set.c

Vxworks下的C例子程序

#include <stdio.h>
#include "set.h"

/* 
 * set.c from the Set of Integers example, 
 * Ch. 7, C: A Reference Manual 5/e
 * Samuel P. Harbison III and Guy L. Steele Jr.
 */


int cardinality(SET x)
{	
/* The following loop body is executed once for every 1-bit
   in the set x.  Each iteration, the smallest remaining
   element is removed and counted. The expression (x & -x)
   is a set containing only the smallest element in x, in
   twos-complement arithmetic. */
	int count = 0;
	while (x != emptyset) {
		x ^= (x & -x);
		++count;
	}
	return count;
}

SET next_set_of_n_elements(SET x)
{
/* This code exploits many unusual properties of unsigned
   arithmetic.  As an illustration:
     if x               == 001011001111000, then
     smallest           == 000000000001000
     ripple             == 001011010000000
     new_smallest       == 000000010000000
     ones               == 000000000000111
     the returned value == 001011010000111 
   The overall idea is that you find the rightmost
   contiguous group of 1-bits. Of that group, you slide the
   leftmost 1-bit to the left one place, and slide all the
   others back to the extreme right. 
   (This code was adapted from HAKMEM.) */
	SET smallest, ripple, new_smallest, ones;
	if (x == emptyset) return x;
	smallest     = (x & -x); 
	ripple       = x + smallest;
	new_smallest = (ripple & -ripple);
	ones         = ((new_smallest / smallest) >> 1) - 1;
	return (ripple | ones);
} 

void printset(SET z)
{
	int first = 1;
	int e;
	forallelements(e, z) {
		if (first) printf("{");
		else printf(", ");
		printf("%d", e);
		first = 0;
	}
	if (first) printf("{"); /* Take care of emptyset */
	printf("}");            /* Trailing punctuation */
}

#define LINE_WIDTH 54
void print_k_of_n(int k, int n)
{
	int count = 0;
	int printed_set_width = k * ((n > 10) ? 4 : 3) + 3;
	int sets_per_line = LINE_WIDTH / printed_set_width;
	SET z = first_set_of_n_elements(k);
	printf("\nAll the size-%d subsets of ", k);
	printset (first_set_of_n_elements(n));
	printf(":\n");
	do {			/* Enumerate all the sets. */
		printset(z);
		if ((++count) % sets_per_line) printf (" ");
		else printf("\n");
		z = next_set_of_n_elements(z);
	}while ((z != emptyset) && !element(n, z));
	if ((count) % sets_per_line) printf ("\n");
	printf("The total number of such subsets is %d.\n",
		count);
}