functions in any situation where there may
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be some uncertainty about the expected result type.
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The following example is contrived because it would be easier to write the
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constant `"12"` instead of the type conversion in this case, but shows how to
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use [`tostring`](/docs/language/functions/tostring.html) to explicitly convert a number to
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a string.
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```hcl
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var.example ? tostring(12) : "hello"
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```
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