For expressions for transformation in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a 'for expression' in Terraform?

A 'for expression' in Terraform is a way to transform or create new collections by looping over existing ones, similar to a simple loop that builds a new list or map.

beginner

How do you create a new list by doubling each number in an existing list using a for expression?

You write:
[for n in var.numbers : n * 2]
This loops over each number 'n' in 'var.numbers' and multiplies it by 2, creating a new list.

intermediate

What is the difference between a for expression that creates a list and one that creates a map?

A list for expression returns a list of values.
A map for expression returns key-value pairs using the syntax:
{ for k, v in var.map : k => v }
This creates a new map by transforming keys and values.

intermediate

How can you filter elements in a for expression?

You add an 'if' condition after the colon:
[for n in var.numbers : n if n > 5]
This creates a list of numbers greater than 5 only.

beginner

Why use for expressions in Terraform configurations?

For expressions help you write less repetitive code by transforming data collections dynamically, making your infrastructure code cleaner and easier to maintain.

What does this Terraform expression produce?
[for s in ["a", "b", "c"] : upper(s)]

A Error, 'upper' is not valid

B ["A", "B", "C"]

C {"a" => "A", "b" => "B", "c" => "C"}

D ["a", "b", "c"]

The expression loops over each string and applies 'upper' to convert to uppercase, producing a list of uppercase letters.

How do you write a for expression that creates a map from a list of strings where keys are strings and values are their lengths?

A [ for s in var.strings : length(s) ]

B { for s in var.strings : length(s) => s }

C { for s in var.strings : s => length(s) }

D [ for s in var.strings : s ]

Using curly braces and 'key => value' syntax creates a map where each string maps to its length.

Which of these filters elements in a list to only include even numbers?

A [for n in var.numbers : n if n % 2 == 0]

B [for n in var.numbers : n]

C {for n in var.numbers : n => n if n % 2 == 0}

D [for n in var.numbers : n if n > 0]

The 'if' condition after the colon filters the list to include only even numbers.

What type of collection does this expression produce?
{ for k, v in var.map : k => v * 2 }

A An error

B A list of doubled values

C A list of keys

D A map with values doubled

Curly braces with 'key => value' syntax produce a map; values are doubled here.

Can you use for expressions inside resource blocks in Terraform?

A Yes, to generate multiple resources or attributes dynamically

B No, for expressions are only for variables

C Only in provider blocks

D Only in output blocks

For expressions can be used anywhere in Terraform configuration to create dynamic values.

Explain how to use a for expression to transform a list of strings into a list of their uppercase versions.

Think about looping over each item and changing it.

Describe how to filter elements in a list using a for expression in Terraform.

Filtering happens after the colon with an 'if' statement.

Practice

1. What does a for expression in Terraform primarily do?

easy

A. Creates a new resource in the cloud

B. Transforms each item in a list or map into a new list or map

C. Deletes resources based on a condition

D. Runs a shell command on the local machine

Solution

1. Step 1: Understand the purpose of for expressions

   For expressions in Terraform are used to transform collections like lists or maps into new collections by applying a transformation to each item.

2. Step 2: Differentiate from other Terraform actions

   Creating or deleting resources and running shell commands are not done by for expressions; they are handled by resource blocks or provisioners.

3. Final Answer:

   Transforms each item in a list or map into a new list or map -> Option B

4. Quick Check:

   For expressions = transform collections [OK]

Hint: For expressions change each item in a list/map to a new form [OK]

Common Mistakes:

• Thinking for expressions create or delete resources
• Confusing for expressions with provisioners
• Assuming for expressions run commands

2. Which of the following is the correct syntax for a simple for expression transforming a list [1, 2, 3] to their squares in Terraform?

easy

A. [for n in [1, 2, 3] : n * n]

B. [for n : n * n in [1, 2, 3]]

C. [for n in [1, 2, 3] n * n]

D. [for n in [1, 2, 3] => n * n]

Solution

1. Step 1: Recall Terraform for expression syntax

   The correct syntax is: [for <variable> in <list> : <expression>]. [for n in [1, 2, 3] : n * n] matches this exactly.

