KotlinHow-ToBeginner · 3 min read

How to Count Occurrences in String Kotlin - Simple Guide

In Kotlin, you can count occurrences of a character or substring in a string using the count function with a condition or by using windowed for substrings. For example, str.count { it == 'a' } counts how many times 'a' appears in str.

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Syntax

The main way to count occurrences of a character in a string is using the count function with a condition inside curly braces. For substrings, you can use windowed to check each part of the string.

string.count { it == 'char' } counts a single character.
string.windowed(substring.length).count { it == substring } counts a substring.

kotlin

val str = "banana"
val countChar = str.count { it == 'a' }
val countSub = str.windowed(2).count { it == "an" }

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Example

This example shows how to count the number of times the character 'a' appears and how many times the substring "an" appears in the string "banana".

kotlin

fun main() {
    val str = "banana"
    val countChar = str.count { it == 'a' }
    val countSub = str.windowed(2).count { it == "an" }
    println("Number of 'a': $countChar")
    println("Number of 'an': $countSub")
}

Output

Number of 'a': 3 Number of 'an': 2

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Common Pitfalls

A common mistake is trying to count substrings using count directly on the string, which only works for characters. Also, overlapping substrings are not counted by default with windowed. For example, counting "ana" in "banana" requires special handling.

kotlin

fun main() {
    val str = "banana"
    // Wrong: counts characters, not substrings
    // val wrongCount = str.count { it == 'an' } // Error: 'it' is Char, not String

    // Right: use windowed for substrings
    val correctCount = str.windowed(3).count { it == "ana" }
    println("Count of 'ana': $correctCount")
}

Output

Count of 'ana': 1

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Quick Reference

Task	Kotlin Code Example
Count character 'a'	str.count { it == 'a' }
Count substring "an"	str.windowed(2).count { it == "an" }
Count substring with length 3 "ana"	str.windowed(3).count { it == "ana" }

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Key Takeaways

Use count with a condition to count characters in a string.

Use windowed with count to count substrings.

Counting overlapping substrings requires careful use of windowed with the correct size.

Do not try to count substrings directly with count as it works on characters.

Always test your counting logic with examples to avoid off-by-one errors.