Thread: Decryption problem

Originally Posted by hjen

So I've made a program to decrypt a textfile without user interaction. This is done by counting the letters in the input file and using the fact that the most common letter is "e" (not fool-proof of course, but usually works for large txt files). Everything seems to work except for the decryption itself. It is just a Caesar Cipher, so the code for it should be alright. However, it does not seem to return the necessary string. I have spent a lot of time trying to figure out where the mistake is but, alas, without success. By now, however, I am pretty sure the problem is to be found within the last method (i.e. the one called "caesar"). I would love for someone to have a look at it and tell me where I've gone wrong. The code:

import java.util.*;
import java.io.*;
 
 
public class Caesardec{	// beginning of class
	public static void main (String [] args) { // beginning of main
	File infile;
	PrintStream output = null;
 
	if (args.length > 1){ // beginning of if
		try{ // beginning of try
		output = new PrintStream(new File(args [1]));
		} // end of try
		catch(Exception e) { // beginning of catch
System.out.println("nah");
		} // end of catch
 	} // end of if
 
	infile = new File( args[0] );
	String input = "";
	try { // beginning of try
	 BufferedReader br = new BufferedReader(new FileReader(infile));
         StringBuilder sb = new StringBuilder();
         String line = br.readLine();
     	   while ( line != null){ // beginning of while
        	  sb.append(line + "\n");
       		  line = br.readLine(); 
          } // end of while
	 br.close();
      	input = sb.toString();
    	} // end of try
	catch (Exception e) { // beginning of catch
      e.printStackTrace();
      System.err.println(infile);
      System.err.println(e);
	} // end of catch
 
	int key = deknogle(input);
 
		if( args.length == 1 ){ // beginning of if
			System.out.println( caesar( input, key ));
		} // end of if
		else { // beginning of else
		output.println( caesar( input, key));
		output.close();
		System.out.println("Writing to output-file.");
		}	// end of else		
 
 
	} // end of method
 
	public static int deknogle(String input){ // beginning of method
			String lower = input.toLowerCase();
		HashMap lettercount = new HashMap();
		for (int i = 0; i < lower.length(); i++) { // beginning of for
			if(Character.isLetter(lower.charAt(i))){ // beginning of if
            	 		 char c = lower.charAt(i);   
				if(lettercount.get(c)!=null){ // beginning of if
				lettercount.put(c, (Integer)(lettercount.get(c))+1);
				} // end of if
				else{ // beginning of else
				lettercount.put(c, 1);
				} // end of else
			} // end of if
 
 		} // end of for
		HashMap map = new LinkedHashMap();
		List nogler = new ArrayList(lettercount.keySet());
		List vardier = new ArrayList(lettercount.values());
		TreeSet sorteret = new TreeSet(vardier);
		Object[] sort = sorteret.toArray();
		int size = sort.length;
 
		for (int i=0; i<size; i++) { // beginning of for
		   map.put
		      (nogler.get(vardier.indexOf(sort[i])),
		       sort[i]);
		} // end of for
		Character[] keys = new Character[map.size()]; 
				map.keySet().toArray(keys);
		int nogl = keys[25]-'a';
		int tildek = nogl - 4;      //4 corresponding to the letter "e".
		int enkey = 26-tildek;
		tildek = enkey;	
		return tildek;
	}	// end of method
 
		public static String caesar(String input, int key) { // beginning of method
		String decrypt = "";
		for (int i = 0; i < input.length(); i++) { // beginning of for
			char d = 0;
			int c = input.charAt(i);
			if (97 <= c && c <= 122){ // beginning of if
				d = (char) ((c+key-97)%26+97);
 
			}  // end of if
else{ // beginning of else
			if (65 <= c && c <= 90){
			d = (char) ((c+key-65)%26+65);
			} // end of if
			else { // beginning of else
			d = (char) c;
 
			} // end of else} // end of else
			decrypt = decrypt + d;
		} // end of for
		return decrypt;
 
	} // end of method
 
} // end of class

As I understand it, the method caesar does not return the decrypt string i create, but simply the empty string as first defined. If this is indeed the problem, do any of you guys have an idea as to how I can avoid it? Any help would be greatly appreciated.

Well, it looks like you're adding a char to a String. Try type casting it to a String or something.

It's because you're adding a char to a String that's causing the problem I think.

I assume you mean by defining a new string as Character.toString(d). That doesn't work either though. If you mean something different, then please elaborate. I have roughly one week of programming experience, so I apologize if I seem a little dim on the subject.

d = (char) ((c+key-97)%26+97);


Will add c to key and then subtract 97.

Will then divide value in parenthesis by 26 and add 97 to the remainder of that division and then make the whole thing into a char.

If I add the line "System.out.println(decrypt);" right after the line "decrypt = decrypt + d" it actually prints out the right thing, but it does it over and over again in sort of an endless loop. Either way it's correct, except for the fact that it wont return the value. It seems like the return statement is unaware of what's going on inside the 'for' statement. And of course I can't move the return statement up, because then it wont compile because of "missing return statement".