given list of subnets, find out which one are public and which are private

terraform-subnets.tf

variable "subnets" {
  default = "subnet-xxxxx,subnet-yyyyy,subnet-zzzzzz"
}

locals {
  subnets         = ["${split(",", replace(var.subnets, "/(,$|\\s)/", ""))}"]

  private_subnets = "${compact(null_resource.subnets.*.triggers.private)}"
  public_subnets  = "${compact(null_resource.subnets.*.triggers.public)}"
}

data "aws_route_table" "selected" {
  count     = "${length(local.subnets)}"
  subnet_id = "${element(local.subnets, count.index)}"
}

data "aws_route" "selected" {
  count = "${length(local.subnets)}"

  route_table_id         = "${element(data.aws_route_table.selected.*.id, count.index)}"
  destination_cidr_block = "0.0.0.0/0"
}

resource "null_resource" "subnets" {
  count = "${length(local.subnets)}"

  triggers = {
    public  = "${element(data.aws_route.selected.*.gateway_id, count.index) != "" ? "${element(local.subnets, count.index)}" : ""}"
    private = "${element(data.aws_route.selected.*.gateway_id, count.index) == "" ? "${element(local.subnets, count.index)}" : ""}"
  }
}

output "sub" {
  value = "${data.aws_route.selected.*.gateway_id}"
}

output "private_subnets" {
  value = "${local.private_subnets}"
}

output "public_subnets" {
  value = "${local.public_subnets}"
}