How to generate random numbers without repetition

Question

I generate a random number with this code :

int randomNumber = (int) (Math.random() * 7);

I want to stop generating duplicate numbers and I want to generate a new number after my app opens again.

how can I do ?

Answer 1

Store the numbers you generate in a Set . Then every time you generate a new random number, test that it's not already present in this Set .

Set<Integer> nums = new HashSet<Integer>();

// Initialize set on app boot

int rnum;
do {
  rnum = (int) (Math.random() * 7);
} while (!nums.contains(rnum));

// Update the set
nums.add(rnum);

// Use rnum now..

You'll also need to persist the Set values before your app shuts down; in onPause() or onStop() method. You could use SharedPreferences or simply serialize/deserialize the Set . Using a database would be an overkill.

Answer 2

Here's one way you could ensure there are no duplicates;

public int generateRandomNumber(){
SharedPreferences sp = PreferenceManager.getDefaultSharedPreferences(this);
final int lastNumber = sp.getInt("lastNumber", 0);
int randomNumber = (int) (Math.random() * 7);
while(randomNumber == lastNumber){
randomNumber = (int) (Math.random() *7);
}
sp.edit().putInt("lastNumber", randomNumber).apply();
return randomNumber;
}

Edit: Here is a dumb way to do what you want:

String _p = sp.getString("previousNumbers", "");
String[] previousNumbers = _p.split(";");
int randomNumber = (int) (Math.random() * 7);
while(Arrays.asList(previousNumbers).contains(Integer.toString(randomNumber))
{
randomNumber = (int) (Math.random() * 7);
}
_p.concat(Integer.toString(randomNumber) + ";");
sp.edit().putString("previousNumbers",_p).apply();
return randomNumber;

Answer 3

To generate distinct value at every app open you can use

System.currentTimeMillis()

This will return a long instead of int

Answer 4

Step 1: create array

Step 2: Fill array with proper numbers

Step 3: Shuffle array.

private Integer[] randomNumbersRange(int numberRange) {
    //Create and shuffle array
    Integer[] randomNumbers = new Integer[numberRange];
    for (int i = 0; i < randomNumbers.length; i++) {
        randomNumbers[i] = 1 + i;
    }
    Collections.shuffle(Arrays.asList(randomNumbers));
    return randomNumbers;
}

Answer 5

Perhaps check out seeding your number with the system timestamp, or some other value you persist outside the application (in storage / a db / etc).

https://developer.android.com/reference/java/util/Random.html#setSeed(long)