Swift-function which counts the letters, numbers, spaces, special chars in a given string

Question

Task description: Write a function, which counts the letters, spaces, number and special characters in string.

My implementation:

fun getPartsCount(str: String): Array<Int> {
    val nums = "[0-9]".toRegex().findAll(str)
    val letters = "[a-zA-Z]".toRegex().findAll(str)
    val spaces = " ".toRegex().findAll(str)

    var others = str.count() - nums.count() - letters.count() - spaces.count()

    return arrayOf(nums.count(), letters.count(), spaces.count(), others)
}

fun main() {
    print("Please enter a string > ")
    val userInput = readLine()

    if (!userInput.isNullOrEmpty()) {
        val results = getPartsCount(userInput)
        println("Numbers: ${results[0]}, Letters: ${results[1]}, " +
                "Spaces: ${results[2]}, Others: ${results[3]}")

    } else {
        println("No input provided.")
    }
}

I solved the task using Regular Expressions, but is there a better way in Kotlin for accomplishing the task?

What's your opinion about returning the results in an array? What would you do different and why?

Answer 1

Returning an array

What's your opinion about returning the results in an array?

Why would you do that? Because it's an easy way to return the result? Because you don't know how else to do it?

How are users expected to know which index is 0, 1, 2, and 3 respectively?

It would be much better to return a class.

data class PartsCountResult(
    val numbers: Int,
    val letters: Int,
    val spaces: Int,
    val others: Int,
)

Creating a class in Kotlin is really easy. Do it when you need it. You don't even have to put it in a separate file!

---

Using Regex

I solved the task using Regular Expressions, but is there a better way in Kotlin for accomplishing the task?

As each character is only in one group and are easily distinguishable, you can loop over the string once and count each char.

var letters = 0
var numbers = 0
var spaces = 0
var others = 0
for (c in str) {
    when (c) {
        in 'a'..'z', in 'A'..'Z' -> letters++
        in '0'..'9' -> numbers++
        ' ' -> spaces++
        else -> others++
    }
}

Alternative solution

val letters = str.count { it in 'A'..'Z' || it in 'a'..'z' }
val digits = str.count { it in '0'..'9' }
val spaces = str.count { it == ' ' }
val others = str.length - letters - digits - spaces

Answer 2

Two more ways in addition to Simon's excellent answer:

The Char class has convenient methods like isDigit(), isLetter(), etc.:

val digits = str.count { it.isDigit() }
val letters = str.count { it.isLetter() }
val whitespaces = str.count { it.isWhitespace() }
val others = str.count() - digits - letters - whitespaces

Packed into one loop using list destructuring to assign the result in one go to the four variables:

val (digits, letters, spaces, others) = str
  .fold(mutableListOf(0, 0, 0, 0)) { acc, c ->
    when (true) {
      c.isDigit()      -> acc[0]++
      c.isLetter()     -> acc[1]++
      c.isWhitespace() -> acc[2]++
      else             -> acc[3]++
    }
    acc
  }