Conditional expressions (ternary) in Terraform - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a conditional expression (ternary) in Terraform?

A conditional expression in Terraform chooses one of two values based on a condition. It works like a simple if-else, written as: condition ? value_if_true : value_if_false.

beginner

How do you write a conditional expression to set a variable to "small" if size is less than 10, otherwise "large"?

You write: size < 10 ? "small" : "large". This means if size is less than 10, use "small", else use "large".

intermediate

Can Terraform conditional expressions be nested?

Yes, you can nest conditional expressions inside each other to handle multiple conditions, but keep it simple to avoid confusion.

intermediate

What types of values can be returned by Terraform conditional expressions?

Terraform conditional expressions can return any type: strings, numbers, booleans, lists, or maps, as long as both possible values are the same type.

beginner

Why use conditional expressions in Terraform configurations?

They let you write flexible and reusable code by choosing values based on conditions, like environment type or resource size, without repeating code.

What is the correct syntax for a Terraform conditional expression?

A condition then true_value else false_value

B if (condition) { true_value } else { false_value }

C condition ? true_value : false_value

D condition ? false_value : true_value

Terraform uses the syntax condition ? true_value : false_value for conditional expressions.

In Terraform, what happens if the condition in a ternary expression is false?

A The expression returns the value before the question mark (?)

B The expression returns the value after the colon (:)

C Terraform throws an error

D The expression returns null

If the condition is false, Terraform returns the value after the colon (:).

Can you use different data types for the true and false values in a Terraform conditional expression?

A No, both must be the same type

B Yes, any types are allowed

C Only if one is a string

D Only if one is a number

Terraform requires both true and false values to be the same type for conditional expressions.

Which of these is a valid use case for Terraform conditional expressions?

A Choosing resource size based on environment

B Writing loops over resources

C Defining variables without conditions

D Creating new resource types

Conditional expressions are used to select values like resource size based on conditions such as environment.

Is it recommended to nest many conditional expressions in Terraform?

A Terraform does not support nesting

B Yes, always nest as much as possible

C Only if you use more than 5 conditions

D No, it can make code hard to read

Nesting many conditional expressions can make Terraform code hard to read and maintain.

Explain how a Terraform conditional expression works and give a simple example.

Think of it like a simple if-else that picks one value.

Describe best practices when using conditional expressions in Terraform configurations.

Focus on readability and type consistency.

Practice

1. What does this Terraform conditional expression do?
var.is_production ? "prod" : "dev"

easy

A. Returns true or false

B. Always returns "prod"

C. Returns "prod" if var.is_production is true, otherwise "dev"

D. Causes a syntax error

Solution

1. Step 1: Understand the condition

   The expression checks if var.is_production is true or false.

2. Step 2: Evaluate the ternary expression

   If true, it returns "prod"; if false, it returns "dev".

3. Final Answer:

   Returns "prod" if var.is_production is true, otherwise "dev" -> Option C

4. Quick Check:

   Condition ? true_value : false_value = C [OK]

Hint: Remember: condition ? value_if_true : value_if_false [OK]

Common Mistakes:

• Thinking it returns a boolean
• Confusing true and false branches
• Assuming it always returns the first value

2. Which of these is the correct syntax for a Terraform conditional expression?

easy

A. condition : true_val ? false_val

B. condition ? true_val : false_val

C. condition ? (true_val, false_val)

D. if (condition) then true_val else false_val

Solution

1. Step 1: Recall Terraform conditional syntax

   Terraform uses the format: condition ? true_val : false_val.

2. Step 2: Compare options

   Only condition ? true_val : false_val matches this syntax exactly.

3. Final Answer:

   condition ? true_val : false_val -> Option B

4. Quick Check:

   Correct ternary syntax = D [OK]

Hint: Look for question mark and colon in correct order [OK]

Common Mistakes:

• Using if-then-else syntax from other languages
• Swapping question mark and colon
• Using commas inside the expression

3. Given this Terraform code:

variable "env" { default = "test" }
output "message" {
  value = var.env == "prod" ? "Deploy to production" : "Deploy to staging"
}

What will be the output value?

medium

A. Error: unknown variable

B. "Deploy to production"

C. "test"

D. "Deploy to staging"

Solution

1. Step 1: Check variable value

   var.env is set to "test" by default.

2. Step 2: Evaluate condition

   var.env == "prod" is false, so the false branch "Deploy to staging" is chosen.

3. Final Answer:

   "Deploy to staging" -> Option D

4. Quick Check:

   Condition false -> false branch output = A [OK]

Hint: Check variable value before condition evaluation [OK]

Common Mistakes:

• Assuming default is "prod"
• Confusing true and false branches
• Expecting variable value as output

4. Identify the error in this Terraform conditional expression:

output "result" {
  value = var.enabled ? "yes" "no"
}

medium

A. Missing colon ':' between true and false values

B. Variable var.enabled is undefined

C. Quotes around values are incorrect

D. Ternary operator not supported in outputs

Solution

1. Step 1: Review ternary syntax

   The correct syntax requires a colon ':' between true and false values.

2. Step 2: Identify missing colon

   The expression has "? \"yes\" \"no\"" missing the ':' separator.

3. Final Answer:

   Missing colon ':' between true and false values -> Option A

4. Quick Check:

   Syntax error due to missing ':' = A [OK]

Hint: Check for '?' and ':' pair in ternary expressions [OK]

Common Mistakes:

• Omitting the colon ':'
• Misplacing quotes
• Assuming ternary not allowed in outputs

5. You want to assign a resource name based on environment, but if the environment variable is empty or null, use "default" instead. Which expression correctly does this in Terraform?

var.env == "" || var.env == null ? "default" : var.env

hard

A. var.env == "" || var.env == null ? "default" : var.env

B. var.env == "" && var.env == null ? "default" : var.env

C. var.env ? "default" : var.env

D. var.env == null ? var.env : "default"

Solution

1. Step 1: Understand the requirement

   If var.env is empty string or null, use "default"; else use var.env.

2. Step 2: Analyze each option

   var.env == "" || var.env == null ? "default" : var.env checks if var.env is empty or null, then returns "default", else var.env, matching the requirement exactly.

3. Final Answer:

   var.env == "" || var.env == null ? "default" : var.env -> Option A

4. Quick Check:

   Check empty or null -> default else env = B [OK]

Hint: Check both empty string and null explicitly in condition [OK]

Common Mistakes:

• Using && instead of || causing wrong logic
• Assuming var.env alone checks for empty or null
• Swapping true and false branches