Collection functions (length, flatten, merge) in Terraform - Step-by-Step Execution

Process Flow - Collection functions (length, flatten, merge)

Start with collections

↓

Apply length()

↓

Apply flatten()

↓

Apply merge()

↓

Get final result

Start with collections, then use length() to count items, flatten() to make nested lists flat, and merge() to combine maps.

Execution Sample

Terraform

locals {
  list_nested = [[1, 2], [3, 4]]
  map1 = {a = 1, b = 2}
  map2 = {b = 3, c = 4}
  len_list = length(local.list_nested)
  flat_list = flatten(local.list_nested)
  merged_map = merge(local.map1, local.map2)
}

This code counts items in a nested list, flattens it, and merges two maps.

Process Table

Step	Function	Input	Output	Explanation
1	length	[[1, 2], [3, 4]]	2	Count top-level elements in the nested list
2	flatten	[[1, 2], [3, 4]]	[1, 2, 3, 4]	Flatten nested list into single list
3	merge	{a=1,b=2}, {b=3,c=4}	{a=1,b=3,c=4}	Merge maps; later keys overwrite earlier ones

💡 All functions applied; final outputs obtained.

Status Tracker

Variable	Start	After length	After flatten	After merge
list_nested	[[1, 2], [3, 4]]	[[1, 2], [3, 4]]	[[1, 2], [3, 4]]	[[1, 2], [3, 4]]
len_list	undefined	2	2	2
flat_list	undefined	undefined	[1, 2, 3, 4]	[1, 2, 3, 4]
merged_map	undefined	undefined	undefined	{a=1,b=3,c=4}

Key Moments - 3 Insights

Why does length([[1, 2], [3, 4]]) return 2, not 4?

length() counts only the top-level elements. Here, the list has two elements: [1, 2] and [3, 4]. See execution_table step 1.

How does flatten() change the nested list?

flatten() removes one level of nesting, combining inner lists into a single list. See execution_table step 2.

When merging maps with overlapping keys, which value is kept?

The value from the later map overwrites the earlier one. In step 3, key 'b' changes from 2 to 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output of length() at step 1?

A 1

B 2

C 4

D 0

💡 Hint

Check the 'Output' column in execution_table row with Step 1.

At which step does the list become flat?

A Step 2

B Step 1

C Step 3

D No flattening occurs

💡 Hint

Look at the 'Function' and 'Output' columns in execution_table for flatten().

If map2 was {b=5, c=6} instead, what would merged_map's 'b' value be?

A 2

B 3

C 5

D Undefined

💡 Hint

Refer to execution_table step 3 where later map keys overwrite earlier ones.

Concept Snapshot

length(collection) returns number of top-level items.
flatten(list_of_lists) merges nested lists into one list.
merge(map1, map2, ...) combines maps; later keys overwrite earlier.
Use these to count, simplify, and combine collections easily.

Full Transcript

This visual trace shows how Terraform collection functions work step-by-step. We start with a nested list and two maps. length() counts the top-level items in the list, returning 2 because there are two inner lists. flatten() then merges these inner lists into one flat list with four numbers. Finally, merge() combines two maps; when keys overlap, the value from the later map replaces the earlier one. This helps beginners see exactly how these functions change data collections in Terraform.