RLE Compression method

Question

Console.WriteLine("Enter the string you would want to compress using RLE format");

string UserInput = Console.ReadLine();
int repitions = 1;

for (int i = 0; i < UserInput.Length; i++)
{
    if (UserInput[i] == UserInput[i + 1])
    {
        repitions++;
    }
    else
    {
        Console.WriteLine("{0}{1}", repitions, UserInput[i]);
        repitions = 1;
    }
}

In line 5, there is an error where it says

index was outside the bounds.

My aim is to use this compression and output the number of times the characters repeats and the type of character then move onto the next. Im not sure what is wrong here.

Im expecting a proper working RLE algorithm.

In the last iteration i will have the highest possible index value (e.g. 9 when the string has a length of 10). With `UserInput[i + 1]` you read beyond the length of the string. — jps, Dec 23 '22 at 16:20
`index was outside the bounds` - right, `for (int i = 0; i < UserInput.Length; i++)` and then `UserInput[i + 1]` will do that. — 500 - Internal Server Error, Dec 23 '22 at 16:20
You would probably do something along the lines of `pendingChar = UserInput[0]; count = 1;` and then `for int i = 1; i < UserInput.Length; i++) { if (UserInput[i] != pendingChar) { Console.WriteLine(...); pendingChar = UserInput[1]; count = 1; } else count++; }`. — 500 - Internal Server Error, Dec 23 '22 at 16:25
`if (i < UserInput.Length - 1 && UserInput[i] == UserInput[i + 1]) {...}` — Dmitry Bychenko, Dec 23 '22 at 16:47

Dmitry Bychenko · Answer 1 · 2022-12-23T17:16:37.163

The immediate reason of the exception is in

if (UserInput[i] == UserInput[i + 1])

comparions: on last character of the UserInput, i == UserInput.Length - 1 and we have index out of range at UserInput[i + 1]. We have to add an extra check for the last character:

Console.WriteLine("Enter the string you would want to compress using RLE format");
string UserInput = Console.ReadLine();

for (int i = 0, count = 1; i < UserInput.Length; ++i) 
  if (i < UserInput.Length - 1 && UserInput[i] == UserInput[i + 1])
    count += 1;
  else {
    // You, probably, want to get one line string (Write instead of WriteLine)
    Console.Write($"{count}{UserInput[i]}");
    
    count = 1;
  }

A more maintable approach is to extract method for RLE:

private static string ToRle(string value) {
  if (string.IsNullOrEmpty(value))
    return value;

  StringBuilder sb = new SringBuilder(value.Length * 2);

  for (int i = 0, count = 1; i < UserInput.Length; ++i) 
    if (i < UserInput.Length - 1 && UserInput[i] == UserInput[i + 1])
      count += 1;
    else {
      sb.Append(count);
      sb.Append(UserInput[i]);
    
      count = 1;
    }
  }

  return sb.ToString();
}

And then use it:

Console.WriteLine("Enter the string you would want to compress using RLE format");
string UserInput = Console.ReadLine();

Console.WriteLine(ToRle(UserInput));

On the last character `i == UserInput.Length - 1` – E. Shcherbo Dec 23 '22 at 17:14 — E. Shcherbo, Dec 23 '22 at 17:14
@E. Shcherbo: You are quite right, I'm sorry for the typo – Dmitry Bychenko Dec 23 '22 at 17:18 — Dmitry Bychenko, Dec 23 '22 at 17:18

RLE Compression method

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