Java字符串压缩输出错误的字符串。 我在这里想念什么？

Question

I have a string "aaabbaaaavvvd", it should be compressed to a3b2a4v3d But when I run my code it prints out b3a2v4d3

Strangely, it starts with b instead of a

public class compression {
    public String compress(String str){
        char chararr[] = str.toCharArray();
        StringBuilder sb = new StringBuilder();
        int count=1;
        char previous = chararr[0];
        for(int i=1; i<chararr.length; i++) {
             char current = chararr[i]; 
             if(current == previous){
                 count++;
             } else {
                 sb.append(current).append(count);
                 count = 1;
             }
             previous = current;
        }
        System.out.println(sb.toString());
        return sb.toString();
    }

    public static void main(String args[]){
        compression test = new compression();
        String str = "aaabbaaaavvvd";
        test.compress(str);
    }
}

Answer 1

Error on line:

sb.append(current).append(count);

Think about the if statement

if(current == previous){
    count++;
} else {
    sb.append(current).append(count);
    count = 1;
}

As your code steps through each character, the if statement is only false if the current character is not equal to the last (ie the current character is the start of a new sequence of characters).

Stepping through one-by-one:

Index 1: previous == 'a', current == 'a', if-statement: true - increments
Index 2: previous == 'a', current == 'a', if-statement: true - increments
Index 3: previous == 'a', current == 'b', if-statement: false - prints and reset

Note how upon the third index, current is b rather than the required character a , outputting b3 as opposed to a3 .

Replace the append chain with previous to fix:

if(current == previous){
    count++;
} else {
    sb.append(previous).append(count);
    count = 1;
}

Answer 2

You have 3 issues:

1. When current == previous you should add 1 to counter and then continue
2. When you exit the loop - print the last char that you process
3. You should append the previous char - not the current sb.append(previous)

your code should look like this:

        public String compress(String str){
        char chararr[] = str.toCharArray();
        StringBuilder sb = new StringBuilder();
        int count=1;
        char previous = chararr[0];
        for(int i=1; i<chararr.length; i++) {
            char current = chararr[i];
            if(current == previous){
                count++;
                continue;
            } else {
                sb.append(previous).append(count);
                count = 1;
            }
            previous = current;
        }

        sb.append(previous).append(count);
        System.out.println(sb.toString());
        return sb.toString();
    }

Answer 3

This code sample works. I have changed sb.append(current) to sb.append(previous) and added a sb.append(previous).append(count) outside the for loop to account for the last character

public class compression {
public String compress(String str){
    char chararr[] = str.toCharArray();
    StringBuilder sb = new StringBuilder();
    int count=1;
    char previous = chararr[0];
    for(int i=1; i<chararr.length; i++) {
         char current = chararr[i]; 

         if(current == previous){
             count++;
         } else {
             sb.append(previous).append(count);
             count = 1;
         }

         previous = current;
    }

    sb.append(previous).append(count);

    System.out.println(sb.toString());
    return sb.toString();
}

public static void main(String args[]){
    compression test = new compression ();
    String str = "aaabbaaaavvvd";
    test.compress(str);
}

}

Answer 4

Try this:

public class Compression {
    public String compress(String str) {
        char chararr[] = str.toCharArray();
        StringBuilder sb = new StringBuilder();
        int count = 0;
        char previous = chararr[0];
        for (int i = 0; i < chararr.length; i++) {
            char current = chararr[i];
            if (current == previous) {
                count++;
            } else {
                sb.append(previous);
                if (count != 1) {
                    sb.append(count);
                }
                count = 1;
            }
            previous = current;
        }

        sb.append(previous);
        if (count != 1) {
            sb.append(count);
        }
        return sb.toString();
    }

    public static void main(String args[]) {
        Compression test = new Compression();
        System.out.println(test.compress("aaabbaaaavvvd"));
        System.out.println(test.compress("aaabbaaaavvvdddd"));
    }
}

Output:

a3b2a4v3d
a3b2a4v3d4

Answer 5

Try something like below:

public String compress(String str){

    String result="";
    StringBuilder sb = new StringBuilder(str);


    while(sb.length() != 0){
        int count = 0;
        char test = sb.charAt(0);
        while(sb.indexOf(test+"") != -1){
            sb.deleteCharAt(sb.indexOf(test+""));
            count++;
        }
        result=result+test+count;
    }
    System.out.println(result); 

    return sb.toString();
   }