Replace multiple occurrences of a character

Question

I try to replace multiple occurrences of a character with a single character.

Input:

hhiii!!! hooowww aaareee yyyooou???

Output:

hi! how are you?

public class CharSingleOccurrence {
    public static void main(String... args){
        charSingleOccurrence("hhiii!!!  hooowww   aaareee   yyyooou???");
    }
    public static void charSingleOccurrence(String str){
        int i=0;
        int j=0;
        char arr[]=new char[50];
        while (i<str.length()-1) {            
            if(str.charAt(i)!=str.charAt(i+1)){
                arr[j]=str.charAt(i);
                i++;
                j++;
            }else{
                i++;
            }
        }
        arr[j]=str.charAt(str.length()-1);
        for(char c:arr){
            System.out.print(c);
        }
    }
}

Here I need review for

• char arr[]=new char[50];
• arr[j]=str.charAt(str.length()-1);

Answer 1

I think this is oddly complicated...though, I'm 99% sure this can be done with a single line of RegEx, I don't know how from the top of my head¹.

This can be shortened to a much simpler and shorter solution, without the need of arrays at all.

String input = "hhiii!!!  hooowww   aaareee   yyyooou???";
StringBuilder output = new StringBuilder();

// Append the first character of the string.
// We can also let the loop start at 0, but then
// we'd need an additional if inside the loop, which
// I'd like to avoid.
output.append(input.charAt(0));

// Start the loop at 1, because we already have the first character.
// We can not 
for (int idx = 1; idx < input.length(); idx++) {
    // Check the current against the previous character.
    if(input.charAt(idx) != input.charAt(idx-1)) {
        // If it is not the same, append it.
        output.append(input.charAt(idx));
    }
 }

Of course you'd need to protect yourself from invalid input (empty or null String), and it will also break words with allowed double-characters: foobar, tool, support etc..

Some further comments on your code:

• Function name: It's not directly visible what the function does. It should be called something along the lines of removeDoubleChars or removeMultipleCharacters.
• Function type: The function shouldn't directly output the result to System.out but rather return it.
• Loops: Use appropriate loops, f.e. your while should be a for.
• Variable names: I know it's partly taught, but I get the creeps (or become a Creeper) if I see variables which are named i, j or arr. Give your variables meaningful names! In any modern language the name of the variable is neither limited nor does it matter for performance. Write your code primarily for humans, and only secondary for machines...the coder which has to maintain your code after you will thank you.
• Whitespaces: Use 'em! if(str.charAt(i)!=str.charAt(i+1)){ is harder to read then if(str.charAt(i) != str.charAt(i + 1)) {.
• Code duplication: Avoid duplicate code. In this case it's minor, but the i++ should be moved outside the if, so that it's not necessary to have an else part.

¹: But John does.

Answer 2

Maybe I am wrong but I prefers to use the regex for this kind of text manipulation. Here is my code,

System.out.println("hhiii!!!  hooowww   aaareee   yyyooou???"
                      .replaceAll("(.)\\1+","$1"));

Answer 3

A StringBuilder is good for building strings. I suggest that you keep track of the last character that you put in the builder to avoid the repeated characters.

This is C# code, but I think that it should translate to Java easily enough:

public static string CharSingleOccurance(string value) {
  StringBuilder result = new StringBuilder();
  char? last = null;
  foreach (char c in value) {
    if (!last.HasValue || c != last.Value) {
      result.Append(c);
      last = c;
    }
  }
  return result.ToString();
}

Edit:

Here is Bobbys suggestion for a Java translation:

public static String charSingleOccurance(String value) {
     StringBuilder result = new StringBuilder();
     char last = '\u0000';

    for (char c : value.toCharArray()) {
         if (last == '\u0000' || c != last) {
             result.append(c);
             last = c;
         }
     } 

    return result.toString();
}