Iterating collections with forEach in Kotlin - Step-by-Step Execution

Concept Flow - Iterating collections with forEach

Start with collection

↓

Call forEach on collection

↓

Take first element

↓

Execute lambda with element

↓

Take next element?

No→End

↩Back to Execute lambda

The forEach function takes each element from a collection one by one and runs the given code block (lambda) on it until all elements are processed.

Execution Sample

Kotlin

val numbers = listOf(1, 2, 3)
numbers.forEach { n ->
    println(n * 2)
}

This code goes through each number in the list and prints double its value.

Execution Table

Step	Current Element (n)	Action	Output
1	1	Execute lambda: println(1 * 2)	2
2	2	Execute lambda: println(2 * 2)	4
3	3	Execute lambda: println(3 * 2)	6
4	-	No more elements, forEach ends	-

💡 All elements processed, forEach stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
n	-	1	2	3	-

Key Moments - 2 Insights

Why does the variable 'n' change each step inside forEach?

Because forEach takes each element from the collection one by one and assigns it to 'n' before running the lambda, as shown in execution_table rows 1 to 3.

Does forEach return a new collection or change the original?

No, forEach only runs the code on each element and returns Unit (nothing). It does not create or change collections, as seen by no changes in variable_tracker except 'n' during iteration.

Visual Quiz - 3 Questions

Look at the execution table, what is the output when 'n' is 2?

A 4

B 2

C 6

D 8

💡 Hint

Check row 2 in the execution_table where n=2 and output is printed

At which step does forEach stop running the lambda?

A Step 3

B Step 4

C Step 1

D Step 2

💡 Hint

Look at the exit note and step 4 in execution_table where no more elements remain

If the list had one more element 4, how many times would the lambda run?

A 3 times

B 5 times

C 4 times

D 1 time

💡 Hint

forEach runs once per element, so check variable_tracker for count of elements

Concept Snapshot

forEach iterates over each element in a collection.
Syntax: collection.forEach { element -> /* code */ }
Runs the code block for every element.
Does not return a new collection.
Useful for actions like printing or updating UI.

Full Transcript

This example shows how to use forEach in Kotlin to go through each item in a list. The code takes each number and prints double its value. The flow starts by calling forEach on the list, then for each element, it runs the lambda code. The variable 'n' holds the current element and changes each step. The execution table shows each step with the current element and output. The process stops when all elements are done. forEach does not create or change collections, it just runs code on each item.