Java: Generate the alphabet in random order, no duplicates

[-Noah-]

Hi guys,

I am taking an introductory programming course and need a bit of help. The assignment is to have the Java program read a message (I am assuming from a .dat file, not quite sure). The program creates substitutes each random letter with a different letter chosen at random by the computer. If the letter M is chosen to replace the letter P in the message, then M replaces P in every instance of the message.

Right now, I just need help with generating a set of the 26 letters ordered randomly without duplicates. Here is what I have:

import java.util.Random;

public class encrypt {

public static void main(String[] args) {

char [] alphabet =

{'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};

char [] substitute =

{'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};

boolean done=false;

Random generator = new Random();

int i, j, k, random;

while(done==false) {

for(i=0; i<=25; i++) {

random = generator.nextInt(25);

substitute=alphabet[random];

for(j=0; j<=i; j++) {

while(substitute[j]==substitute) {

random = generator.nextInt(25);

if(substitute[j]!=substitute) {

substitute=alphabet[random];

}

}

}

}

for(k=0; k<=25; k++) {

System.out.print(substitute[k] + ", ");

}

done=true;

}

}

}

Right now the program does nothing. If I remove the last for loop with j as the counter and everything within that for loop, I get an output of 26 letters randomly chosen, but there are duplicates. The last for loop is supposed to say something like, "If the randomly generated letter (substitute) has appeared before in substitute[], then generate a new random number until it is no longer a duplicate of a previous number, at which point the letter which corresponds to the random number is stored in substitute.

So if you could give me some suggestions it would be much appreciated. If I am overcomplicating things and there is a much easier way to do this, please guide me in the right direction.

Thanks,

Noah

EDIT: Spacing in the program code isn't working. Looks fine while I'm editing it though. What's wrong?

EDIT 2: See here for my code, correctly formatted.

Edited May 26, 2009 by -Noah-

[chrismaddern]

These solutions are all overkill...

- Make an ArrayList containing all 26 letters.

- generate random number from 0 to List.size()-1.

- List.get(randomNumber)

- List.remove(randomNumber)

loop and restart at step 2 until List.size() returns 0.

import java.util.*;

class alphaRandom {

	public static void main(String[] args) {

	ArrayList<Character> al = new ArrayList<Character>();
	Random gen = new Random();
	int num;

	for(int i = 65; i <= 90; i++) {
		al.add((char)i);
	}

	while(al.size() > 0) {
		num = gen.nextInt(al.size());
		System.out.print(al.get(num));
		al.remove(num);
	}
 }
}

There you go.

Was about to post but saw that... that's the right idea!

Chris

[sialivi]

thanks for repeating what someone else said captain obvious

The only thing "obvious" about it was the fact that I had missed that post, but anyway, terribly sorry for trying to help.

[-Noah-]

Tech God (and bmaher): That's awesome! Only thing I don't understand is this line:

num = gen.nextInt(al.size());

As the size gets smaller, and eventually reaches 1, the equivalent would be:

num = gen.nextInt(1);

Wouldn't this be telling the program to generate a random integer from 0 to 0? Other than that, sweet!

[-Noah-]

The Teej: Thanks for the reply, but unfortunately, the same problem occurs. :huh: I decided to modify it a bit. I figured there was no reason for a do while over a while, and that I could eliminate the for loop, and put a counter in the new while loop. Here is the result:

import java.util.Random;

public class encrypt {

  public static void main(String[] args) {
	char [] alphabet =  {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};
	char [] substitute = {'.','.','.','.','.','.','.','.','.','.','.','.','.','.','.','.','.','.','.','.','.','.','.','.','.','.'};
	int [] numbers = {-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1};
	Random generator = new Random();
	int random, j=0;
	boolean done=false;
	random=generator.nextInt(26);

	while (done==false) { //While the program has not generated all of the letters of the alphabet in random order without duplicates
		if (numbers[random] == -1) { //If the random number has not yet been used
			numbers[random] = random; //then assign that position in the array that number (i.e. position 10 in the array will now have the number 10), so it's considered to be used
			substitute[j] = alphabet[random];  //assign the current position of the substitute alphabet array the random letter from the real alphabet
			j++; //Counter goes up by one because a letter has been successfully chosen
			random=generator.nextInt(26); //Generate a new random number for the next iteration of the while loop
			if (j==26) {
				done=true; //If the counter has reached 26, stop the while loop, as all 26 letters of the alphabet have been generated
			}
		} else { //If the random number has been used already
			random = generator.nextInt(26); //then generate a new random number for the next iteration of the while loop
		}
	}

	for(int i=0; i<=25; i++) {
		System.out.print(substitute[i] + ", "); //Prints out the randomized alphabet
	}
  }
}

It works, too! Thanks for leading me in the right direction. Wonder why your code doesn't work, though. :rofl:

@ All:

Thanks for all the helpful posts. I will make sure to show my teacher some of the posts on the defining of the integer within the for-loop. :D

EDIT: Figured out how to use code tags! Thought you guys used quote tags 'cause the boxes look similar. :blush:

Edited May 27, 2009 by -Noah-

[Tech God]

Tech God (and bmaher): That's awesome! Only thing I don't understand is this line:

num = gen.nextInt(al.size());

As the size gets smaller, and eventually reaches 1, the equivalent would be:

num = gen.nextInt(1);

Wouldn't this be telling the program to generate a random integer from 0 to 0? Other than that, sweet!

yep, so you'd have no choice except to get zero. The last element left.

[-Noah-]

yep, so you'd have no choice except to get zero. The last element left.

Got it now. So it chooses a number at random, removes it from the array, and then repeats the process until there are no numbers left. Awesome. I'm gonna have to remember ArrayLists. :D

[+M2Ys4U Subscriber¹]

Got it now. So it chooses a number at random, removes it from the array, and then repeats the process until there are no numbers left. Awesome. I'm gonna have to remember ArrayLists. :D

Familiarise yourself with the Java Collections Framework, it has all kinda of handy classes/interfaces like ArrayList :)