Generating an array of random numbers without duplicates

Question

I have been banging my head for two days now trying to make sure the random generator method won't generate a duplicate. This I had to do with loops only without importing any libraries. I have come to this solution, will this code generate a duplicate on the long run? If yes please help.

int[] vargu1 = new int[5];

for (int i = 0; i < vargu1.length; i++) {
    int numriSekret = (int) (Math.random() * 10) + 1;
    vargu1[i] = numriSekret;
}

System.out.println(vargu1[i]);
System.out.println();

for (int i = 0; i < vargu1.length; i++) {
    for (int j = 0; j < i; j++) {
        if (vargu1[i] == vargu1[j]) {
            vargu1[i]++;
        }
        System.out.println(vargu1[i]);
    }
}

if it's not able to generate duplicates, it's no longer a 'random generator', so what exactly are you trying to do? — Stultuske, Jan 28 '16 at 08:51
@Stultuske I am trying to make sure that the random method won't give me the same number twice. — ermal gashi, Jan 28 '16 at 08:52
If you think you need "random numbers without duplicates", then you don't understand the problem. What are you actually trying to accomplish? — Lee Daniel Crocker, Jan 28 '16 at 20:32

ParkerHalo · Answer 1 · 2016-01-28T09:05:49.233

Since you said without any libraries you could reimplement the Collections.shuffle(...) method for arrays.

int [] randoms = new int[5];

// creating an array containing the numbers 1-10
int [] shuffleArray = new int[10];
for (int i = 1; i <= 10; i++) {
    shuffleArray[i-1] = i;
}

// shuffling that array
Random random = new Random();
for (int i = 0; i < 10; i++) {
    int j = random.nextInt(10);
    int tmp = shuffleArray[i];
    shuffleArray[i] = shuffleArray[j];
    shuffleArray[j] = tmp;
}

// assigning the first 5 values to the random array
for (int i = 0; i < randoms.length; i++) {
    randoms[i] = shuffleArray[i];
}

Although i have to remark that Random needs an import too (needs a library too)

score 1 · Answer 2 · answered Jan 28 '16 at 08:55

As far as I understand what you are doing, a better way would/could be:

List<Integer> myList = new ArrayList<Integer>();
for ( int i = 0; i < expectedNumberOfUniques; i++){
  Integer a = generateRandom();
  while ( myList.contains(a)){
    a = generateRandom();
  }
  myList.add(a);
}

A few remarks, though: the more elements in myList, the less efficiënt it might be less efficiënt.

But: as soon as you limit it to 'no duplicates allowed', we are no longer talking about 'random' numbers.

I know it won't be called a random if I limit it, but I have to make sure to not generate the same number twice. I have to do this without importing any libraries. — ermal gashi, Jan 28 '16 at 08:58

Generating an array of random numbers without duplicates

2 Answers2

Linked

Related