unsigned const number = minimum + (rand() % (maximum - minimum + 1))
I know how to (easily) generate a random number within a range such as from 0 to 100. But what about a random number from the full range of int (assume sizeof(int) == 4), that is from INT_MIN to INT_MAX, both inclusive?
I don't need this for cryptography or the like, but a approximately uniform distribution would be nice, and I need a lot of those numbers.
The approach I'm currently using is to generate 4 random numbers in the range from 0 to 255 (inclusive) and do some messy casting and bit manipulations. I wonder whether there's a better way.
On my system RAND_MAX is 32767 which is 15 bits. So for a 32-bit unsigned just call three times and shift, or, mask etc.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(void){
unsigned rando, i;
srand((unsigned)time(NULL));
for (i = 0; i < 3; i++) {
rando = ((unsigned)rand() << 17) | ((unsigned)rand() << 2) | ((unsigned)rand() & 3);
printf("%u\n", rando);
}
return 0;
}
Program output:
3294784390
3748022412
4088204778
For reference I'm adding what I've been using:
int random_int(void) {
assert(sizeof(unsigned int) == sizeof(int));
unsigned int accum = 0;
size_t i = 0;
for (; i < sizeof(int); ++i) {
i <<= 8;
i |= rand() & 0x100;
}
// Attention: Implementation defined!
return (int) accum;
}
But I like Weather Vane's solution better because it uses fewer rand() calls and thus makes more use of the (hopefully good) distribution generated by it.
We should be able to do something that works no matter what the range of rand() or what size result we're looking for just by accumulating enough bits to fill a given type:
// can be any unsigned type.
typedef uint32_t uint_type;
#define RAND_UINT_MAX ((uint_type) -1)
uint_type rand_uint(void)
{
// these are all constant and factor is likely a power of two.
// therefore, the compiler has enough information to unroll
// the loop and can use an immediate form shl in-place of mul.
uint_type factor = (uint_type) RAND_MAX + 1;
uint_type factor_to_k = 1;
uint_type cutoff = factor ? RAND_UINT_MAX / factor : 0;
uint_type result = 0;
while ( 1 ) {
result += rand() * factor_to_k;
if (factor_to_k <= cutoff)
factor_to_k *= factor;
else
return result;
}
}
Note: Makes the minimum number of calls to rand() necessary to populate all bits.
Let's verify this gives a uniform distribution.
At this point we could just cast the result of rand_uint() to type int and be done, but it's more useful to get output in a specified range. The problem is: How do we reach INT_MAX when the operands are of type int?
Well... We can't. We'll need to use a type with greater range:
int uniform_int_distribution(int min, int max)
{
// [0,1) -> [min,max]
double canonical = rand_uint() / (RAND_UINT_MAX + 1.0);
return floor(canonical * (1.0 + max - min) + min);
}
As a final note, it may be worthwhile to implement the random function in terms of type double instead, i.e., accumulate enough bits for DBL_MANT_DIG and return a result in the range [0,1). In fact this is what std::generate_canonical does.
