Kotlin operators and expressions

Calculations, comparisons, boolean logic, assignment, and expression readability.

Chapter goal: Calculations, comparisons, boolean logic, assignment, and expression readability.

Simple explanation

An expression is a small calculation or question. Operators such as +, -, and == tell the program what work to perform.

In Kotlin, this chapter is about combining values to produce a new value or decision. 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

Most bugs in beginner programs come from one wrong comparison, assignment, or boolean condition. 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 expressions when you calculate totals, compare states, build filters, or decide what should happen next.

Example code

val subtotal = 40
val discount = 5
val isMember = true

val total = subtotal - discount
val canCheckout = total > 0 && isMember

Line-by-line explanation

• val subtotal = 40 — This stores or updates subtotal. The value on the right is worked out first, then saved under that name.
• val discount = 5 — This stores or updates discount. The value on the right is worked out first, then saved under that name.
• val isMember = true — This stores or updates isMember. The value on the right is worked out first, then saved under that name.
• val total = subtotal - discount — This stores or updates total. The value on the right is worked out first, then saved under that name.
• val canCheckout = total > 0 && isMember — This stores or updates canCheckout. The value on the right is worked out first, then saved under that name.

What the output means

No visible output: `total` becomes 35 and `canCheckout` becomes true.

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 subtotal = null // operator order is assumed instead of made clear
val discount = 5
val isMember = true

val total = subtotal - discount
val canCheckout = total > 0 && isMember

This version intentionally shows how operator order is assumed instead of made clear. The changed assignment stores a missing value, or a required line is removed, so later code cannot complete its job safely.

Fixed version

val subtotal = 40
val discount = 5
val isMember = true

val total = subtotal - discount
val canCheckout = total > 0 && isMember

The corrected version restores the real value or required operation. It fixes the chapter-specific problem: operator order is assumed instead of made clear.

Common mistakes

• Confusing assignment with equality checks.
• Writing long conditions without parentheses.
• Forgetting operator precedence.

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

• Price calculators.
• Game score rules.
• Search and filter conditions.

Practice exercise

1. Add tax to a price.
2. Write a comparison that checks whether a score is at least 50.
3. Use parentheses to make one calculation unambiguous.

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 does == check?
2. Which operation runs first in 2 + 3 * 4?
3. When should parentheses be added?

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 conditions and decisions. Before opening it, explain this chapter out loud in under one minute.