Terraform 0.12 - Skip specific indexes in a list passed into a resource

Terraform
1

Hello,

I have a variable i am passing into my autoscalegroup resource that contains a list of subnet ids:

variable "subnet_ids" {
  type = list(string)
}

the variable subnet_ids would contain a list such as [subnet_001, subnet_002, subnet_003, subnet_004]

Each subnet are in a specific AWS availability zone:

subnet_001 - us-east-1a
subnet_002 - us-east-1b
subnet_003 - us-east-1c
subnet_004 - us-east-1d

I am using the subnet_ids variable in my aws_autoscaling_group resource as:

resource "aws_autoscaling_group" "my_asg" {
  count                     = length(var.subnet_ids)
  name                      = "${var.environment}-${var.role}-${element(var.subnet_ids, count.index)}"
  max_size                  = var.max_size
  min_size                  = var.min_size
  desired_capacity          = var.desired_capacity
  health_check_grace_period = var.hc_grace_period
  health_check_type         = var.hc_check_type
  force_delete              = var.force_delete
  vpc_zone_identifier       = [element(var.subnet_ids, count.index)]
  termination_policies = var.asg_term_policy
  enabled_metrics      = var.enabled_metrics

  launch_template {
    id      = aws_launch_template.my_lt.id
    version = "$Latest"
  }

However I will occassionally run into an issue where the instance type is not supported in the AZ:

Error: Error creating AutoScaling Group: ValidationError: You must use a valid fully-formed launch template. Your requested instance type (c5.2xlarge) is not supported in your requested Availability Zone (us-east-1e). Please retry your request by not specifying an Availability Zone or choosing us-east-1a, us-east-1b, us-east-1c, us-east-1d, us-east-1f.

Is there a way to refactor my module so that I can exclude the unsupported subnets/AZs?

Thank you