Using Random.range to generate level no without repetition

Question

I tried to use recursion for the problem at hand as follows,

int newlevelgen()
{
    int exampleno = Random.Range (1,4);
    if (exampleno != lastlevelno)
    {
        lastlevelno = exampleno;
        return exampleno;
    }
    else
    {
        newlevelgen();
    }
    return exampleno;
}

This is my code above, what I want to do is generate new number without repeating the previous one, but this simply does not work. Help!

Answer 1

Let me preface this by saying that the following is an answer to the problem with the original method that was posted. It is far from that best way to get the desired result but that's not the focus of this answer. Yes there is a chance that the recursion will go on for a few calls. 33% chance to need another recursive call, though that does not mean a 33% for an infinite loop as the likelihood of needing X recursive calls is 0.33^X so the likelihood of reaching 3 recursive calls is only 0.33^3 ~= 0.036. Still, a different method is advised. See Joe Blow's answer for example.

There's a problem with your recursion. You aren't using the new value that the recursive call to newlevelgen() would give you in case exampleno is the same as lastlevelno and always returning the (possibly duplicate) exampleno value. Change it to:

int newlevelgen()
{
    int exampleno = Random.Range (1,4);
    if (exampleno != lastlevelno)
    {
        lastlevelno = exampleno;
        return exampleno;
    }
    else
    {
        return newlevelgen();
    }
}

Answer 2

The idea is that you get a value between a and b. If that value is greater of equal to previous value, then you increase that and return it. Other case, you return as is.

Think of it, it does work.

public int GetUniqueLevelInclusiveOrdinal(int a , int b, int previous){

    // given the ordinal numbers from a to b INCLUSIVE, 
    // so including a and b,
    // (*** NOTE, no random int calls in Unity or any system
    // work inclusively so take care ***)
    // choose one of the numbers from a to b inclusive
    // but do not choose "previous"

    // which top index to use with Random.Range which is exclusive?
    int top = b+1;

    // everyday algorithm for excluding one item from a random run
    int topExcludeOne = top-1;

    int value = Random.Range(a, topExcludeOne);
    if (value >= previous) return value+1;
    else return value;
}

---

This is an extremely well-known, basic, pattern in programming...

int result = UnityEngine.Random.Range(0,highest-1);
if (result>=exclude)
   return result+1;
else
   return result;

In Unity you must use extensions:

public static int RandomIndexButNotThis(this int highest, int exclude)
    {
    if (highest<2) Debug.Break();

    int result = UnityEngine.Random.Range(0,highest-1);
    if (result>=exclude)
       return result+1;
    else
       return result;
    }

To get a random index 0 to 99

Random.Range(0,100)

To get a random index 0 to 99, but excluding 61

100.RandomIndexButNotThis(61)

To get a random index 0 to 9

Random.Range(0,10)

To get a random index 0 to 9, but excluding 8

10.RandomIndexButNotThis(8)

If new to Unity, intro to extensions

Answer 3

I created utility to easely handle random without repetitions, flat distributed random, and weighted lists (universal property drawer included!). You can find it on GtiHub and use freely: https://github.com/khadzhynov/RandomUtils