Terraform: How to find the index of an element in a list

Overview

Finding the index of an element in a list is a common task in many programming languages and environments. Terraform, the popular infrastructure as code (IaC) tool, has its own approach to managing such tasks. Whether you’re new to Terraform or looking to polish your skills, understanding how to manipulate lists can be incredibly valuable in scripting dynamic and flexible infrastructure setups. This tutorial will take you through various methods to find the index of an element in a list in Terraform, from basic to advanced examples.

Introduction to Terraform Lists

Before diving into finding element indexes, it’s crucial to grasp what lists are in Terraform and how they’re used. A list in Terraform is an ordered collection of values that are all of the same type. Lists are commonly used to manage multiple resources of a similar type or to configure a resource with multiple values. Defining a list in Terraform is straightforward:

variable "example_list" {
  type    = list(string)
  default = ["a", "b", "c"]
}

Now, let’s proceed to find the index of an element within a list.

Basic Example

To find the index of a specific element, Terraform offers the index function. This function searches for an element in a list and returns its position as a zero-based index. If the element is not found, Terraform will return an error. Here’s how you use it:

element_index = index(var.example_list, "b")

The code above returns 1, as ‘b’ is the second element in the list.

Handling Non-Existent Elements

What if the element is not in the list? By default, Terraform will exit with an error. To handle this scenario gracefully, you can use a combination of Terraform functions to avoid an immediate error exit:

element = "d"
element_index = contains(var.example_list, element) ? index(var.example_list, element) : null

This approach will set element_index to null if ‘d’ is not found in the list, instead of causing an error.

Advanced Strategies

For more complex scenarios where you might be dealing with lists of objects or needing to find multiple indexes, you will need to employ more advanced techniques.

Lists of Objects

Consider you have a list of objects, and you want to find the index of an object based on one of its attributes. For this, you’ll need to use a combination of Terraform’s for expression and the find function:

variable "object_list" {
  type = list(object({
    id   = string
    name = string
  }))
  default = [
    {id = "1", name = "ObjectA"},
    {id = "2", name = "ObjectB"}
  ]
}

object_to_find = {id = "2", name = "ObjectB"}
object_index = [for object in var.object_list : object.id if object == object_to_find]

Here, object_index will be a list of indices where ‘ObjectB’ is found. Note, this example assumes a direct match is possible. In reality, you might need to adapt it based on your exact requirements.

Multiple Indexes

If you’re looking to find multiple indexes where a condition is met, you can employ Terraform’s flatten and for functions in combination:

element_indexes = flatten([
  for element in var.example_list : [
    for i, value in var.example_list : i if value == element
  ]
])

This code snippet will result in a list of indexes for all occurrences of each element in var.example_list.

Dynamic Index Finding

In more dynamic scenarios, where elements and conditions might change, it’s useful to create modular and reusable pieces of code. Consider creating a module or using local values to abstract complex index-finding logic. This not only makes your Terraform code cleaner but also more adaptable to changes.

Performance Considerations

When working with large lists or complex operations, keep in mind the performance impact of your solutions. Terraform’s declarative nature means it’s sometimes less efficient at iterating over lists or performing numerous conditional checks compared to traditional programming languages. Optimize where possible, and consider refactoring into smaller, more manageable pieces if performance becomes an issue.

Conclusion

Finding the index of an element in a list in Terraform requires a good understanding of its built-in functions and expressions. Starting with the basic index function, we explored handling non-existent elements gracefully and moved towards more advanced strategies involving lists of objects and finding multiple indexes. Each method has its use case, and choosing the right one depends on your specific requirements and the complexity of your Terraform configuration.