begin Alexandra Thorn <[EMAIL PROTECTED]>
>
> I've been looking around for a C library function that will shuffle the
> elements of an arbitrarily long array. I'd been hoping to turn up
> something that would randomly shuffle the elements of an array. The array
> that I want to shuffle is made up of a class of structs that I've created.
>
> A few hours of googling and fiddling with the man pages hasn't shed too
> much light on the issue
after a few hours, any solution is better than googling! (-:
here's some code i quickly whipped up. it's not the best code you can
write, but i just wanted to show an algorithm. there are many
permutations you can make on the same algorithm; some better than
others.
i apologize if there's a mistake here. i ran it once to make sure the
output looked sane. looked random to me. (-:
pete
ps- some newlines in case you don't want an answer right away.
#include <stdio.h>
#include <stdlib.h>
unsigned int SeedRandomGenerator(void);
int Irand(int low, int high);
/* Example of shuffling an integer array. We are going to take elements of
* of from[], shuffle them, and place them into to[]. We're also going to
* use another array, been_there[] to keep track of which elements of to[]
* are free to accept an element of to[].
*
* There are many many ways to optimize this code, but this is simply an
* example of the algorithm.
*/
int main(void)
{
int n = 5;
int i, from[n], to[n], been_there[n], myrand;
unsigned int seed;
/* Seed the random number generator */
seed = SeedRandomGenerator();
printf("seed is %u\n", seed);
/* initialize from[] and been_there[]. if been_there[j] == 0, then
* we haven't put anything in to[j]. if been_there[j] == 1, then
* we've already put something in to[j].
*/
for (i=0; i<n; ++i) {
from[i] = i;
been_there[i] = 0;
}
/* Loop over each element of from[]. */
for (i=0; i<n; ++i)
{
// Pick one of the empty slots of to[].
do {
myrand = Irand(0, n-1);
} while (been_there[myrand] == 1);
// Make a note that this element of to[] has been filled
been_there[myrand] = 1;
// Switch the elements
to[myrand] = from[i];
}
// Print the results side by side.
for (i=0; i<n; ++i)
printf("%d %d\n", from[i], to[i]);
return 0;
}
/* Seed the random number generator
*/
unsigned int SeedRandomGenerator(void)
{
FILE *fp;
unsigned int seed;
size_t retval;
if((fp = fopen("/dev/random", "r")) == NULL) {
fprintf(stderr, "Can't open /dev/random for reading.");
exit(0);
}
retval = fread(&seed, 1, sizeof(unsigned int), fp);
if (retval != sizeof(unsigned int)) {
fprintf(stderr, "SeedRandomGenerator: fread wanted %d but got %d",
sizeof(unsigned int), retval);
exit(0);
}
fclose(fp);
srand(seed);
return(seed);
}
/* Returns an integer between high and low (inclusive)
*/
int Irand(int low, int high)
{
return low + (int)( (double)(high-low+1) * rand()/(RAND_MAX + 1.0) );
}
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