tcpl_ex-2-7.c

/*
 * Exercise 2-7. Write a function  invert(x,p,n) that returns x with the n 
 * bits that begin at position p inverted (i.e., 1 changed into 0 and vice 
 * versa), leaving the others unchanged.  
 */
#include <stdio.h>

unsigned int invert(unsigned int x, int p, int n);

unsigned int invert(unsigned int x, int p, int n) {
	return (x & ~(~(~0 << n) << (p + 1 - n))) | 
		((~((x >> (p + 1 - n)) & ~(~0 << n)) & ~(~0 << n)) << (p + 1 - n));
}

int main(void)
{
	unsigned int x = 0xc9;
	unsigned int r = 0xc7;
	int p = 3;
	int n = 3;
	if (r == invert(x, p, n))
		printf("assert success!\n");
	else
		printf("assert fail!\n");

	return 0;
}