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Terraform

Terraform for_each: Get List Index & Element Access

By Filip on 10/05/2024

Learn how to access the index of elements within a Terraform for_each loop to efficiently manage and reference your infrastructure resources.

Terraform for_each: Get List Index & Element Access

Table of Contents

Introduction

In Terraform, the for_each construct is powerful for iterating over collections, but it doesn't directly provide an index during iteration. This can be limiting if you need to access elements by their position. This article presents several methods to work around this limitation and achieve index-based functionality in your Terraform code. We'll explore using count with local variables, transforming data structures, leveraging range and element, and handling sets. Additionally, we'll touch upon important considerations and potential drawbacks of relying heavily on indices.

Step-by-Step Guide

Terraform's for_each doesn't directly provide an index when iterating. However, you can achieve index-based functionality using alternative methods.

1. Using count with local Variables:

  • Use count to create multiple instances of a resource.
  • Within the resource, define a local variable and use the count.index to represent the index.

2. Transforming Data Structures:

  • If you have a list and need indices, you can transform it into a map using for expressions.
  • Create a map where keys are indices (starting from 0) and values are the original list elements.
  • Use this new map with for_each.

3. Leveraging range and element:

  • Use range(length(your_list)) to generate a sequence of numbers representing indices.
  • Use element(your_list, index) within the for_each loop to access list elements by index.

4. Working with Sets:

  • When using for_each with a set, each.key and each.value are identical.
  • To get an index-like behavior, you need to iterate over the set's values as a list using values(your_set).

Important Considerations:

  • Terraform doesn't guarantee the order of elements in a set. If you need a specific order, use a list.
  • When using dynamic blocks, there's no direct way to get the index. Consider restructuring your code or using alternative approaches.

Remember that using indices can sometimes make your code less readable and maintainable. Explore other Terraform features and best practices to achieve your desired outcome without relying heavily on indices.

Code Example

The Terraform code demonstrates various techniques for iterating and transforming data structures. It showcases using count with local variables to create multiple subnets, transforming a list into a map for indexed access during instance creation, leveraging range and element to iterate over a list of ports, and achieving index-like behavior with sets for subnet creation in different availability zones. These techniques provide flexibility in managing resources and data manipulation within Terraform configurations.

# Using count with local variables
resource "aws_subnet" "example" {
  count = length(var.subnet_cidr_blocks)
  cidr_block = var.subnet_cidr_blocks[count.index]

  # Accessing index within the resource
  locals {
    subnet_index = count.index
  }
  tags = {
    Name = "Subnet-${local.subnet_index}"
  }
}

# Transforming data structures
locals {
  # Original list
  instance_names = ["web-1", "web-2", "db-1"]

  # Transforming into a map with indices as keys
  instance_map = { for i, name in local.instance_names : i => name }
}

resource "aws_instance" "example" {
  for_each = local.instance_map

  # Accessing index and value from the map
  tags = {
    Name = "Instance-${each.key}-${each.value}"
  }
}

# Leveraging range and element
resource "aws_security_group_rule" "example" {
  for_each = { for i in range(length(var.allowed_ports)) : i => var.allowed_ports[i] }

  # Accessing index and port from the list
  from_port = each.value
  to_port   = each.value
  description = "Allowing port ${each.value} - Index: ${each.key}"
}

# Working with sets
locals {
  # Set of zones
  zones = toset(["us-east-1a", "us-east-1b", "us-east-1c"])
}

resource "aws_subnet" "example" {
  for_each = { for i, zone in values(local.zones) : i => zone }

  # Accessing zone from the set using index-like behavior
  availability_zone = each.value
}

Explanation:

  1. count with local Variables: This example creates multiple subnets based on the length of subnet_cidr_blocks. Inside the resource, count.index is used to access the current index and store it in the subnet_index local variable.

  2. Transforming Data Structures: Here, a list of instance_names is transformed into a map (instance_map) where keys are indices and values are the original names. This map is then used with for_each to create instances, allowing access to both index and name.

  3. Leveraging range and element: This example iterates over a list of allowed_ports using range to generate indices. element is used to access the port value at the specific index within the loop.

  4. Working with Sets: This demonstrates how to achieve index-like behavior when working with sets. Since sets don't have inherent order, values(local.zones) converts the set into a list, allowing iteration with indices.

Remember to choose the method that best suits your specific use case and prioritize code readability and maintainability.

Additional Notes

General Considerations:

  • Readability and Maintainability: While these methods provide workarounds, always prioritize code clarity. Overly complex indexing logic can hinder understanding and future modifications.
  • Alternative Solutions: Before resorting to indices, explore if Terraform features like lookup, merge, or data sources can achieve the desired outcome more elegantly.
  • Performance: Transforming large data structures within Terraform configurations can impact performance. Consider pre-processing data externally if it becomes a bottleneck.

Specific to Methods:

  • count with local Variables:
    • Suitable for simple scenarios where the index is used within the same resource.
    • Can become cumbersome for complex resource configurations.
  • Transforming Data Structures:
    • Offers flexibility in creating custom maps with desired key-value pairs.
    • Useful when you need to associate additional data with each element for iteration.
  • Leveraging range and element:
    • Straightforward approach for accessing elements by their position in a list.
    • Can be combined with conditional logic within the loop for more complex scenarios.
  • Working with Sets:
    • Remember that sets are inherently unordered. If order matters, use a list.
    • Converting a set to a list using values() provides index-like access but loses the set's uniqueness property.

Additional Tips:

  • Use meaningful variable names: Clearly indicate the purpose of indices and transformed data structures to improve code readability.
  • Comment your code: Explain the logic behind index manipulation, especially when using less intuitive methods.
  • Test thoroughly: Verify that your index-based logic works as expected, especially when dealing with dynamic data or complex transformations.

By understanding the trade-offs and applying these best practices, you can effectively leverage for_each and achieve index-like functionality in your Terraform code while maintaining clarity and maintainability.

Summary

While Terraform's for_each doesn't directly provide an index, you can achieve index-based functionality through these methods:

| Method | Description

Conclusion

In conclusion, while Terraform's for_each doesn't inherently provide an index, several techniques offer workarounds for index-based resource management. These include using count with local variables, transforming data structures into maps, leveraging range and element for list iteration, and handling sets by converting them to lists. When choosing a method, prioritize code readability and maintainability. Consider alternative Terraform features before resorting to complex indexing. Remember to use meaningful variable names, comment your code, and test thoroughly to ensure your logic functions as expected. By understanding these techniques and best practices, you can effectively utilize for_each for flexible and robust infrastructure management with Terraform.

References

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