Kotlin loops and repetition

For loops, while loops, collection iteration, counters, and stopping conditions.

Chapter goal: For loops, while loops, collection iteration, counters, and stopping conditions.

Simple explanation

A loop is a repeating checklist. It performs the same small job for each item instead of copying the code many times.

In Kotlin, this chapter is about doing the same operation safely for many values. Start with the idea above. Then connect each symbol to a value or action in the example.

Do not try to remember every symbol. First ask what data the program has, what it does with that data, and what result it creates. Technical words become easier when you connect them to those three questions.

Why this topic is important

Loops turn one idea into useful automation. In Kotlin, the syntax may look different from other languages, but the thinking skill transfers: name the data, choose the right operation, and make the next step obvious.

When to use it

Use loops when processing lists, retrying work, scanning data, or running repeated game/app updates.

Example code

val scores = listOf(90, 64, 35)

for (score in scores) {
  println(score + 5)
}

Line-by-line explanation

• val scores = listOf(90, 64, 35) — This stores or updates scores. The value on the right is worked out first, then saved under that name.
• for (score in scores) { — This starts a loop. The program repeats the following block for each item or while the condition remains true.
• println(score + 5) — This is the visible output line. It shows the final value after the earlier work is complete.

What the output means

Pass
Retry
Retry

The output is evidence that the program followed the instructions. If your result is different, read from the first line and write down how each value changes. That is debugging, not failure.

Mistake example

val scores = null // the stopping rule or collection boundary is not checked

for (score in scores) {
  println(score + 5)
}

This version intentionally shows how the stopping rule or collection boundary is not checked. The changed assignment stores a missing value, or a required line is removed, so later code cannot complete its job safely.

Fixed version

val scores = listOf(90, 64, 35)

for (score in scores) {
  println(score + 5)
}

The corrected version restores the real value or required operation. It fixes the chapter-specific problem: the stopping rule or collection boundary is not checked.

Common mistakes

• Forgetting to update the loop condition.
• Mutating a collection while iterating it carelessly.
• Using a loop when a collection helper would be clearer.

Warning: Change one part at a time. If you change many lines together, it becomes harder to learn which change caused the result.

Real use cases

• Print reports.
• Validate every form field.
• Update all items in a cart or inventory.

Practice exercise

1. Print only even numbers.
2. Stop when a value reaches 10.
3. Explain what changes during every repetition.

Tip: If the exercise feels too large, complete only steps 1 to 3. Small working code teaches more than a large unfinished project.

Mini quiz

1. What changes on each repetition?
2. What causes an infinite loop?
3. When should the loop stop?

How to read AI-generated code

Do not copy AI code first. Read it like a detective. Find the data, follow the changes, and locate the final output. Ask AI to explain a line only after you have made your own guess.

• What data goes in?
• What values are stored?
• What calculation or decision happens?
• What is printed, displayed, saved, or returned?
• What can go wrong?
• Can you rename one value and still explain the code?

Language reading check

Find the program entry point, follow function calls one at a time, and keep track of each value's type. Do not jump into a class or helper until you know who calls it.

Before you move on

• I can explain this topic in my own words.
• I can read the small example without AI.
• I can change the example and predict the new result.
• I can find and fix one simple mistake.
• I can name one real project that uses this idea.

Next topic

Next, learn functions and reusable logic. Before opening it, explain this chapter out loud in under one minute.