Preface

Terraform’s HCL language does not provide us with a double for loop out of the box. However this is still possible albeit in a more complicated way using the power of math! 

Tutorial

1 Create two list variables

For better understanding we will use list_1 and list_2 in this example. list_1 containing colors and list_2 containing shapes. 

Create your variables.tf

variable "list_1" {
  type    = "list"
  default = ["Red", "Blue", "Yellow", "Red"]
}

variable "list_2" {
  type    = "list"
  default = ["Square", "Triangle", "Circle", "Rectangle"]
}

2 Create your resource

This will be the resource where you want use your nested loop. We will be applying two tags onto a VPC – List1 and List2. I realize this creates an unreasonable amount of VPC but we are only looking at the logic of what we are trying to accomplish. 

Since there are a list of 4 items in both list_1 and list_2 there will be 16 resources created with every item of list_1 mapping to every item in list_2. 

Create your main.tf

resource "aws_vpc" "aws_vpc_test" {
  count = "${length(var.list_1) * length(var.list_2)}"

  cidr_block = "10.${count.index}.0.0/16"

  tags = {
    List1 = "${var.list_1[floor(count.index / length(var.list_2))]}"
    List2 = "${var.list_2[count.index % length(var.list_2)]}"
  }
}

If you do a terraform plan 16 different VPC resources will be planned for creation with each one having a unique mapping of list_1 and list_2.

Output:

  + aws_vpc.aws_vpc_test[0]
      
      ...
      cidr_block:                       "10.0.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Red"
      tags.List2:                       "Square"

  + aws_vpc.aws_vpc_test[1]
      ...
      cidr_block:                       "10.1.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Red"
      tags.List2:                       "Triangle"

  + aws_vpc.aws_vpc_test[2]
      ...
      cidr_block:                       "10.2.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Red"
      tags.List2:                       "Circle"

  + aws_vpc.aws_vpc_test[3]
      ...
      cidr_block:                       "10.3.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Red"
      tags.List2:                       "Rectangle"

  + aws_vpc.aws_vpc_test[4]
      ...
      cidr_block:                       "10.4.0.0/16"    
      tags.%:                           "2"
      tags.List1:                       "Blue"
      tags.List2:                       "Square"

  + aws_vpc.aws_vpc_test[5]
      ...
      cidr_block:                       "10.5.0.0/16" 
      tags.%:                           "2"
      tags.List1:                       "Blue"
      tags.List2:                       "Triangle"

  + aws_vpc.aws_vpc_test[6]
      ...
      cidr_block:                       "10.6.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Blue"
      tags.List2:                       "Circle"

  + aws_vpc.aws_vpc_test[7]
      ...
      cidr_block:                       "10.7.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Blue"
      tags.List2:                       "Rectangle"

  + aws_vpc.aws_vpc_test[8]
      ...
      cidr_block:                       "10.8.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Yellow"
      tags.List2:                       "Square"

  + aws_vpc.aws_vpc_test[9]
      ...
      cidr_block:                       "10.9.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Yellow"
      tags.List2:                       "Triangle"

  + aws_vpc.aws_vpc_test[10]
      ...
      cidr_block:                       "10.10.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Yellow"
      tags.List2:                       "Circle"

  + aws_vpc.aws_vpc_test[11]
      ...
      cidr_block:                       "10.11.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Yellow"
      tags.List2:                       "Rectangle"

  + aws_vpc.aws_vpc_test[12]
      ...
      cidr_block:                       "10.12.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Red"
      tags.List2:                       "Square"

  + aws_vpc.aws_vpc_test[13]
      ...
      cidr_block:                       "10.13.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Red"
      tags.List2:                       "Triangle"

  + aws_vpc.aws_vpc_test[14]
      ...
      cidr_block:                       "10.14.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Red"
      tags.List2:                       "Circle"

  + aws_vpc.aws_vpc_test[15]
      ...
      cidr_block:                       "10.15.0.0/16"
      tags.%:                           "2"
      tags.List1:                       "Red"
      tags.List2:                       "Rectangle"


Plan: 16 to add, 0 to change, 0 to destroy.