Implementing the random generator
Hello, i have posrted this question in the forum yesterday, and didnt get a reply, so i am reposting, hope i havent violated any of the forum rules? If i have can a mod tell me, delete it and ill try and find my old post
The problem i am having is trying to implement thwe random generator in to my code, it hasnt been done correctly as its not printing out a list of matches based on the conditons, can someone tell me what is wrong? thanks
import java.util.Random;
public class Matchlist
private studentdetails sd = new studentdetails();
/** matchList StringBuilder stores the match list in progress */
private StringBuilder matchList = new StringBuilder();
private Random studentPicker = new Random();
private int loop = 0;
private int matches = 0;
public Matchlist()
sd.createstudentdetails();
/** Method to create the actual match list, returns the list as a string */
public String CreateMatch()
int game;
int yellowStudent = 0;
int currentGame = 0;
int matchAttempt = 0;
while (matches < 70)
makeMatches:
for (game = 0; game < 4; game++)//g = game
for (int greenStudent = 0; greenStudent < 17; greenStudent++)
while (sd.getgh(greenStudent).getGame(game).equalsIgnoreCase(""))
matchAttempt++;
if (matchAttempt > 800)
sd = new studentdetails();
game = 0;
matches = 0;
break makeMatches;
yellowStudent = studentPicker.nextInt(17);
if (sd.getyh(yellowStudent).getGame(game).equalsIgnoreCase(""))
if (sd.getgh(greenStudent).getC_lass() != sd.getyh(yellowStudent).getC_lass())
/** Check to see if the person has played the other person */
if (sd.getgh(greenStudent).checkPlayed(sd.getyh(yellowStudent).getName()) == false)
/** Set the game to the name of the opponent played */
sd.getyh(yellowStudent).changeGame(game, sd.getgh(greenStudent).getName());
sd.getgh(greenStudent).changeGame(game, sd.getyh(yellowStudent).getName());
/** Build the match list step by step using append with \n at the end to create a new line */
matchList.append(sd.getyh(yellowStudent).getName() + " vs " + sd.getgh(greenStudent).getName() + "\n");
matches++;
currentGame++;
if (currentGame == 18)
currentGame = 0;
break;
/** Convert the stringbuilder into an actual string, then return it */
String completeMatchList = matchList.toString();
System.out.println(matches);
for (int i = 0; i < 18; i++)
sd.getyh(i).getEmptyMatches();
sd.getgh(i).getEmptyMatches();
return completeMatchList;
}
Where is it occuring? What are the values of the variabels at that point? We can't even run your code to find out because it could be in the studentdetails class which you haven't posted. I suggest you use an IDE to set breakpoints and step through it in a debugger which will allow you to examine the state of the program a any given point and identify where the NPE is coming from.
For Netbeans
http://www.netbeans.org/kb/55/using-netbeans/debug.html
For Eclipse
http://pages.cs.wisc.edu/~cs302/resources/EclipseDebugTutorial/
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How to use the random generator in java
hey peeps, this is the class in which i am trying to implement the random generator in;
public class Matchlist
private studentdetails sd = new studentdetails();
/** matchList StringBuilder stores the match list in progress */
private StringBuilder matchList = new StringBuilder();
private int loop = 0;
private int matches = 0;
public Matchlist()
sd.createstudentdetails();
/** Method to create the actual match list, returns the list as a string */
public String CreateMatch()
int g;
int y;
for (g = 0; g < 4; g++)//g = game
for (y = 17; y > -1; y--)//y = green house
/** Check to see if the game is empty */
if (sd.getgh(y).getGame(g).equalsIgnoreCase(""))
for (int x = 0; x < 18; x++) //x = yellow house
if (sd.getyh(x).getGame(g).equalsIgnoreCase(""))
if (sd.getgh(y).getC_lass() != sd.getyh(x).getC_lass())
/** Check to see if the person has played the other person */
if (sd.getgh(y).checkPlayed(sd.getyh(x).getName()) == false)
/** Set the game to the name of the opponent played */
sd.getyh(x).changeGame(g, sd.getgh(y).getName());
sd.getgh(y).changeGame(g, sd.getyh(x).getName());
/** Build the match list step by step using append with \n at the end to create a new line */
matchList.append(sd.getyh(x).getName() + " vs " + sd.getgh(y).getName() + "\n");
matches++;
break;
/** Convert the stringbuilder into an actual string, then return it */
String completeMatchList = matchList.toString();
System.out.println(matches);
for (int i = 0; i <18; i++)
sd.getyh(i).getEmptyMatches();
sd.getgh(i).getEmptyMatches();
return completeMatchList;
}what i dont understand is how to implement it to pick my matches at random using the http://java.sun.com/j2se/1.4.2/docs/api/java/util/Random.html java tutorials from here
regardsHow to use Random ?
