Quick Answer
The correct answer is toset(module.my_module.ids) because Terraform’s for_each meta-argument requires a map or a set of strings, not a list. A list is ordered and allows duplicates, which for_each cannot iterate over directly, so the toset function converts the list into an unordered, deduplicated set that satisfies this requirement. On the HashiCorp Terraform Associate TF-003 exam, this question tests your understanding of the fundamental difference between list and set data types in the context of resource iteration—a common trap is assuming for_each accepts a list, leading to errors like “The given for_each argument value is unsuitable.” Remember the memory tip: “for_each needs a set, not a list—use toset to get it kissed.”
This TF-003 practice question tests your understanding of read, generate and modify configuration. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A developer has a module that outputs a list of subnet IDs. They want to use this list to create an EC2 instance in each subnet using for_each. Which for_each expression is correct?
module.my_module.ids
Why wrong: for_each requires a set or map, not a list.
toset(module.my_module.ids)
Converts the list to a set, valid for for_each.
{ for id in module.my_module.ids : id => id }
Why wrong: This creates a map, but using toset is simpler and more idiomatic.
module.my_module.ids[*]
Why wrong: Splat returns a list, which cannot be used directly with for_each.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
toset(module.my_module.ids)
Correct C: toset converts the list to a set, which is required by for_each. A is a list, B is a map but not necessary, D is a splat expression returning a list.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
module.my_module.ids
Why it's wrong here
for_each requires a set or map, not a list.
toset(module.my_module.ids)
Why this is correct
Converts the list to a set, valid for for_each.
Related concept
CIDR notation defines the prefix length.
{ for id in module.my_module.ids : id => id }
Why it's wrong here
This creates a map, but using toset is simpler and more idiomatic.
module.my_module.ids[*]
Why it's wrong here
Splat returns a list, which cannot be used directly with for_each.
Common exam traps
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
What to study next
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related TF-003 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Read, generate and modify configuration — This question tests Read, generate and modify configuration — CIDR notation defines the prefix length..
The correct answer is: toset(module.my_module.ids) — Correct C: toset converts the list to a set, which is required by for_each. A is a list, B is a map but not necessary, D is a splat expression returning a list.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related TF-003 subnetting questions on CIDR, address ranges, and subnet selection.
CIDR notation defines the prefix length.
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Last reviewed: Jun 24, 2026
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