Java run length decoding (expanding a compressed string)

Question

public static String decompressString (String text) {
    int count = 0;
    StringBuilder result = new StringBuilder () ;
    for (int i = 0; i < text.length(); i++) {
        char c = text.charAt(i);
        if (Character.isDigit(c)) {
            count = count * 10 + c - '0';
        } else { 
            while (count >0){ 
                result.append(c);
                count--;
            }
        }

    }
    return result.toString();
}

The program is supposed to take in a run length encoded string from the main method such as 5A5Bcd and return the string in a run length decoded format. 5A5Bcd -> AAAAABBBBBcd. The problem i am running into is that the code seems to ignore characters that are not preceeded by a digit. In the example above i am returning AAAAABBBBB instead of AAAAABBBBBcd; the 'c' and 'd' are not preceeded by a digit and therefore arent recognized. Any ideas, I have been stuck at this point for quite some time now.

Ian W. · Accepted Answer · 2018-03-04 23:09:03Z

2

Your count variable is not going to be nonzero when you encounter the "c" and "d" characters in your example, because it will have been decremented to zero after processing the "5B".

The simplest fix I see in your code is to add a check before the while loop:

if (Character.isDigit(c)) {
    // ...
} else {
    if (count == 0) {
        // Single-run-length characters have an implicit "1" prepended
        count = 1;
    }
    while (count > 0) {
        // ..
    }
}

answered Mar 4, 2018 at 23:09

Ian W.

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Comments

T. Schalke · Accepted Answer · 2018-03-04 23:11:03Z

0

Whenever you start working on a new character, count is at 0, therefore nothing will be appended. You want count to be 1 at the beginning of the loop and set it to 1 after the while(count > 0) loop. Can you explain why you do count = count * 10 + c - '0'; instead of count = c (That will have to be changed as well)?

answered Mar 4, 2018 at 23:11

T. Schalke

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1 Comment

cise Over a year ago

The count = count * 10 + c - '0' accounts for digits that are more than one digit such as 10, 100 , 1000 etc. Ian and you have both helped me and the code is working as wanted now. Thank you both.

Satinath Mondal · Accepted Answer · 2021-02-27 05:41:02Z

You can solve this problem as below

private static String decode(String encodedString) {
        String decodedString = null;
        //aaabbbcccccdd
        //3a3b5c2d
        
        int n = encodedString.length();
        StringBuilder sb= new StringBuilder();
        for (int i = 0; i < n; i++) {
            if(i+1 <n && i%2 ==0)
            sb.append(repeat(Integer.parseInt(String.valueOf(encodedString.charAt(i))),encodedString.charAt(i+1)));
        }
        
        return sb.toString();
                
    }

    private static String repeat(int length, char charAt) {
        StringBuilder sb = new StringBuilder();
        for (int j = 0; j < length; j++) {
            sb.append(charAt);
        }
        return sb.toString();
    }

Suraj Rao · Accepted Answer · 2022-03-31 06:33:37Z

0

public class RunLengthDecode {
    public static void main(String[] args) {
        
        String string="9a8b8c5d";
        String resultString="";
        

        for (int i = 0; i < string.length(); i+=2) {
            int count=Character.getNumericValue(string.charAt(i));
            
            for(int j=0;j<count;j++) {
            resultString+=string.charAt(i+1);
            }
        }
        System.out.println(resultString);
        
    }
}

edited Mar 31, 2022 at 6:33

Suraj Rao

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answered Mar 31, 2022 at 6:19

Praveen Joma

1

2 Comments

Suraj Rao Over a year ago

Welcome to Stack Overflow! While this code may solve the question, including an explanation of how and why this solves the problem would really help to improve the quality of your post. Remember that you are answering the question for readers in the future, not just the person asking now. Please edit your answer to add explanations and give an indication of what limitations and assumptions apply.

Community Over a year ago

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