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.

Practice

1. What does the Terraform function length() do when applied to a list?

easy

A. Returns the number of items in the list

B. Combines all items into a single string

C. Flattens nested lists into one list

D. Merges the list with another list

Solution

1. Step 1: Understand the purpose of length()

   The length() function counts how many elements are in a list, map, or string.

2. Step 2: Apply to a list context

   When used on a list, it returns the total number of items inside that list.

3. Final Answer:

   Returns the number of items in the list -> Option A

4. Quick Check:

   length(list) = count of items [OK]

Hint: Count items with length() to get list size fast [OK]

Common Mistakes:

• Thinking length() merges lists
• Confusing length() with flatten()
• Assuming length() returns the list itself

2. Which of the following is the correct syntax to merge two maps map1 and map2 in Terraform?

easy

A. length(map1, map2)

B. flatten(map1, map2)

C. map1 + map2

D. merge(map1, map2)

Solution

1. Step 1: Identify the function for combining maps

   The merge() function is used to combine two or more maps into one.

2. Step 2: Check syntax correctness

   The correct syntax is merge(map1, map2). Other options either use wrong functions or invalid operators.

3. Final Answer:

   merge(map1, map2) -> Option D

4. Quick Check:

   merge() combines maps correctly [OK]

Hint: Use merge() with parentheses and commas for maps [OK]

Common Mistakes:

• Using + operator to merge maps
• Confusing flatten() with merge()
• Passing maps without commas

3. What is the output of this Terraform expression?

flatten([["a", "b"], ["c"], ["d", "e"]])

medium

A. [["a", "b"], ["c"], ["d", "e"]]

B. ["a", "b", "c", "d", "e"]

C. ["a", "b", ["c"], "d", "e"]

D. Error: flatten() only works on maps

Solution

1. Step 1: Understand flatten() function

   The flatten() function takes a list of lists and combines all inner lists into one single list.

2. Step 2: Apply flatten() to the given nested list

   Input is [["a", "b"], ["c"], ["d", "e"]]. Flattening merges all inner lists into ["a", "b", "c", "d", "e"].

3. Final Answer:

   ["a", "b", "c", "d", "e"] -> Option B

4. Quick Check:

   flatten(nested list) = single list [OK]

Hint: flatten() removes inner list brackets to one list [OK]

Common Mistakes:

• Thinking flatten() returns original nested list
• Expecting flatten() to work on maps
• Confusing flatten() with merge()

4. Given the code snippet:

locals {
  map1 = { a = 1, b = 2 }
  map2 = { b = 3, c = 4 }
  result = merge(map1, map2)
}

What is the value of local.result?

medium

A. { a = 1, b = 3, c = 4 }

B. { a = 1, b = 2, c = 4 }

C. { a = 1, b = 5, c = 4 }

D. Error: duplicate keys not allowed in merge

Solution

1. Step 1: Understand merge() behavior with duplicate keys

   When keys overlap, the value from the last map argument overrides earlier ones.

2. Step 2: Apply merge to map1 and map2

   map1 has b=2, map2 has b=3. The merged map uses b=3 from map2, so result is { a=1, b=3, c=4 }.

3. Final Answer:

   { a = 1, b = 3, c = 4 } -> Option A

4. Quick Check:

   merge() overrides duplicate keys with last map [OK]

Hint: Last map's keys override earlier ones in merge() [OK]

Common Mistakes:

• Assuming values add up for duplicate keys
• Expecting error on duplicate keys
• Confusing merge() with flatten()

5. You have a list of maps:

list_of_maps = [
  { name = "Alice", age = 30 },
  { name = "Bob", age = 25 },
  { name = "Carol", age = 27 }
]

Which Terraform expression correctly merges all maps into one map with all keys and values combined?

hard

A. length(list_of_maps)

B. flatten(list_of_maps)

C. merge(list_of_maps...)

D. merge(list_of_maps)

Solution

1. Step 1: Understand how to merge multiple maps in a list

   To merge all maps inside a list, you use the splat operator ... to unpack the list as arguments to merge().

2. Step 2: Check each option

   merge(list_of_maps...) uses merge(list_of_maps...) which correctly merges all maps. flatten(list_of_maps) tries flatten on maps (invalid), C returns count, D passes list directly (invalid).

3. Final Answer:

   merge(list_of_maps...) -> Option C

4. Quick Check:

   Use merge() with ... to combine list of maps [OK]

Hint: Use ... to unpack list when merging maps [OK]

Common Mistakes:

• Passing list directly to merge() without ...
• Using flatten() on maps
• Confusing length() with merge()