# random number within a specified range?

Don nospam at nospam.com.au
Thu Oct 9 06:33:49 PDT 2008

```Jarrett Billingsley wrote:
> On Sun, Oct 5, 2008 at 7:34 PM, Michael P. <baseball.mjp at gmail.com> wrote:
>> Hi,
>>
>> How would I go about getting a random number that was in a specified range of 2 numbers?
>> For example, getting a number inbetween 1000 and 2000.
>>
>> -Michael P.
>>
>
> lo + rand() % (hi - lo)
>
> assuming you're using Phobos, where rand() returns a number in the
> range [0, uint.max].
>
> This will get you a number in the range [1000, 2000); that is, it will
> be at least 1000 and at most 1999.  If you need 2000 to be one of the
> numbers generated, use 2001 as the hi.
The random numbers will not be evenly distributed, though. You'll get
more which are closer to 1000 than to 2000.

(Imagine uint.max = 1000. Then 0 and 1000 map to 1000, but every other
number has only one number which maps to it).

Simple way to fix this is to ignore numbers from the non-uniform part:

uint rmax = uint.max - (uint.max % (hi-lo));
uint r;
do {
r = rand();
} while (r>rmax);
return lo + r % (hi - lo);
```

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