Terraform get the length of list

Question

I'm experimenting with Terraform and I came across locals. What I'm trying to do is to get the length of the list based on another varibale (env).

how can to make terrform to evaluate the variable before getting trying to evaluate the length?

This is my code:

locals {
  env = "${terraform.workspace}"

  subnet_names = {
    "default" = ["default_sub1"]
    "dev"     = ["dev_sub1", "dev_sub2", "dev_sub3"]
    "prod"    = ["prod_sub1", "prod_sub2", "prod_sub3"]
  }

}

resource "azurerm_subnet" "subnet" {
  name                      = "${lookup(local.subnet_names, local.env, count.index)}"
  virtual_network_name      = "${azurerm_virtual_network.network.name}"
  resource_group_name       = "${azurerm_resource_group.terraform.name}"
  address_prefix            = "10.0.1.0/24"
  network_security_group_id = "${azurerm_network_security_group.security_group.id}"
  count                     =  "${length(local.subnet_names, local.env)}"
}

When I try to validate the code I get this length: expected 1 arguments, got 2 in: ${length(local.subnet_names, local.env)}

What is the trick here?

Answer 1

Your local.subnet_names is not a list, it is a map and can be accessed as explained in the interpolation syntax:

${length(local.subnet_names[local.env])}

EDIT:

As for the name variable, the right way to make it is using the element interpolation:

name = "${element(local.subnet_names[local.env], count.index)}"

This is due to the fact that local.subnet_names[local.env] will return a list. As an example, if local.env is "dev", it will return

["dev_sub1", "dev_sub2", "dev_sub3"]

and to get an element in a certain index in a list we use element.