Declaring list and map variable values in terraform.tfvars - variables

Question

I recently moved over the values for my variables within my terraform code to terraform.tfvars. I am now getting an error that is due to how I am declaring my list and map variables. The code where I am getting the error is replicated below:
image_id = var.web_amis[var.region]
this is how I have these variables specified in terraform.tfvars:
web_amis = ["ami-0dacb0c129b49f529", "ami-00068cd7555f543d5", ]
this is the error code I am getting:
Error: Invalid index
on autoscaling.tf line 3, in resource "aws_launch_configuration" "web_lc":
3: image_id = var.web_amis[var.region]
|----------------
| var.region is "us-east-2"
| var.web_amis is tuple with 2 elements
The given key does not identify an element in this collection value: a number
is required.

Answer 1

You're trying to access a list element with a non index key instead of by position.
What you probably want instead is to have your web_amis variable be a map that is keyed by the region name:
main.tf
variable "region" {}
variable "web_amis" {}
resource "foo_bar" "baz" {
# ...
image_id = var.web_amis[var.region]
}
terraform.tfvars
web_amis = {
us-east-2 = "ami-0dacb0c129b49f529"
us-west-2 = "ami-00068cd7555f543d5"
}
But, this is a very old school and inelegant way of doing things with Terraform nowadays. Instead you could use the aws_ami data source to look up the AMI for the region based on filters such as tags or the name of the AMI.
A basic example is given in the aws_instance resource documentation:
provider "aws" {
region = "us-west-2"
}
data "aws_ami" "ubuntu" {
most_recent = true
filter {
name = "name"
values = ["ubuntu/images/hvm-ssd/ubuntu-trusty-14.04-amd64-server-*"]
}
filter {
name = "virtualization-type"
values = ["hvm"]
}
owners = ["099720109477"] # Canonical
}
resource "aws_instance" "web" {
ami = data.aws_ami.ubuntu.id
instance_type = "t2.micro"
tags = {
Name = "HelloWorld"
}
}