Working with strings

I am making a decoder for a cipher called the Caesar Shift Cipher. I am making a first build of it and it will not be efficent at all so please ignore that. Here is my code so far:

import java.util.Scanner;
public class decoder {

Code:

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        public static void main(String[] args) {
                Scanner number = new Scanner(System.in);
                System.out.println("Please enter your shift number");
                int shift = number.nextInt();
                int counter = 1;
               
                if (shift == 1)
                        while (counter  > 0) {
                                System.out.println("Enter a letter");
                                String letter = number.next();
                                if (letter == "a")
                                        System.out.println("z");
                                else if (letter == "b")
                                        System.out.println("a");

---

I thought this would work but whenever I run it it does not print out the new letter. I have no previous work with stings and I would really appriciate any information. Thanks.

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db

Quote:

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Originally Posted by gman1199

I am making a decoder for a cipher called the Caesar Shift Cipher. I am making a first build of it and it will not be efficent at all so please ignore that. Here is my code so far:

Code:

---

      if (shift == 1)
                        while (counter  > 0) {
                                System.out.println("Enter a letter");
                                String letter = number.next();
                                if (letter == "a")
                                        System.out.println("z");
                                else if (letter == "b")
                                        System.out.println("a");

---

I thought this would work but whenever I run it it does not print out the new letter. I have no previous work with stings and I would really appriciate any information.

---

1. You don't have a closing bracket for your class.

2. You don't have a closing bracket for your while-loop.

3. A letter will only print out if the user enters a or b.

4. A letter will only print out if the user enters 1 for the shift.

5. Inside your while-loop counter is never decremented, so the while-loop (if it had a closing bracket) would run forever.

6. You compare strings using the == operator when you should use the equals() method instead.

7. No precautions against invalid user input are taken.

8. You don't have a closing bracket for your main method.

Also, I don't understand the connection between the shift number and the actual cipher.


Since you haven't worked with Strings before, I recommend that you read this: Strings (The Java™ Tutorials > Learning the Java Language > Numbers and Strings)

awinston -

Thank you. The reason for a lot of the errors you pointed out is the program is not finished and I only posted a small section because it is, as I say in the post, very lengthy. The shift number is basically the amount of letters that are moved from the back of the alphabet to the front. For example, a cipher where the shift number is 4 would look like this: a=w, b=x, c=y, d=z, e=a, ... z=v. The cipher was used by Caesar to communicate with his generals. It's very easily solved because there are only 25 possible combinations.

That's interesting!

Why are you choosing to go about this in an inefficient manner (as you pointed out)?

Sorry for not getting back sooner, out of town for a few weeks. The answer to your question is I have no idea how else to do it. Also, I was working on my code today, trying to make it analise a whole string, and I came up with this (using that web page you gave me) :

Code:

---


import java.util.Scanner;
public class decoder {

        public static void main(String[] args) {
                Scanner number = new Scanner(System.in);
                System.out.println("Please enter your shift number");
                int shift = number.nextInt();
                int counter = 0;
                int i = 0;
                System.out.println("Enter a string");
                String cipher = number.next();
                int len = cipher.length();
                if (shift == 1)
                        while (counter < len) {
                                Character letter = cipher.charAt(i);
                                if (letter.equals("a"))
                                        System.out.println("z");
                                else if (letter.equals("b"))
                                        System.out.println("a");
                               
                                counter++;
                                i++;       
                        }
        }
}

---

I thought this would work, but when I ran it, nothing was printed out. I originally got this error -

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 3
at java.lang.String.charAt(Unknown Source)
at decoder.main(decoder.java:15)

- but this was fixed when I changed to while loop to while (counter < len). Any thoughts you have on this would be appriciated, I have no idea what is wrong.

Thanks,
gman1199

Can you post the code that was giving you the error?

Also, it looks like you can use either counter or i without using both of them. They are always the same value, so you only need one.

Code:

---

import java.util.Scanner;
public class decoder {
 
    public static void main(String[] args) {
        Scanner number = new Scanner(System.in);
        System.out.println("Please enter your shift number");
        int shift = number.nextInt();
        int counter = 0;
        int i = 0;
        System.out.println("Enter a string");
        String cipher = number.next();
        int len = cipher.length();
        if (shift == 1)
            while (counter < len) {
                Character letter = cipher.charAt(i);
                if (letter.equals("a"))
                    System.out.println("z");
                else if (letter.equals("b"))
                    System.out.println("a");
               
                counter++;
                i++;   
            }
    }
}

---

This is what the error was coming from

I can compile and run that code with no errors. Didn't you say that you were receiving the error before you changed the condition of the while-loop to (counter < len)? What was the condition before?

Also, you should be aware that using the equals() method to compare a Character with a String will return false.

Sorry, I forgot to change it from while (counter < len) to while (counter <= len). It was (counter <= len) that was causing the error. Also, is there a way to fix that equals() thing?

Quote:

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is there a way to fix that equals() thing?

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Yeah, just don't use a character.

In java, a literal 'a' is the character a where as "a" is a string. As long as you are not using a literal character, you're fine. The alternative is to use only characters in which case the == will work. You're current solution seems fine though!

So if I replace "a" with a (or is it 'a'?) it should work?

Thank you so much! I replaced everything with the '' and it works!

Quote:

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Originally Posted by gman1199

Sorry, I forgot to change it from while (counter < len) to while (counter <= len). It was (counter <= len) that was causing the error.

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Yes, (counter <= len) will allow counter to equal length. Since the last character of any String has an index of length-1, this will cause the StringIndexOutOfBoundsException to be thrown.

Quote:

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Originally Posted by gman1199

it works!

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Congrats, but here's a tip on making your code more efficient.

Code:

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System.out.println((char) 'b' - 1);

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The only issue is how to wrap 'a' back around to 'z'.

Quote:

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So if I replace "a" with a (or is it 'a'?) it should work?

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It's the ' vs the " that determines wether a literal is a char or a String in java.
So:

Code:

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char someCharacter = 'a';
String someString = "a";

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