Generate Random Integer from Min to Max?

Question

I want to generate a random Integer that is:

1. Inside the range [Min, Max] inclusive
2. The range can be [5,20], [-29, -3] or [-13, 13] (It can be in any range, positive or negative or in between)
3. The code is working fine in Android

What I got so far is this, but it seems not working with negative ranges !

1 + (int)(Math.random() * ((Max - Min) + 1));

Answer 1

Try this

int min = -100;
int max = 100;
Random rand = new Random();
return rand.nextInt(max - min + 1) + min;

Answer 2

I'm pretty sure you want

Min+(int)(Math.random()*((Max-Min) + 1));

However, I should point out that the range [-3,-29] has its min and max reversed. (And the same with [5,-13] as was pointed out by Merlyn.)

If you want to just put in any two numbers for the range, a and b then use the code

int Min = Math.min(a,b);
int Max = Math.max(a,b);

That way you won't have to worry about the order. This will even work for a==b.

Answer 3

/**
 * @param bound1 an inclusive upper or lower bound
 * @param bound2 an inclusive lower or upper bound
 * @return a uniformly distributed pseudo-random number in the range.
 */
public static int randomInRange(int bound1, int bound2) {
    int min = Math.min(bound1, bound2);
    int max = Math.max(bound1, bound2);
    return min + (int)(Math.random() * (max - min + 1));
}

---

If the caller can guarantee that bound1 will be less or equal to bound2 than you can skip the step of figuring out the minimum and maximum bounds; e.g.

/**
 * @param min the inclusive lower bound
 * @param max the inclusive upper bound
 * @return a uniformly distributed pseudo-random number in the range.
 */
public static int randomInRange(int min, int max) {
    return min + (int)(Math.random() * (max - min + 1));
}

---

I haven't tested this on Android, but it should work on any Java or Java-like platform that supports those methods in conformance to the standard (Sun) Java SE specifications.

Answer 4

You would simply calculate the difference between min and max example -30 and 30 to get: delta <-- absolute value of (30 - (-30)) then find a random number between 0 and delta.

There is a post related to this already. afterwards translate the number along the number-line by your constant min.

If using the Random class: 1) is added to the equation here for Random.nextInt(someInt) because nextInt returns: someVal < someInt so you need to be sure to include the boundaries in the functions output.

Something about this code is makes me cautious:

return min + 
(int)(Math.random() * (max - min + 1));
}

When casting rounding doubles to cast to ints:

int someRandomDoubleRoundedToInteger = (int)(someDouble + 0.5)

Does it work? This is for my benefit, maybe some other newbies will be amused, or maybe I made a blunder so:

lets choose 1 and 10 to begin with. Pick a number between 1 and 10 (Inclusive) I pick 10

Math.Random Excludes 0 and 1. so 0.9999999999999999 * (10 - 1 + 1) = 9.999999999999999 cast to int, so lob off everything after the decimal produces 9 the we add 1 to 9 to return 10. (using random anything from (.9 to .999999999999) also will produce 10

I want to choose 1 from between 1 and 10.

In this case the random method puts out it's closest value to zero say: 0.00000000000000001 * (10 - 1 + 1) cast to int is zero 0 We return 1+0. So that works.

Huh, seems to work. Looks like a very, very, very minor bias against 1 since we are never including "0" as a possibility but it should be acceptable.

The method works, If the random generator evenly covers the entire area between 0 and 1. How many different numbers can be represented between the {0,1} interval, are they evenly spaced?

I think it works.

Answer 5

Has anyone mentioned that his min and max are reversed? Because that's what is screwing up his code.

EDIT

The other thing that's messing up his code: it should be Min + not 1 +