Read and validate console input

A console report can ask for a score, but typed text is not yet a valid mark. Input can end, contain non-numeric words or represent a number outside the accepted range. Handle those cases before allowing the value into report calculations.

What you’ll learn

Read, parse and validate console input.
Handle invalid text and end of input.

Read and validate console input in the example

1
Read text
A console report can ask for a score, but typed text is not yet a valid mark.
```
Read text
```
→2
Parse possible integer
ReadlnOrNull returns null at end of input.
```
Parse possible integer
```
→3
Validate score range
With sample input 80 followed by a newline, parsing returns a present Int and range validation succeeds.
```
Validate score range
```

With sample input 80 followed by a newline, parsing returns a present Int and range validation succeeds. The safe calls skip processing if input is absent. Invalid or out-of-range input selects the explanatory alternative.

Understand read and validate console input

ReadlnOrNull returns null at end of input. ToIntOrNull parses a compatible integer string without throwing for invalid text. Presence, parsing and domain validation are separate stages; trimming outer spaces is a deliberate input policy rather than a guarantee about every string.

Connect this to nullable types and safe calls and numbers and explicit conversions.

Follow read and validate console input

Save the complete source as Main.kt and use the Kotlin/JVM environment. Provide sample standard input 80 followed by a newline.

Kotlin/JVM · complete program

fun main() {
  val text = readlnOrNull()
  val score = text?.trim()?.toIntOrNull()
  if (score != null && score in 0..100) println("Score: $score")
  else println("Enter a score from 0 to 100")
}

Expected output

Score: 80

Follow the execution

With sample input 80 followed by a newline, parsing returns a present Int and range validation succeeds. The safe calls skip processing if input is absent. Invalid or out-of-range input selects the explanatory alternative.

Try this in the Kotlin compiler by pasting the complete program.

Practice: Read and validate console input

Your task: What should the program display for input eighty?

Kotlin/JVM · complete program

fun main() {
  val text = readlnOrNull()
  val score = text?.trim()?.toIntOrNull()
  if (score != null && score in 0..100) println("Score: $score")
  else println("Enter a score from 0 to 100")
}

Worked answer

Enter a score from 0 to 100, because numeric parsing returns null and the success condition fails.