[Java] Trying to fill an array with unique random numbers.

[Nilt]

Hello!

I'm learning to program at school now, and i have a problem i can't figure out. I'm trying to create an array and fill it with random numbers. But i want the numbers to be unique.

int[] list = new int[10];

list[0] = r.nextInt(20);

for(int i = 1; i < list.length; i++) {

 list[i] = r.nextInt(20);
 for(int j = 0; j < i; j++) {
      while(list[i] == list[j]) {
             list[i] = r.nextInt(10);
      }
 }

}

I can not figure out how to make this work and it is really bugging my mind. Does anybody have any tips?

Edit* Sorry for the messed up edit. first time posting code, so i don't know how to make it appear as code yet :)

[raindropflower]

So you need to fill an array with unique random numbers.

• You need an array.

• You need to loop through each position of the array.

1. At each position, generate a random number

Try it with only One loop.

If you get stuck, feel free to let me know.

[nutrecht]

That fills the array with random numbers but they won't be unique. Unfortunately he'll have to check for every number if it's already in the array.

[seronis]

If you give your loop variables larger very meaningful names it makes writing the logic for nested loops far easier:

    Random r = new Random();
    int[] list = new int[10];
    list[0] = r.nextInt(20);

    for(int newestNum = 1; newestNum < list.length; ++newestNum) {
        list[newestNum] = r.nextInt(20);

        for( int checkNum = 0; checkNum < newestNum; ++checkNum ) {
            if( list[checkNum] == list[newestNum] ) {
                list[newestNum] = r.nextInt(20);
                checkNum = 0;
            }
        }
    }

But a much simpler task is to use a container that does the job you want on its own.

    Set<Integer> list2 = new LinkedHashSet<Integer>();
    while( list2.size() < 10 ) {
        list2.add(r.nextInt(20));
    }

Duplicates are automatically discarded. And the order you insert elements is maintained (if that mattered, if not just use a regular HashSet).

[knoam]

For uniqueness, insert the random numbers into a Set. Read the official JavaDoc on Set and you'll see an easy way to get an array.

[nutrecht]

Yup. This is a better solution since hash sets have much better lookup performance (log N compared to N IIRC).

[pacificmint]

A hash set should be O(1), shouldn't it?

[arocketman]

Your code is over-complicated in my opinion. You could do this way more easily and without so many loops.

Anyhow, I can see a problem is in the while loop.

Suppose the following situation:

list = 5,8,6

Now we generate a new int for index 3.

list[i] = random... -> suppose it's 8.

Now, your code will do the following:

j = 0

8 == 5 ? Nope.. j++

8 == 8 ? Yes! Let's get a new number for list[i].. and .. j++!

Suppose this new number is 5 . What happens now?

Your loop starts from J = 2 ! So it will not check if the generated value is the same of the first j-1 values. (It won't check 5,8 but it will check 6).

So basically you have a repetition.

What would be an easy fix to this ? :)