Question

In the following code the for loop is never executed. I have attempted to fix the problem with breakpoints and watches. The correct length for the cipher text is returned, however the for loop does not increment until int i >= ciphertext.length(). In fact, nothing seems to execute past the ‘Debug’ message.

private void decrypt_btnActionPerformed(java.awt.event.ActionEvent evt) {                                            


    String alphabet= "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
    String ciphertext="fRcXFBxXTRJ";


    status_label.setText( "Decryption has begun" );

    JOptionPane.showMessageDialog(null,"ciphertext-length: " + ciphertext.length() + "\n" + ciphertext,"Debug", JOptionPane.INFORMATION_MESSAGE);

    for (int i = 0; i>=ciphertext.length(); i--){

        System.out.println("inc:" + i);

        String cipher_current_char = getLetterAtIndex(ciphertext, i);
        int pos_char_in_alphabet = getIndexAtLetter(alphabet, cipher_current_char);


        if(pos_char_in_alphabet-2<0){

            plain_ta.setText(getLetterAtIndex(alphabet, (alphabet.length()+(pos_char_in_alphabet -2)+1 ) ));

            status_label.setText( 100/i + "%");



        }else{

            cipher_current_char = getLetterAtIndex(ciphertext, i);

            pos_char_in_alphabet = getIndexAtLetter(alphabet, cipher_current_char);

            plain_ta.setText(getLetterAtIndex(alphabet, (pos_char_in_alphabet-2)));

            status_label.setText( 100/i + "%");

        }
    }
}                          

Answer 1

for (int i = ciphertext.length()-1; i>=0; i--){

You need to work backwards. Also note the -1, you need it to avoid out of bounds exceptions (indexes start at 0 and go to length -1 ).

Always trace your loop out loud with paper and pencil when you’re stuck, it always helps.