The beauty of Terraform

vpc.tf

# "data" keyword allow you to pull existing resources from your AWS account
# In this case, we pull the list of availability zones from the current AWS region
data "aws_availability_zones" "available" {}

# Local values assign names to expressions so you can re-use them
# multiple times in the current module (here the number of AZs for the current region)
locals {
  nb_azs = "${length(data.aws_availability_zones.available.names)}"
}

# The "resource" keyword will create a new resource in our AWS account
resource "aws_subnet" "public_subnets" {
  # The "count" parameter allows you to loop and create a resource a variable number of times
  # We create one subnet in each AZ for our current region. Since we are in eu-west-2,
  # this will effectively create 3 subnets, one in each of eu-west-2a, eu-west-2b, eu-west-2c.
  count             = "${local.nb_azs}"

  vpc_id            = "${aws_vpc.vpc.id}"

  # Note the use of the Terraform helper function "cidrsubnet" which calculates non-overlapping
  # CIDR blocks for each subnet
  cidr_block        = "${cidrsubnet(aws_vpc.vpc.cidr_block, 8, count.index)}"

  # We loop over the array of AZs initialised before 
  availability_zone = "${data.aws_availability_zones.available.names[count.index]}"
}