Types

Int represents whole numbers (no decimals). Arithmetic uses the usual operators, plus % for the remainder:

val a: Int = 10
val b = 3                // type inferred as Int

a + b      // 13
a - b      // 7
a * b      // 30
a / b      // 3   ← integer division: drops the fractional part
a % b      // 1   ← remainder of a / b

Note a / b truncates toward zero — 10 / 3 is 3, not 3.33. Use Double if you need decimals.

Implement area(width, height): Int returning width * height.

Starter code

fun area(width: Int, height: Int): Int {
    return 0
}

Double represents decimal numbers (64-bit floating point). Same operators as Int but / is real division, not truncating:

val pi: Double = 3.14159
val r = 2.5              // type inferred as Double (because of the decimal)

pi * r * r     // 19.6349375
10.0 / 3.0     // 3.3333333333333335

Mixing Int and Double produces a Double:

1 + 2.0        // 3.0   (Double)

Decimal literals (2.0, 0.5) are Double by default.

Implement average(a, b): Double returning (a + b) / 2.0.

Starter code

fun average(a: Double, b: Double): Double {
    return 0.0
}

String holds text. Some everyday operations:

val name = "Ada"
name.length                   // 3
name.uppercase()              // "ADA"
name.lowercase()              // "ada"
name + "!"                    // "Ada!"   (concatenation)
"Hi, $name!"                  // template (idiomatic)

Indexing returns a Char:

name[0]                       // 'A'   (Char, not String)
name.first()                  // 'A'

Strings are immutable — these methods return new strings, they never modify the original.

Implement shout(s) returning s in uppercase followed by an exclamation mark. Hint: s.uppercase() + "!".

Starter code

fun shout(s: String): String {
    return s
}

Boolean is the simplest type — only two values, true and false.

You rarely write a Boolean literal directly; instead, comparisons produce one:

val older = age >= 18         // Boolean
val same = name == "Ada"      // Boolean
val empty = name.isEmpty()    // Boolean

The full set of comparison operators: ==, !=, <, >, <=, >=. They work on numbers and (for equality) on strings and other types.

The next lesson covers how to combine booleans with &&, ||, and !.

Implement isPositive(n) returning true if n is greater than zero. Use a comparison.

Starter code

fun isPositive(n: Int): Boolean {
    return false
}