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Kotlin Conversions

A conversion changes a value from one type into another type. Kotlin writes a conversion as a short call placed after the value.

A Double becomes an Int

The call toInt() drops the part after the point. The second print shows the stored value itself is unchanged.

fun main() {
    val price = 3.9
    println(price.toInt())
    println(price)
}

An Int becomes a Double

The call toDouble() adds a decimal point to the value. The new print shows 7.0 instead of 7.

fun main() {
    val whole = 7
    println(whole)
    println(whole.toDouble())
}

A String becomes a number

The call toInt() reads the digits in the text as a number. The changed value can then join arithmetic.

fun main() {
    val text = "42"
    println(text.toInt() + 1)
    println(text.toInt() * 2)
}

A number becomes text

The call toString() turns a number into a String. The String note on the variable makes the compiler check the new type. Changing the text back with toInt() and adding 1 gives 1235.

fun main() {
    val count = 1234
    val text: String = count.toString()
    println(text)
    println(text.toInt() + 1)
}

A Double cannot sit in an Int variable

Placing a Double value into a variable marked Int does not compile. The program fails with this error.

fun main() {
    val price = 9.99
    val whole: Int = price
    println(whole)
}

Example

fun main() {
    val scoreText = "85"
    val score = scoreText.toInt()
    println(score + 5)
    println(score.toDouble() / 2)
    val label: String = score.toString()
    println(label)
}

Key points

  • Use toInt() to change a value into an Int.
  • Use toDouble() to change a value into a Double.
  • Use toString() to change a value into a String.
  • toInt() on a Double drops the part after the point.
  • toInt() on a String reads the digits in the text as a number.

In this example, scoreText holds the text 85 and toInt() changes it into the number 85. Adding 5 gives 90. Changing the score to a Double and dividing by 2 gives 42.5. The last print changes 85 back into text; the String note on label makes the compiler check that the value really is text.

Note

Kotlin does not change a value into another type on its own, so the call must be written out.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Kotlin Playground

Exercise

A program sets val x = 6.8 and prints x.toInt(). What prints?