Question

can anybody help me in making a method to generate random number without repetition in Android?
The maximum number is: prjcts.size(); it’s my JSON Array. And the return value should be in integer.

What I’ve already had is:
int i = (int)(prjcts.size() * Math.random()); I casted the method 3 times, because I need 3 random generated numbers. It works, but I don’t know how to make it without repetition. So those 3 numbers won’t be the same between each other.

Thank you

Answer 1

Have you tried just using Math.random()?

Just do some casting magic and you’ll be good to go:

int index = (int)((double)prjcts.size() * Math.random());

Edit:

If you want prevent repetition, you could create a list with all the possible indices.

int max = prjcts.size();
List<int> indices = new ArrayList<int>(max);
for(int c = 0; c < max; ++c)
{
    indices.add(c);
}

Then each time you want a random index, just pick a random item from the list, removing it after from the list when you’re done

int arrIndex = (int)((double)indices.size() * Math.random());
int randomIndex = indices.get(arrIndex);
indices.remove(arrIndex);

randomIndex is now guaranteed to be a never-before used index of of your JSON list.