Collection functions (length, flatten, merge) in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the length() function do in Terraform collections?

The length() function returns the number of items in a list, map, or string. It helps you count how many elements are inside a collection.

beginner

How does the flatten() function work in Terraform?

The flatten() function takes a list of lists and combines all inner lists into a single flat list. It removes one level of nesting.

beginner

What is the purpose of the merge() function in Terraform?

The merge() function combines two or more maps into one map. If keys overlap, later maps overwrite earlier ones.

beginner

Example: What is the output of length(["apple", "banana", "cherry"])?

The output is 3 because there are three items in the list.

beginner

Example: What does flatten([[1, 2], [3, 4], [5]]) return?

It returns [1, 2, 3, 4, 5], a single list with all elements from the inner lists combined.

What does length(["a", "b", "c"]) return?

A 3

B 2

C 1

D Error

The length function counts the number of items in the list, which is 3.

What is the result of flatten([["x"], ["y", "z"]])?

A ["x", "y", "z"]

B [["x"], ["y", "z"]]

C ["x"]

D Error

Flatten removes one level of nesting, combining all inner lists into one list.

What does merge({a=1, b=2}, {b=3, c=4}) return?

A {a=1, b=2, c=4}

B {a=1, b=3, c=4}

C {a=1, b=2}

D Error

The second map's value for key 'b' (3) overwrites the first map's value (2).

Which function would you use to count items in a list?

A concat()

B merge()

C flatten()

D length()

length() counts the number of items in a collection.

If you have a list of lists, which function helps you make it a single list?

A length()

B merge()

C flatten()

D split()

flatten() combines nested lists into one flat list.

Explain how the length(), flatten(), and merge() functions work in Terraform collections.

Think about counting, combining lists, and combining maps.

Describe a real-life example where you might use flatten() and merge() in Terraform.

Imagine organizing lists or settings from different sources.

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()