Terraform
8. Expressions
For Iterations

for Expressions

A for expression in Terraform is like a loop that helps you transform or process lists and maps of data. Think of it as a way to say "for each item in this collection, do something with it". For more information about for expression check here (opens in a new tab)

Here are some common use cases with simple examples:

Creating a new list from an existing list

Convert a list of domains into edge hostnames.

main.tf
variable "domains" {
  default = ["example.com", "api.example.com"]
}
 
locals {
  edge_hostnames = [for domain in var.domains : "${domain}.edgesuite.net"]
  # Result: [
  #   "example.com.edgesuite.net",
  #   "api.example.com.edgesuite.net"
  # ]
}

Creating a map from a list

Converting a list of hostnames into a map containing the respective edge hostnames.

main.tf
locals {
  edge_hostnames = {for domain in var.domains : "${domain}" => "${domain}.edgesuite.net"}
  # Result: {
  #   "api.example.com" = "api.example.com.edgesuite.net"
  #   "example.com"     = "example.com.edgesuite.net"
  # }
}

Filtering items

Filter out the example.com domain from the list.

main.tf
variable "all_names" {
  default = ["example.com", "api.example.com", "www.example.com"]
}
 
locals {
  hostnames = [for item in var.all_names : item if item != "example.com"]
}
 
output "hostname" {
  value = local.hostnames
}
# Result: [
#   "api.example.com",
#   "www.example.com"
# ]

The main benefits of for expressions are:

  • They help you avoid repetitive code
  • They make it easier to process lists and maps
  • They allow you to transform data into the format you need
  • They're great for creating multiple similar resources

Exercise

Configure

Create the following variable in your variables.tf:

variables.tf
variable "apps" {
  default = [ "www", "api", "blog", "shop", "cdn"]
}

Update

In your main.tf and with the use of the for expression, convert each app name above into a hostname by appending the example.com domain. The new local variable containing the new hostnames list can be named app_hostnames and we will use it in later exercises.

Output

Output the app_hostnames local to confirm the logic is applied.

Plan

Run terraform plan. You could run terraform apply too, but because we are not modifying any resources no changes will be applied anyway.

Commit

Commit your change and push it to GitHub.

ConditionalsDynamic Blocks