»flatten Function

flatten takes a list and replaces any elements that are lists with a flattened sequence of the list contents.

»Examples

If any of the nested lists also contain directly-nested lists, these too are flattened recursively:

Indirectly-nested lists, such as those in maps, are not flattened.

»Flattening nested structures for for_each

The resource for_each and dynamic block language features both require a collection value that has one element for each repetition.

Sometimes your input data structure isn't naturally in a suitable shape for use in a for_each argument, and flatten can be a useful helper function when reducing a nested data structure into a flat one.

For example, consider a folder that declares a variable like the following:

The above is a reasonable way to model objects that naturally form a tree, such as top-level networks and their subnets. The repetition for the top-level networks can use this variable directly, because it's already in a form where the resulting instances match one-to-one with map elements:

However, in order to declare all of the subnets with a single resource block, we must first flatten the structure to produce a collection where each top-level element represents a single subnet:

The above results in one subnet instance per subnet object, while retaining the associations between the subnets and their containing networks.

»Related Functions

  • setproduct finds all of the combinations of multiple lists or sets of values, which can also be useful when preparing collections for use with for_each constructs.