Problem - Kevin Tafuro

Discussion
In all cases, you will start with a function that gives you a random unsigned number
that can be any value, such as spc_rand_uint( ) from Recipe 11.10. You will mold
numbers returned from this function into numbers in a specific range.
If you need random numbers in a particular range, the general approach is to get a
number between zero and one less than the number of values in the range, then add
the result to the smallest possible value in the range.
Ideally, when picking a random number in a range, you would like every possible
value to be equally likely. However, if you map from an arbitrary unsigned integer
into a range, where the range does not divide evenly into the number of possible integers,
you are going to run into problems.
Suppose you want to create a random number in a range using an unsigned 8-bit
type. When you get a random unsigned 8-bit value, it can take on 256 possible values,
from 0 to 255. If you are asking for a number between 0 and 9 inclusive, you
could simply take a random value and reduce it modulo 10.
The problem is that the numbers 0 through 5 are more likely values than are 6
through 9. 26 possible values will reduce to each number between 0 and 5, but only
25 values will yield 6 through 9.
In this example, the best way to solve this problem is to discard any random numbers
that fall in the range 250-255. In such a case, simply get another random value and try
again. We took this approach in implementing the function spc_rand_range( ). The
result will be a number greater than or equal to a minimum value and less than or
equal to maximum value.
#include
#include
Some programmers may expect this function to exclude the upper
limit as a possible value; however, we implement this function in such
a way that it is not excluded.
int spc_rand_range(int min, int max) {
unsigned int rado;
int range = max - min + 1;
if (max < min) abort( ); /* Do your own error handling if appropriate.*/
do {
rado = spc_rand_uint( );
} while (rado > UINT_MAX - (UINT_MAX % range));
return min + (rado % range);
}
Getting a Random Integer in a Range | 607
This is the Title of the Book, eMatter Edition
Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.