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qwerty

Fri May 28, 2004 5:15 pm


How to generate random numbers without repetition?

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I want to generate 22 random numbers between 1 and 52 and store them into an array, but each integer stored in the array must be different, i.e, there has to be repetition. If anyone know how to do it I will appretiate her/his help. Thanks. 

Note: I am writing my program in Java.


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zylum

Fri May 28, 2004 5:49 pm



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you can take an array of the 52 numbers, scramble it and then choose the first 22 elements...

int[] numbers = new int[52];
int randNumbers = new int[22];
for (int i = 0; i < 52; i ++) numbers[i] = i;
for (int i = 0; i < 52; i ++) {
    int idx = Math.random()*52 //i forgot how to do random in java so im using the action script way which is prolly the same way
    int temp = numbers[idx];
    numbers[idx] = numbers[i];
    numbers[i] = temp;
}
for (int i = 0; i < 22; i ++) randNumbers[i] = numbers[i];


edit:

what my code does is basically the same as bugz's except that i dont know if java has a shuffle method so i made one manually...  my code first fills an array 1..52 then it goes through each index and swaps values with another random index (this is the shuffle part) then i fill the randNumber array with the first 22 elements of the number array.. if you want you could fill it up with every other element but it doesnt really matter..


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wtd

Sat May 29, 2004 12:05 am



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Funny, I just write some code to do this in C++.  It's a bit easier there, but it can be done in pretty much any language with some work.

int[] possibles = new int[52];
for (int i = 0; i < 52; i++) possibles[i] = i + 1;

int[] alreadyChosen = new int[52];

for (int i = 0; i < 52; i++)
{
   boolean hasInt = false;
   int randIndex;
   do
   {
      randIndex = Math.random() * 52;
      for (int j = 0; j < i && !hasInt; j++)
      {
         if (possibles[randIndex] == alreadyChosen[j])
         {
            hasInt = true;
         }
      }
   } while (hasInt);
   alreadyChosen[i] = possibles[randIndex];
}

Basically, start out with a full deck of cards.  Choose one.  If it's already in the destination deck, choose another one.  If not, throw it on the destination deck and try again.  Keep doing this until the destination deck has 52 cards.


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Tony

Sat May 29, 2004 11:36 am



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well zylum got the right idea... but I don't really understand his code at the moment.

The way I do it is to randomly pick a number from the array of remaining values rether then randomly guessing each time. [url=http://www.compsci.ca/v2/viewtopic.php?t=2184]link to code


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bugzpodder

Sat May 29, 2004 11:58 am



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this is the way to do it in C++

#include
int main(){
     int arr[52];
     for (int i=0;i