Count All Occurrences of a char Within a String in Scala

1. Introduction

In this tutorial, we’ll learn how to count all occurrences of a given char in a String in Scala.

For the purpose of the tutorial, we’re going to focus on the methods that are Scala-specific and available in the standard library.

2. Using count()

The most idiomatic way to perform the task is to use the count() method. The count() method accepts a function returning Boolean and automatically counts all elements for which the given function is true:

private def countWithCount(string: String, char: Char): Int = string.count(_ == char)

3. Using  filter()

We can also use filter() combined with size to achieve our goal:

private def countWithFilter(string: String, char: Char): Int = string.filter(_ == char).size

4. Using groupBy()

Another way we can count the occurrences of a char in a String is to compose three different methods groupBy(), mapValues(), and reduce():

def countWithGroupBy(string: String, char: Char): Int =
    string.groupBy(identity).mapValues(_.map(_ => 1).reduce(_+_))(char)

The above pattern was used so often that, in Scala 2.13, a new method – mapGroupReduce() – was introduced to simplify its usage.

5. Using Recursion

Although Scala offers some simple out-of-the-box solutions to solve our problem, it’s also possible to use tail-recursion for that:

 def countRecursive(string: String, char: Char): Int = {
    @tailrec
    def countRec(string: String, char: Char, count: Int): Int = {
      if(string.isEmpty) {
        count
      } else {
        val newCount = if(string.head == char) count + 1  else count
        countRec(string.tail, char, newCount)
      }
    }
    countRecursive(string, char)
  }

6. Summary

In this tutorial, we’ve learned different ways to count all occurrences of a char inside a String in Scala.

Because of the interoperability between Java and Scala, we can also use any of the methods described in our related article for Java.