Having difficulty with logic in basic Java RLE program

Question

Learning Java at the moment, having difficulty with this fairly basic RLE program. I think the error is to do with when the character count is being reset? It seems to be outputting the previous values that should have been moved on from.

Please help! I feel so dumb that I can't figure out how to fix it haha

e.g. for formatting: AABCEDDDGHIIIII would be converted to A2B1C1E1D3G1H1I5

import java.util.Scanner;

class Main {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    
    System.out.println("Enter string for encoding: ");
    String uString = input.next();

    int count = 1;
    String eString = "";

    for (int i = 0; i < uString.length(); i++){
      if ((i != 0) && (uString.charAt(i) == uString.charAt(i-1))){
        count++;
      }else if(i!= 0){
        eString = eString + uString.charAt(i) + String.valueOf(count);
        count = 1;
      }
      System.out.println(String.valueOf(uString.charAt(i)) + String.valueOf(i) + String.valueOf(count)); //just to see what's happening in my code
    }System.out.println("Unencoded string's length: " + uString.length() + "\nEncoded string:" + eString + ", encoded string's length: " + eString.length());
  }
}```

Answer 1

The main error in the logic is that this code is missing current character for each count is incremented.

So, in the line eString = eString + uString.charAt(i) + String.valueOf(count); the next character is added to the result string.

The logic may be fixed by adding the first character before the loop, starting the loop from index 1, and adding the counter after the loop.

Conditions if (i != 0) may be removed altogether then:

String eString = "";

if (!uString.isEmpty()) {// there's something to count

    int count = 1;
    String eString = "" + uString.charAt(0);

    for (int i = 1, n = uString.length(); i < n; i++) {
        if (uString.charAt(i) == uString.charAt(i-1)) {
            count++;
        } else {
            // append count for previous character and then the current char
            eString = eString + count + uString.charAt(i);
            count = 1;
        }
        System.out.println(String.valueOf(uString.charAt(i)) + i + count); //just to see what's happening in my code
    }
    eString = eString + String.valueOf(count);
}
System.out.println("Unencoded string's length: " + uString.length() + "\nEncoded string:" + eString + ", encoded string's length: " + eString.length());

---

Another approach could be to use an empty nested loop (and a StringBuilder to concatenate the characters and counter more efficiently):

StringBuilder sb = new StringBuilder();
for (int i = 0, n = uString.length(); i < n; i++) {
    int count = 1;
    char c = uString.charAt(i);
    // loop without body just to increment counter
    for (int j = i + 1; j < n && uString.charAt(j) == c; j++, i++, count++);

    sb.append(c).append(count);
}
String eString = sb.toString();