Java: generating random number in a range

Question

Hello,

I am trying to generate a random number with Java, but random in a specific range. For example, my range is 5-10, meaning that 5 is the smallest possible value the random number can take, and 10 is the biggest. Any other number in between these numbers is possible to be a value, too.

In Java, there is a function random() in the Math class, which returns a double value between 0.0 and 1.0. In the class Random there is a function nextInt(int n), which returns a random value in the range of 0 (inclusive) and n (exclusive). I couldn't find a method, which returns a random value between two numbers.

I have tried the following things, but I still have problems: (minimum and maximum are the smallest and biggest numbers).

Solution 1 :

randomNum = minimum + (int)(Math.random()*maximum);

problem: randomNum takes is assinged values numbers bigger that maximum

Solution 2 :

Random rn = new Random();
int n = maximum - minimum + 1;
int i = rn.nextInt() % n;
randomNum =  minimum + i;

problem: randomNum takes is assigned values smaller than minimum.

Could you suggest how to solve my problem, or point me to some references? I have tried also browsing through the archive, and found:

• http://stackoverflow.com/questions/137783/given-a-function-which-produces-a-random-integer-in-the-range-1-to-5-write-a-fu
• http://stackoverflow.com/questions/288739/generate-random-numbers-uniformly-over-entire-range

but I couldn't solve the problem.

Thank you.

Answer 1

how about minimum + rn.nextInt(maxValue - minvalue + 1) ?

Answer 2

The standard way to do this is as follows:

// Example assumes these variables have been initialized
// above, e.g. as method parameters or otherwise
Random rand;
int min, max;

// nextInt is normally exclusive of the top value,
// so add 1 to make it inclusive
int randomNum = rand.nextInt(max - min + 1) + min;

Answer 3

Try

rand.nextInt((max+1) - min) + min;

Answer 4

I wonder if any of the random number generating methods provided by an Apache Commons library would fit the bill.

For example: nextInt or nextLong

Answer 5

int random = minimum + Double.valueOf(Math.random()*(maximum-minimun)).intValue();

Or take a look to RandomUtils from apache commons

http://commons.apache.org/lang

Answer 6

You can edit your second code example to:

Random rn = new Random();
int range = maximum - minimum + 1;
int randomNum =  rn.nextInt(range) + minimum;

Answer 7

One standard pattern for accomplishing this is:

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

The java Math library function Math.random() generates a double value in the range [0,1). Notice this range does not include the 1.

In order to get a specific range of values first you need to multiply by the magnitude of the range of values you want covered.

Math.random() * ( Max - Min )

This returns a value in the range [0,Max-Min).

For example if you want [5,10] you need cover 5 integer values so you use

Math.random() * 5

This would return a value in the range [0,5)

Now you need to shift this range up to the range that you are targeting. You do this by adding the Min value.

Min + (Math.random() * (Max - Min))

You now will get a value in the range [Min,Max). But this is still doesn't include max and you are getting a double value. In order to get the max value included, you need to add 1 to your range parameter (Max - Min) and then truncate the decimal part by casting to an int. This is accomplished via:

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

And there you have it. A random integer value in the range [Min,Max].

Answer 8

Thanks a lot for the replies!

Answer 9

When you need a lot of random number I do not recommend the Random class in the API. It has just a too small period. Try MersenneTwister instead. You can get a java implementation here.

Answer 10

The Math.Random class in java is 0-based. So, if you write something like

Random rand = new Random();
int x = rand.NextInt(10);

x will be between 0-9 inclusive.

So given the following array of 25 items, the code to generate a random number between 0 (the base of the array) and array.length would be:

String[] i = new String[25];
Random rand = new Random();
int index = 0;

index = rand.NextInt(i.Length)

Since i.Length will return 25, the NextInt(i.Length) will return a number between the range of 0-24. The other option is going with the Math.Random which works in the same way.

   index = (int)Math.floor(Math.random()*i.length);

For a better understanding, check out this post.

Answer 11

    Random ran = new Random();

int x = ran.nextInt(5) + 5;

the integer x is now the random that has a possible outcome of 5-10.

Answer 12

Thanx for the solution... it's help me very much....

Answer 13

use : in case of rolling a dice it would be ramdom number between 1 to 6 (not 0 to 6) so face = 1 + randomNumbers.nextInt(6);

Answer 14

rand.nextInt((max+1) - min) + min;

this is working fine.........