For expressions for transformation in Terraform - Step-by-Step Execution

Process Flow - For expressions for transformation

Start with input list/map

↓

Apply for expression

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Transform each item

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Collect transformed items

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Output new list/map

For expressions take each item from a list or map, change it, and make a new list or map with the changed items.

Execution Sample

Terraform

locals {
  names = ["alice", "bob", "carol"]
  upper_names = [for n in local.names : upper(n)]
}

This code takes a list of names and creates a new list with all names in uppercase.

Process Table

Step	Input Item (n)	Transformation	Output Item	Output List State
1	"alice"	upper("alice")	"ALICE"	["ALICE"]
2	"bob"	upper("bob")	"BOB"	["ALICE", "BOB"]
3	"carol"	upper("carol")	"CAROL"	["ALICE", "BOB", "CAROL"]
4	No more items	End loop	Final list	["ALICE", "BOB", "CAROL"]

💡 All items processed, for expression completes with transformed list.

Status Tracker

Variable	Start	After 1	After 2	After 3	Final
n	none	"alice"	"bob"	"carol"	none
upper_names	[]	["ALICE"]	["ALICE", "BOB"]	["ALICE", "BOB", "CAROL"]	["ALICE", "BOB", "CAROL"]

Key Moments - 3 Insights

Why does the output list grow after each step?

Because each input item is transformed and added to the output list, as shown in execution_table rows 1 to 3.

What happens if the input list is empty?

The for expression produces an empty list immediately, no iterations happen, similar to execution_table row 4 but with no items.

Can the transformation be more complex than a simple function call?

Yes, any expression can be used to transform each item, like conditionals or combining values, as long as it produces a valid output.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output list after step 2?

A ["ALICE"]

B ["ALICE", "BOB"]

C ["BOB"]

D ["ALICE", "BOB", "CAROL"]

💡 Hint

Check the 'Output List State' column in execution_table row 2.

At which step does the for expression finish processing all items?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Look for the row where 'No more items' is the input in execution_table.

If the input list had 4 names instead of 3, how many rows would the execution_table have before the exit row?

A 4

B 5

C 3

D 6

💡 Hint

Each input item gets one step, so count input items in execution_table rows.

Concept Snapshot

For expressions transform each item in a list or map.
Syntax: [for item in list : expression]
Produces a new list with transformed items.
Useful for changing or filtering data simply.
Runs once per input item, collects results.
Always returns a new list or map.

Full Transcript

For expressions in Terraform let you take each item from a list or map and change it to make a new list or map. For example, you can take a list of names and make a new list with all names uppercase. The process starts with the input list, then for each item, it applies a transformation, and adds the result to a new list. This repeats until all items are processed. The output is a new list with all transformed items. Beginners often wonder why the output list grows step by step, or what happens if the input is empty. The output list grows because each transformed item is added in order. If the input is empty, the output is empty too. You can use any expression to transform items, not just simple functions. The execution table shows each step clearly, tracking input, transformation, and output list state. This helps understand how the for expression works internally.