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Random number generator without replacement?

I tried to program a random number generator that doesn't generate the same random number more than once. But I cannot and cannot understand why. My code looks like this:

public void printNS(){

    System.out.print("Numeros Numeros: ");

    for(int i=0; i < 5 ; i++){
        System.out.print( (int)(Math.random()*50) + ",");
    }

    System.out.print("; Numeros Stars: ");

    for(int i=0; i < 2 ; i++){
        System.out.print( (int)(Math.random()*12)+ ",");    
    }

}
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3 answers


in java 8 you can do the following

 int[] rand = new Random().ints(start, end).distinct().limit(number).toArray();


for more info / options see doc

And before java 8, you can use Set. Generate random numbers until the size of your set is less than the desired number of random numbers.

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So, you want k

great random numbers from 0

to n

(s k < n

).

Two possible approaches:



  • Pick k

    random numbers as you did and store them in the data structure. Each time you choose a number, check if it is contained in the structure: if so, keep collecting until you get a "new" random number. This is a fairly straightforward approach, but the loop can potentially block your application. I suggest using Set

    as it preserves certain elements by definition

    Set<Integer> set = new LinkedHashSet<>(); // unordered
while (set.size() < k){
   set.add((int)(Math.random()*n));
}
System.out.println(set);

  • Create List

    and initialize it with every number between 0

    and n

    . Then shuffle it. The first k

    items in the list are the numbers you want.

    List<Integer> list = new ArrayList<>(n);
for (int i = 0; i < n; i++){
   list.add(i);
}
Collections.shuffle(list);
list.subList(0, k).clear();
System.out.println(list);

I would suggest the second approach as it is cleaner, but I don't know your performance requirements.

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Here:

private printStars(int loops, int factor) {
  for(int i=0; i < loops ; i++){
    System.out.print( (int)(Math.random()*factor) + ",");
  }

And now:

public void printNS(){
  System.out.print("Numeros Numeros: ");
  printStars(5, 50);
  System.out.print("; Numeros Stars: ");
  printStars(2, 12);

Hope it helps. The key point is: when you iterate over the code, look for those elements that are "identical"; and then move them to another method!

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