2. Step 2: Check other options for syntax errors

   [for n : n * n in [1, 2, 3]] has wrong order, [for n in [1, 2, 3] n * n] misses colon, [for n in [1, 2, 3] => n * n] uses => which is invalid in Terraform for expressions.

3. Final Answer:

   [for n in [1, 2, 3] : n * n] -> Option A

4. Quick Check:

   Correct syntax = [for var in list : expr] [OK]

Hint: Remember syntax: for var in list : expression [OK]

Common Mistakes:

• Omitting the colon ':' after the list variable
• Using arrow '=>' instead of colon
• Placing expression before 'in' keyword

3. Given the Terraform expression:

[for s in ["apple", "banana", "cherry"] : upper(s) if length(s) > 5]

What is the resulting list?

medium

A. ["banana", "cherry"]

B. ["APPLE", "BANANA", "CHERRY"]

C. ["BANANA", "CHERRY"]

D. ["APPLE"]

Solution

1. Step 1: Identify filtering condition

   The expression filters strings with length greater than 5. "banana" (6) and "cherry" (6) qualify; "apple" (5) does not.

2. Step 2: Apply transformation function

   For qualifying strings, upper(s) converts them to uppercase: "BANANA", "CHERRY".

3. Final Answer:

   ["BANANA", "CHERRY"] -> Option C

4. Quick Check:

   Filter length > 5 + uppercase = ["BANANA", "CHERRY"] [OK]

Hint: Filter first, then transform inside for expression [OK]

Common Mistakes:

• Including items that don't meet the if condition
• Not applying the upper() function
• Confusing syntax order of filter and expression

4. Identify the error in this Terraform for expression:

[for k, v in {a=1, b=2} : "${k}=${v}" if v > 1]

medium

A. Using if condition outside the for expression

B. Missing colon after the map

C. Incorrect use of string interpolation syntax

D. Using two variables without parentheses in for expression

Solution

1. Step 1: Check syntax for multiple variables in for expressions

   When using two variables (key and value) in a for expression, parentheses are required: for (k, v) in map.

2. Step 2: Identify the error in given expression

   The expression uses 'for k, v in' without parentheses, causing syntax error.

3. Final Answer:

   Using two variables without parentheses in for expression -> Option D

4. Quick Check:

   Two variables need parentheses: for (k, v) in map [OK]

Hint: Use parentheses for multiple variables in for expressions [OK]

Common Mistakes:

• Omitting parentheses around multiple variables
• Placing if condition incorrectly
• Misusing string interpolation syntax

5. You have a map of instance names to their CPU counts:

{"web"=2, "db"=4, "cache"=1}

Write a Terraform for expression that creates a list of instance names with CPU count greater than 1, all in uppercase.

hard

A. [for (name, cpu) in {"web"=2, "db"=4, "cache"=1} : upper(name) if cpu > 1]

B. [for name, cpu in {"web"=2, "db"=4, "cache"=1} : upper(name) if cpu > 1]

C. [for (name, cpu) in {"web"=2, "db"=4, "cache"=1} : name if cpu > 1]

D. [for (name, cpu) in {"web"=2, "db"=4, "cache"=1} : upper(cpu) if cpu > 1]

Solution

1. Step 1: Use parentheses for key-value iteration

   When iterating over a map with two variables, use parentheses: for (name, cpu) in map.

2. Step 2: Apply filter and transformation

   Filter with if cpu > 1 to select "web" and "db". Transform name to uppercase with upper(name).

3. Final Answer:

   [for (name, cpu) in {"web"=2, "db"=4, "cache"=1} : upper(name) if cpu > 1] -> Option A

4. Quick Check:

   Parentheses + filter + upper(name) = [for (name, cpu) in {"web"=2, "db"=4, "cache"=1} : upper(name) if cpu > 1] [OK]

Hint: Parentheses for map keys, filter with if, transform with upper() [OK]

Common Mistakes:

• Omitting parentheses for key-value pairs
• Transforming cpu instead of name
• Not filtering correctly with if condition