示例：反转后几位

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
//提供CHAR_BIT的定义，CHAR_BIT表示每字节的位数
#include <limits.h>

char * itobs(int, char *);
void show_bstr(const char *);
int invert_end(int num, int bits);

int main(int argc, char* argv[])
{
	char bin_str[CHAR_BIT * sizeof(int) + 1];
	int number;

	puts("Enter integers and see them in binary.");
	puts("Non-numeric input terminates program.");
	while (scanf("%d", &number) == 1)
	{
		itobs(number, bin_str);
		printf("%d is\n", number);
		show_bstr(bin_str);
		putchar('\n');
		number = invert_end(number, 4);
		printf("Inverting the last 4 bits gives\n");
		show_bstr(itobs(number, bin_str));
		putchar('\n');
	}
	puts("Bye!");

	system("pause");
	return 0;
}

char * itobs(int n, char * ps)
{
	int i;
	const static int size = CHAR_BIT * sizeof(int);
	for (i = size - 1; i >= 0; i--, n >>= 1)
		//01代表八进制1
		ps[i] = (01 & n) + '0';//1 + '0' 表示 '1'
	ps[size] = '\0';
	return ps;
}

void show_bstr(const char * str)
{
	int i = 0;
	while (str[i]) /* 不是一个空字符 */
	{
		putchar(str[i]);
		if (++i % 4 == 0 && str[i])
			putchar(' ');
	}
}

int invert_end(int num, int bits)
{
	int mask = 0;
	int bitval = 1;
	while (bits-- > 0)
	{
		mask |= bitval;
		bitval <<= 1;
	}
	return num^mask;
}

运行测试