How to print duplicate characters in a String using C#?

Finding duplicate characters in a string is a common programming task in C#. A duplicate character is one that appears more than once in the string. We can solve this by counting the frequency of each character and then displaying those with a count greater than 1.

Using Character Frequency Array

The most efficient approach uses an integer array to store the frequency of each character. Since there are 256 possible ASCII characters, we create an array of size 256 −

using System;

class Demo {
   static int maxCHARS = 256;
   
   static void calculate(String s, int[] cal) {
      for (int i = 0; i < s.Length; i++)
         cal[s[i]]++;
   }

   public static void Main() {
      String s = "Welcometomywebsite!";
      Console.WriteLine("Input String: " + s);
      Console.WriteLine("Duplicate characters:");
      
      int[] cal = new int[maxCHARS];
      calculate(s, cal);

      for (int i = 0; i < maxCHARS; i++)
         if (cal[i] > 1) {
            Console.WriteLine("Character '" + (char)i + "' occurs " + cal[i] + " times");
         }
   }
}

The output of the above code is −

Input String: Welcometomywebsite!
Duplicate characters:
Character 'e' occurs 6 times
Character 'm' occurs 2 times
Character 'o' occurs 2 times
Character 't' occurs 4 times
Character 'w' occurs 2 times

How It Works

The algorithm works in three steps:

• Initialize: Create an integer array of size 256 to store character frequencies

• Count: Loop through each character in the string and increment its frequency in the array

• Display: Print characters whose frequency is greater than 1

Using Dictionary for Better Readability

An alternative approach uses a Dictionary which is more readable and memory-efficient for sparse data −

using System;
using System.Collections.Generic;

class Program {
   public static void Main() {
      string input = "programming";
      Console.WriteLine("Input String: " + input);
      Console.WriteLine("Duplicate characters:");
      
      Dictionary<char, int> charCount = new Dictionary<char, int>();
      
      // Count frequency of each character
      foreach (char c in input) {
         if (charCount.ContainsKey(c))
            charCount[c]++;
         else
            charCount[c] = 1;
      }
      
      // Display duplicates
      foreach (var pair in charCount) {
         if (pair.Value > 1) {
            Console.WriteLine("Character '" + pair.Key + "' occurs " + pair.Value + " times");
         }
      }
   }
}

The output of the above code is −

Input String: programming
Duplicate characters:
Character 'r' occurs 2 times
Character 'g' occurs 2 times
Character 'm' occurs 2 times

Case-Insensitive Duplicate Detection

To find duplicates regardless of case, convert the string to lowercase first −

using System;
using System.Collections.Generic;

class Program {
   public static void Main() {
      string input = "Hello World";
      Console.WriteLine("Input String: " + input);
      Console.WriteLine("Duplicate characters (case-insensitive):");
      
      Dictionary<char, int> charCount = new Dictionary<char, int>();
      string lowerInput = input.ToLower();
      
      foreach (char c in lowerInput) {
         if (c != ' ') { // Skip spaces
            charCount[c] = charCount.ContainsKey(c) ? charCount[c] + 1 : 1;
         }
      }
      
      foreach (var pair in charCount) {
         if (pair.Value > 1) {
            Console.WriteLine("Character '" + pair.Key + "' occurs " + pair.Value + " times");
         }
      }
   }
}

The output of the above code is −

Input String: Hello World
Duplicate characters (case-insensitive):
Character 'l' occurs 3 times
Character 'o' occurs 2 times

Conclusion

Finding duplicate characters in a string can be efficiently done using character frequency counting. The array-based approach is fastest for ASCII characters, while Dictionary provides better readability and handles Unicode characters well.