Collection functions (length, flatten, merge) in Terraform - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of length function on a list?

Given the Terraform expression length(["apple", "banana", "cherry"]), what is the output?

Terraform

length(["apple", "banana", "cherry"])

A 2

B 3

C 0

D Error

💡 Hint

Count the number of items inside the list.

The length function returns the number of elements in the list. Here, there are three fruits, so the output is 3.

❓ query_result

intermediate

What does flatten do to nested lists?

What is the result of flatten([[1, 2], [3, 4], [5]]) in Terraform?

Terraform

flatten([[1, 2], [3, 4], [5]])

A Error

B [[1, 2], [3, 4], [5]]

C [1, 2, 3, 4, 5]

D [1, [2, 3], 4, 5]

💡 Hint

Flatten removes one level of nested lists.

The flatten function merges all inner lists into a single list, so the output is a flat list of all elements.

📝 Syntax

advanced

Which merge expression correctly combines two maps?

Select the Terraform expression that correctly merges two maps {a = 1, b = 2} and {b = 3, c = 4} with the second map overriding keys from the first.

A merge({a = 1, b = 2} + {b = 3, c = 4})

B merge({a = 1, b = 2}, {b: 3, c: 4})

C merge([{a = 1, b = 2}, {b = 3, c = 4}])

D merge({a = 1, b = 2}, {b = 3, c = 4})

💡 Hint

Use merge with two separate map arguments.

The merge function takes multiple maps as separate arguments and combines them. Option D is the correct syntax.

❓ optimization

advanced

Optimizing nested flatten calls

Given a nested list [[[1], [2, 3]], [[4]]], which expression efficiently flattens it into a single list [1, 2, 3, 4]?

A flatten(flatten([[[1], [2, 3]], [[4]]]))

B flatten([1, 2, 3, 4])

C flatten(flatten(flatten([[[1], [2, 3]], [[4]]])))

D flatten([[[1], [2, 3]], [[4]]])

💡 Hint

Use flatten twice to remove two levels of nesting.

The list has three levels of nesting. Using flatten twice removes two levels, resulting in a flat list.

🧠 Conceptual

expert

What is the length of a merged map?

If you merge two maps {x = 10, y = 20} and {y = 30, z = 40}, what is the length of the resulting map?

Terraform

length(merge({x = 10, y = 20}, {y = 30, z = 40}))

A 3

B 4

C 2

D Error

💡 Hint

Keys that appear in both maps are overridden, not duplicated.

The merged map has keys x, y, and z. The key y from the second map overrides the first. So total keys are 3.

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()