Kotlin: count occurrences of `charArray` in `String`

Question

I have two strings

val string1 = "Hello"
val string2 = "Hello world"

I have to count existence of each letter from string1 in string2 in Kotlin

So far, I have written this much code and stuck with regex

val string1_array = string1.toCharArray()
val pattern = Regex("") // Regex pattern here
val matcher = string2

val count = pattern.findAll(matcher).count()

What should be the appropriate Regex pattern to search for charArray? Is there some better way to do in Kotlin

Answer 1

Here are some String extension functions you can use

Occurrences of any char

With the fold extension function:

val string1 = "Hello"
val string2 = "Hello world Hello"

print(
     string2.fold(0) {
         sum: Int, c: Char ->
         if (string1.contains(c))
             sum + 1
         else
             sum
     }   
) 

Or even shorter with sumBy:

string2.sumBy { 
    if (string1.contains(it))
        1
    else
        0
}

And shortest:

string2.count{ string1.contains(it) }

Occurrences of each char individually

With forEach and a MutableMap:

val charsMap = mutableMapOf<Char, Int>()

string2.forEach{
    charsMap[it] = charsMap.getOrDefault(it, 0) + 1
}

print(charsMap)

Occurrences of the whole string1

With the windowed extension function:

string2.windowed(string1.length){
    if (it.equals(string1))
        1
    else
        0
}.sum()

You can browse more String extension functions in the String stblib page

Answer 2

Just use Kotlin's collection functions

val occurrences = string2.filter{ it in string1}
                        .groupingBy { it }
                        .eachCount()

Answer 3

You can use the below higher order method,

val count = string2.count{ string1.contains(it) }
print(count)

Answer 4

Simplified use case when you only want to count occurrences of a single character:

println("needle in a haystack".count{ c -> c == 'a' })
// OUTPUT: 3

Reference:

• https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.text/count.html

Answer 5

You can use chars from the first string as map keys, for example:

val string1 = "Hello"
val string2 = "Hello world"

val chars = HashMap<Character, Int>()

for (char in string1) // fill keys
    chars[char] = 0

for (char in string2) // count occurrences
    chars[char]?.let {
        chars[char] = it + 1
    }

Answer 6

To answer actual the question: the simplest regex pattern would be to use a character class syntax "[" + string1 + "]". Beware, this is not always well behaved. It works well for all ASCii letters and numbers. Special characters like \^-?! must be properly escaped. For the rules see https://www.regular-expressions.info/charclass.html

fun main(args: Array<String>): Unit {
    val string1 = "Hello"
    val string2 = "Hello world Hello"

    val string1_array = string1.toCharArray()
    val pattern = Regex("[" + string1 + "]") // Regex pattern here
    val matcher = string2

    val count = pattern.findAll(matcher).count()
    print(count)
}

How to solve the task: Imho the best way to do this task is to use filter or count instead of regex, because the you don't have to use any special syntax.

fun main(args: Array<String>): Unit {
        val string1 = "Hello"
        val string2 = "Hello world Hello"
        //returns all chars of string2 contained in string1 as list
        val count = string2.filter { string1.contains(it) }.length
        //an other a bit shorter solution with count
        count = string2.count{ string1.contains(it) }
        print(count)
}