java ceaser cipher stanford lectures

Hi,

i am working through some java stanford tutorials on youtube and i copied out some code from ep 13 from the lecture but cant get it to work.

My code doesnt work correctly. It just prints out the system.print.ln text.
I also included the line ceaser s = new ceaser(); and then wrote s.encrypt to call the method but i dont understand why i needed to do this. They didnt do this in the code online but the code only runs when i include these changes.
Thanks for your help.

Code :

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also included the line ceaser s = new ceaser(); and then wrote s.encrypt to call the method but i dont understand why i needed to do this. They didnt do this in the code online but the code only runs when i include these changes.
 
 
Code:
import java.util.Scanner;
public class ceaser {
	public static void main(String args[]){	
 
            Scanner input = new Scanner(System.in);
	System.out.println("Enter a ceaser cipher: ");
	int key = input.nextInt();
	System.out.println("Enter word to encode: ");
	String word = input.nextLine();
	ceaser s = new ceaser();
	String ciphertext = s.encrypt(word, key);
	System.out.println("Encoded text: " + ciphertext);
	String newword = s.encrypt(ciphertext, -key);
	System.out.println("new word: " + newword);
 
	}
 
	private String encrypt(String str, int key){
		if(key<0){
			key = 26 - (-key%26);
		}
 
		String result = "";
		for(int i=0; i<str.length(); i++){
			char ch = str.charAt(i);
			result +=encryptChar(ch, key);
		}
		return result;
	}
 
	private char encryptChar(char ch, int key){
		if(Character.isUpperCase(ch)){
		return( (char)('A' + ((ch - 'A' + key) % 26)) );
		}
		return ch;
	}
 
}

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Quote:

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My code doesnt work correctly

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Can you explain? Show what the code does print out and also show what you think it should output.

the output is below. I typed 2.
____________________
Enter a ceaser cipher:
2
Enter word to encode:
Encoded text:
new word:
_________________

The program will ask for a number, and then shift the alphabet by this number. I then enter a word, and it will output the word in the code.
Eg. If i enter 2, then i enter the word CAT, it will shift the letter by 2 and output ECV

Why isn't there a word to encode entered? Your posted console contents does not show a word being entered.
If nothing goes in, then I would think that nothing would come out.
Try it and enter a word and show what happens.

BTW Caesar was the name of a great Roman general.

It doesnt let me enter a word.
It just lets me enter a digit, then it outputs the last two lines.
Im not really sure why this doesnt work as i copied it exactly from the youtube video
I noticed i also spelt Caesars name wrong. :s

Your problem is with the way the Scanner class works. The nextInt() method reads the next word as a number from the input and leaves the endline from Enter in Scanner's buffer. The nextLine() call returns that endline as an empty String.
Try this for now: type both the number and the word to encrypt on the same line before pressing Enter:
2 CAT
That will give nextLine() the CAT from the buffer.

The fix to your program is to add a call to nextLine() to read the empty line BEFORE you ask for the word.

You are right when i enter the number and word on the same line it will output the encoded word
So what should i add to change this?
Thanks for your help by the way.

The fix to your program is to add a call to nextLine() to read the empty line BEFORE you ask for the word.

Thanks for that it worked.
Also why did i need to write ceaser s = new ceaser(); and write s.encrypt?
I know this is a stupid question but i dont really get it. When i write static in the methods instead it works fine.

What did you want the Caesar class to do for you?
Did it have a method that would do that?

Quote:

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When i write static in the methods instead it works fine.

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Yes, that can work for this specific very small example.
When your programs get larger and when there can be more than one instance of a class, having static variables will be a source of problems. An instance of a class usually needs to have its own variable values. A static variable is shared by all instances and can only have one value for all instances vs a non static variable allowing each instance of the class to have its own value.

That makes sense, thanks for explaining that.

Also when i comment out these two lines from the main method the program works fine. Can you see why these lines of code are included in the program?
String newword = s.encrypt(ciphertext, -key);
System.out.println("new word: " + newword);

Thanks

Quote:

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Can you see why these lines of code are included in the program?

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Look at the output.
Do you see any relationship between the output of these two lines of code and the input to the program?

I thought these two lines of code were needed when a user inputs a negative number but when i comment out these two lines and input a negative value it works fine.

With these two lines included it just repeats the word that was input into the program. I cant see why these two lines are needed.
Here is some example output:
_______________
Enter a ceaser cipher: 2
Enter word to encode: CAT
Encoded text: ECV
new word: CAT
_______________
Enter a ceaser cipher: -2
Enter word to encode: AAA
Encoded text: YYY
new word: AAA
_______________

Do you see how the algorithm works? The original input is encrypted and then decrypted back to the original input.

thanks for your help

Hi, i had another question.

from the code extract below why cant i keep the parameters in the encrypt method as
private String encrypt(String word, int key)

Why do i have to use a new variable called str? I have kept the key variable the same why not the variable word?

Code :

---

 
	String ciphertext = s.encrypt(word, key);
	System.out.println("Encoded text: " + ciphertext);
	String newword = s.encrypt(ciphertext, -key);
	System.out.println("new word: " + newword);
 
	}
 
	private String encrypt(String str, int key){
		if(key<0){
			key = 26 - (-key%26);
		}
 
		String result = "";
		for(int i=0; i<str.length(); i++){
			char ch = str.charAt(i);
			result +=encryptChar(ch, key);
		}
		return result;

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Quote:

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hy cant i keep the parameters in the encrypt method as
private String encrypt(String word, int key)

Why do i have to use a new variable called str? I have kept the key variable the same why not the variable word?

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I'm not sure I understand what you are asking.
You can give just about any name you want to the parameters received by a method:
private String encrypt(String theWordToWorkOn, int theKeyThatWillBeUsed)

It is usually better and less confusing if the variable names local to a method are unique to that method and NOT the same as class variables. If they have the same names, the class variables are "shadowed" and not used. This can be a problem if you forget which variable you are using. You could think you are setting a class variable but are really working with a local one. When the method exits, the class variable is unchanged.
Unique names prevents this problem.