First you open API then you read it, then you use it.
You mention you try to use it but i just see a horrible nested for for for if if if loop.
Restructure code and question and maybe it makes more sense.
Edited by: pgeuens on 10-mrt-2008 22:58 -
I have program that uses random but each time i start the program i get the same sequence. I need to seed and start the rnd-generator but java doesnt support true random. Can anyone tell where i can find such a generator?
True randomness requires a hardware solution that takes input from physical processes (such as radio "white noise"). I think somebody said that certain PCs or chipsets have such hardware, but in general you won't have that option available to you.
java.util.Random and Math.random use a seed you provide, or the system clock if you don't provide a seed. If you use the same seed twice, you'll get the same sequence of numbers both times.
One common error when generating random numbers is to reseed the generator each time in a loop. For instance for (int ix 0; ix < 100; ix++) {
Random rand = new Random();
int i = rand.nextInt(10);
} This will probably produce the same number 100 times, or maybe one number N times and another number 100 - N times. The reason is that every time you create a new Random that way, you're seeding it with the system clock and starting from that sequence's first number. The system clock doesn't have time to change, or maybe changes once, during the time it takes to run through one loop iteration, so you keep starting the same sequence over, or maybe get the first number of two different sequences. The solution is to move the new Random() outside the loop. If it were a member variable, you'd probably want to make it static.
The above doesn't sound like your problem, but I present it because the solution to the problem you are having could lead to the above situation.
As someone already said, if the problem is that you get the same sequence on multiple runs of the program, then just use the system time to seed the PRNG, either explicitly or by not specifying a seed. As long as it takes at least 10 ms between one program startup and the next, you'll get different sequences.
Finally, you might want to look into [url http://java.sun.com/j2se/1.4.2/docs/api/java/security/SecureRandom.html]SecureRandom. It has better random properties than java.util.Random or Math.random, and I think if your computer does have true random hardware attached, it will use that, or can be told to use it.
¶ -
Peeps, to the following match list how do i implent the random generator so that each match is picked at random but still checks the following conditions? help is desperatley needed, i tried using the guide
http://java.sun.com/j2se/1.4.2/docs/api/java/util/Random.html
but to no avail - im REALLY stuck, thanks
public class Matchlist
private studentdetails sd = new studentdetails();
/** matchList StringBuilder stores the match list in progress */
private StringBuilder matchList = new StringBuilder();
private int loop = 0;
private int matches = 0;
public Matchlist()
sd.createstudentdetails();
/** Method to create the actual match list, returns the list as a string */
public String CreateMatch()
int g;
int y;
for (g = 0; g < 4; g++)//g = game
for (y = 17; y > -1; y--)//y = green house
/** Check to see if the game is empty */
if (sd.getgh(y).getGame(g).equalsIgnoreCase(""))
for (int x = 0; x < 18; x++) //x = yellow house
if (sd.getyh(x).getGame(g).equalsIgnoreCase(""))
if (sd.getgh(y).getC_lass() != sd.getyh(x).getC_lass())
/** Check to see if the person has played the other person */
if (sd.getgh(y).checkPlayed(sd.getyh(x).getName()) == false)
/** Set the game to the name of the opponent played */
sd.getyh(x).changeGame(g, sd.getgh(y).getName());
sd.getgh(y).changeGame(g, sd.getyh(x).getName());
/** Build the match list step by step using append with \n at the end to create a new line */
matchList.append(sd.getyh(x).getName() + " vs " + sd.getgh(y).getName() + "\n");
matches++;
break;
/** Convert the stringbuilder into an actual string, then return it */
String completeMatchList = matchList.toString();
System.out.println(matches);
for (int i = 0; i <18; i++)
sd.getyh(i).getEmptyMatches();
sd.getgh(i).getEmptyMatches();
return completeMatchList;
}jermaindefoe wrote:
lol, ok then, the code above is a match list generator that works on a set of conditions, the conditions dont need to be mentioned at the moment as that part of the code works, what does need to be mentioned though is the fact that in another class there is a student list, 2 lots of groups with 18 students each. what i want is to implement the random generator on my code so that players are picked at random and then the conditons are checked, if a match isnt found then another is picked randomly
i just dont no how, i really would like some help if poss?Are you allowed to use java.util.Random? -
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Message Edited by Lucither on 05-10-2010 06:27 AM
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