Question

I have a strange problem involving the following macro:
\[#define shuffle(sequence, len) do \
{ \
fprintf(stderr, "%s\n", TIME_LINE_FILE); \
unsigned int k; \
unsigned int count = (len); \
while (count > 1) \
{ \
k = rand_range(count); \
--count; \
swap((sequence)[count], (sequence)[k]); \
} \
} while (0)
\]

When you pass a number larger than 32767 (what's special about this number is that it is equal to 2^16 - 1), it breaks.

What is wrong with it?

Answer 1

when you pass a number larger than 32767 as len, it breaks*

Answer 2

I've narrowed the problem to this function:
\[#
int random_in_range(unsigned int min, unsigned int max)
{
int base_random, range, remainder, bucket;
do {
do {
base_random = rand();
} while (RAND_MAX == base_random);
range = max - min;
remainder = RAND_MAX % range;
bucket = RAND_MAX / range;
} while (base_random >= (RAND_MAX - remainder));
return min + (base_random / bucket);
}\]

with min equal to 0, and max equal to a number larger than 32767, this function just breaks.

Answer 3

ahh... RAND_MAX happens to equal to 32767

Answer 4

well... now I have a new question.... how do i get a better rand() function in C?

Answer 5

because the default rand() stuff in C just sucks :(

Answer 6

be more specific; what don't you like about the default rand() (and perhaps ask a separate question)?

Answer 7

it only goes from 0 to 2^15 - 1

Answer 8

RAND_MAX is not fixed. In the GNU library it's 2^31-1; not sure what you're using.

You just need to scale it properly. If you need a higher resolution you can always just call it again. E. g.
int my_rand(){
return rand()*32768+rand();
}