Make Random.range not repeat values

Question

I have to spawn different objects and it's crucial that no two objects get repeated. What I mean is, random.range should not get two simultaneous numbers same, otherwise the code spawns same objects sometimes. Here's a simpler code to help you understand what I'm trying.

void Update () {

    j = Random.Range (1, 4); // this should give j values like=(1,2,1,3,2,1)
                              // and not this = (1,1,2,2...) the numbers should not repeat simultaneously.

    Inst();
}

void Inst(){

    if (j == 1) {

        //instantiate obj1
    }
    else if (j == 2) {

        instantiate obj2
    }

    else if (j == 3) {

        instantiate obj3
    }


}

Thank You!

keep track of the previous value, don't let it generate that value on the next go — Jonesopolis, Jan 02 '18 at 13:36
It's look like you are searching for [shuffle](https://stackoverflow.com/q/273313/4685428) — Aleks Andreev, Jan 02 '18 at 13:38
Use Recursion. Every time a random number is generated, add it to a List. If theList.Contains() your random number, recurse your function until a new, unique, number is produced. — Brandon Miller, Jan 02 '18 at 13:41
@BrandonMiller then he will never end up with a result like `1,2,1,3,2` (which is an acceptable outcome per his post) he doesn't want a result like this `1,1,2,3,1` — AresCaelum, Jan 02 '18 at 13:43
Okay, then instead of adding your int to a list, just declare one int and set it every time the function is called. That way it will never be the same thing twice in a row, but will allow for repeated numbers later on. — Brandon Miller, Jan 02 '18 at 13:46
Beware of using `while` loop in the accepted answer to do this. See duplicate for how to do this without a while loop. — Programmer, Jan 02 '18 at 17:24

Nikhil Agrawal · Accepted Answer · 2018-01-02T13:59:55.253

5

How about

int lastValue;
public int UniqueRandomInt(int min, int max)
{
    int val = Random.Range(min, max);
    while(lastValue == val)
    {
        val = Random.Range(min, max);
    }
    lastValue = val;
    return val;
}

void Update() 
{
    UniqueRandomInt(1, 4);
    Inst();
}

void Inst()
{
    if (lastValue == 1)
    {
        //instantiate obj1
    }
    else if (lastValue == 2)
    {
        //instantiate obj2
    }
    else if (lastValue == 3)
    {
        //instantiate obj3
    }
}

edited Jan 02 '18 at 13:59

answered Jan 02 '18 at 13:40

Nikhil Agrawal

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this will become slow for large lists. – Wouter Jan 02 '18 at 13:54
@Wouter:Realised that. Updated answer. – Nikhil Agrawal Jan 02 '18 at 13:55
alternating sequences are valid? – Wouter Jan 02 '18 at 13:56
Okay, when I use this which variable should I use to compare it in the spawn function? – Vivek Negi Jan 02 '18 at 13:57
Alternating sequences are valid. It's just two numbers should not be repeated simultaneously. – Vivek Negi Jan 02 '18 at 13:58
Might be worth adding a fail-safe check to insure that the numbers passed in for `min` and `max` have a range greater than 1. – Draco18s no longer trusts SE Jan 02 '18 at 17:04
2

I don't know why this has so many upvotes. It can lead to many serous issues so such game freezing or even locking. Don't use random number to control a `while` loop unless you are in a coroutine function and you wait for some frames. – Programmer Jan 02 '18 at 17:20

score 2 · Answer 2 · answered Jan 02 '18 at 14:13

Your original statement picks 1 out of 3 possible numbers (1, 2, 3).

j = Random.Range(1, 4);

If you want to avoid the last issued number, then that leaves only 2 possible numbers.

Random.Range(1, 3)

Adding this random number to the previous value of j results in a new value that is equally random, but guaranteed to be different from the previous value of j.

j = j + Random.Range(1, 3);

Of course, you need to wrap values to keep them within the required range {1, 2, 3}:

if (j > 3) j -= 3;

Alternatively, use modulo (%) to make it a one-liner:

j = 1 + (j + Random.Range(0, 2)) % 3;

Works equally well for bigger ranges.

j = min + (j - min + Random.Range(1, max - min)) % (max - min);

This approach is more efficient than 'looping while duplicate'; there is no guaranteed limit to the number of iterations.

Make Random.range not repeat values

2 Answers2