How to solve this Range Error in my code?

Question

I'm trying to create an cipher app on android studio using flutter. Right now I'm working on a simple Atbash Cipher, but I get a range error when trying to test it. These are the encrypt and decrypt codes:

  @override
  String encrypt(String plaintext, {String key}) {
    String alfa = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    String alfaReverso = "";
    for(int i = alfa.length-1; i > -1; i++){
      alfaReverso += alfa[i];
    }

    String encryText = "";
    for (int i = 0; i < plaintext.length; i++){
      if(plaintext.codeUnitAt(i) == 32){
        encryText += " ";
      }
      else{
        int count = 0;
        for(int j = 0; j < alfa.length; j++){
          if(plaintext[i] == alfa[j]){
            encryText += alfaReverso[j];
            break;
          }
        }
      }
    }

    return "ENCRYPT Plain = " + encryText;
  }
}

  @override
  String decrypt(String cyphertext, {String key}) {
    String alfa = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    String alfaReverso = "";
    for(int i = alfa.length-1; i > -1; i++){
      alfaReverso += alfa[i];
    }

    String dencryText = "";
    for (int i = 0; i < cyphertext.length; i++){
      if(cyphertext.codeUnitAt(i) == 32){
        dencryText += " ";
      }
      else{
        int count = 0;
        for(int j = 0; j < alfaReverso.length; j++){
          if(cyphertext[i] == alfaReverso[j]){
            dencryText += alfa[j];
            break;
          }
        }
      }
    }

    return "ENCRYPT Plain = " + dencryText;
  }

When trying to run it this is the range exception I get:

I/flutter ( 6004): RangeError (index): Invalid value: Not in range 0..25, inclusive: 26

I know it has something to do with the alphabetI'm using, but I don't know how to solve it.

Solution

There is an error when you start from the highest index:

 for(int i = alfa.length-1

Your index has to go down and you are using ++.

Use this:

for(int i = alfa.length-1; i > -1; i--)

Comments

[diegoveloper]

Don't worry. I'm glad to help you. Don't forget the accept the answer :)