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

     1  ---
     2  layout: "language"
     3  page_title: "Conditional Expressions - Configuration Language"
     4  description: "Conditional expressions select one of two values. You can use them to define defaults to replace invalid values."
     5  ---
     6  
     7  # Conditional Expressions
     8  
     9  A _conditional expression_ uses the value of a boolean expression to select one of
    10  two values.
    11  
    12  > **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.
    13  
    14  ## Syntax
    15  
    16  The syntax of a conditional expression is as follows:
    17  
    18  ```hcl
    19  condition ? true_val : false_val
    20  ```
    21  
    22  If `condition` is `true` then the result is `true_val`. If `condition` is
    23  `false` then the result is `false_val`.
    24  
    25  A common use of conditional expressions is to define defaults to replace
    26  invalid values:
    27  
    28  ```
    29  var.a != "" ? var.a : "default-a"
    30  ```
    31  
    32  If `var.a` is an empty string then the result is `"default-a"`, but otherwise
    33  it is the actual value of `var.a`.
    34  
    35  ## Conditions
    36  
    37  The condition can be any expression that resolves to a boolean value. This will
    38  usually be an expression that uses the equality, comparison, or logical
    39  operators.
    40  
    41  ## Result Types
    42  
    43  The two result values may be of any type, but they must both
    44  be of the _same_ type so that Terraform can determine what type the whole
    45  conditional expression will return without knowing the condition value.
    46  
    47  If the two result expressions don't produce the same type then Terraform will
    48  attempt to find a type that they can both convert to, and make those
    49  conversions automatically if so.
    50  
    51  For example, the following expression is valid and will always return a string,
    52  because in Terraform all numbers can convert automatically to a string using
    53  decimal digits:
    54  
    55  ```hcl
    56  var.example ? 12 : "hello"
    57  ```
    58  
    59  Relying on this automatic conversion behavior can be confusing for those who
    60  are not familiar with Terraform's conversion rules though, so we recommend
    61  being explicit using type conversion functions in any situation where there may
    62  be some uncertainty about the expected result type.
    63  
    64  The following example is contrived because it would be easier to write the
    65  constant `"12"` instead of the type conversion in this case, but shows how to
    66  use [`tostring`](/docs/language/functions/tostring.html) to explicitly convert a number to
    67  a string.
    68  
    69  ```hcl
    70  var.example ? tostring(12) : "hello"
    71  ```