terraform/website/docs/configuration/expressions/for.html.md

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language	For Expressions - Configuration Language

for Expressions

A for expression creates a complex type value by transforming another complex type value. Each element in the input value can correspond to either one or zero values in the result, and an arbitrary expression can be used to transform each input element into an output element.

For example, if var.list is a list of strings, then the following expression produces a list of strings with all-uppercase letters:

[for s in var.list : upper(s)]

This for expression iterates over each element of var.list, and then evaluates the expression upper(s) with s set to each respective element. It then builds a new tuple value with all of the results of executing that expression in the same order.

The type of brackets around the for expression decide what type of result it produces. The above example uses [ and ], which produces a tuple. If { and } are used instead, the result is an object, and two result expressions must be provided separated by the => symbol:

{for s in var.list : s => upper(s)}

This expression produces an object whose attributes are the original elements from var.list and their corresponding values are the uppercase versions.

A for expression can also include an optional if clause to filter elements from the source collection, which can produce a value with fewer elements than the source:

[for s in var.list : upper(s) if s != ""]

The source value can also be an object or map value, in which case two temporary variable names can be provided to access the keys and values respectively:

[for k, v in var.map : length(k) + length(v)]

Finally, if the result type is an object (using { and } delimiters) then the value result expression can be followed by the ... symbol to group together results that have a common key:

{for s in var.list : substr(s, 0, 1) => s... if s != ""}

For expressions are particularly useful when combined with other language features to combine collections together in various ways. For example, the following two patterns are commonly used when constructing map values to use with the for_each meta-argument:

• Transform a multi-level nested structure into a flat list by using nested for expressions with the flatten function.
• Produce an exhaustive list of combinations of elements from two or more collections by using the setproduct function inside a for expression.