Terraform variable Map Type Explained

terraform-map-variable.tf

Example 1
===============================
Maps are a way to create variables that are lookup tables. An example will show this best. Let's extract our AMIs into a map and add support for the us-west-2 region as well:

variable "amis" {
  type = "map"
  default = {
    "us-east-1" = "ami-b374d5a5"
    "us-west-2" = "ami-4b32be2b"
  }
}

A variable can have a map type assigned explicitly, or it can be implicitly declared as a map by specifying a default value that is a map. The above demonstrates both.

Then, replace the aws_instance with the following:
resource "aws_instance" "example" {
  ami           = var.amis[var.region]
  instance_type = "t2.micro"
}

Example 2
===============================
variable "plans" {
    type = "map"
    default = {
        "5USD"  = "1xCPU-1GB"
        "10USD" = "1xCPU-2GB"
        "20USD" = "2xCPU-4GB"
    }
}
plan = "${var.plans["5USD"]}"

The values matching to their keys can also be used to look up information in other maps. For example, underneath is a short list of plans and their corresponding storage sizes.
variable "storage_sizes" {
    type = "map"
    default = {
        "1xCPU-1GB"  = "25"
        "1xCPU-2GB"  = "50"
        "2xCPU-4GB"  = "80"
    }
}
These can then be used to find the right storage size based on the monthly price as defined in the previous example.

size = "${lookup(var.storage_sizes, var.plans["5USD"])}"

variable "set_password" {
    default = false
}

Example 3
===============================
# Use Case of for_each

variable "account_name" {
   type = "map"
  default = {
      "account1" = "devops1"
      "account2" = "devops2"
      "account3" = "devops3"
}
}
resource "aws_iam_user" "iamuser" {
  for_each = var.account_name
  name = "${each.value}-iam"
}

Example 4
=============================
variable "grps" {
  type = map
  default 	= {
    one = "hello1"
    two = "hello2"
  }
}
resource "azurerm_resource_group" "mapdemo1" {
  name     = var.grps["one"]
  location = "South India"
}

output "resource_group" { 
	value = azurerm_resource_group.mapdemo1.grps["one"]
 }
 
Example 5
===============================
variable "grps" {
  type = map
  default 	= {
    "one" = "hello1"
    "two" = "hello2"
  }
}
resource "azurerm_resource_group" "mapdemo1" {
  name     = var.grps["one"]
  location = "South India"
}

output "resource_group" { 
	value = azurerm_resource_group.mapdemo1.grps["one"]
 }

can you please give an example for variables which did not set up the default values, and type is 'map', how can it be declared?
and how it can be set from the caller for the values?

Is it possible to reference environment variables inside a map type variable?

FYI @devops-school

Examples 4 and 5 are identical.