第 15 章 位操作（invert4）

/*------------------------------------
    invert4.c -- 使用位操作显示二进制
------------------------------------*/

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

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

int main()
{
    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 ", number);

        show_bstr(bin_str);

        putchar('
');

        number = invert_end(number, 4);

        printf("Inverting the last 4 bits gives
");

        show_bstr(itobs(number, bin_str));
        
        putchar('
');
    }

    puts("Bye");

    return 0;
}

char* itobs(int n, char *ps)
{
    const static int size = CHAR_BIT * sizeof(int);

    for (int i(size - 1); i >= 0; --i, n >>= 1)
        ps[i] = (01 & n) + '0';
    
    ps[size] = '';

    return ps;
}

//以4位为一组，显示二进制字符串
void show_bstr(const char *str)
{
    int i = 0;
    while (str[i])        //不是空字符
    {
        fputc(str[i], stdout);

        if (++i % 4 == 0 && str[i])
            fputc(' ', stdout);
    }
}

int invert_end(int num, int bits)
{
    int mask = 0;
    int bitval = 1;

    while (bits-- > 0)
    {
        mask |= bitval;
        bitval <<= 1;
    }

    return num ^ mask;
}