一点一点地初始化错误 - 为什么不正确地设置最后两个变量的最高有效位？

Question

我写这个程序是为了锻炼自己的位运算。在下面的代码行中，我尝试将值赋给三个 short 位。然后我通过 bit_print() 函数打印它们。

        #include <stdio.h>
        #include <limits.h>

        void bit_print(short a)
        {
            int   i;
            int   n = sizeof(short) * CHAR_BIT;       /* in limits.h */
            short   mask = 1 << (n - 1);              /* mask = 100...0 */

            for (i = 1; i <= n; ++i) {
                putchar(((a & mask) == 0) ? '0' : '1');
                a <<= 1;
                if (i % CHAR_BIT == 0 && i < n)
                    putchar(' ');
            }
        }

        int main(void){
            short alice = 0, betty = 0, carol = 0;
            int x;
            char c;

            printf("Put 16 bits for Alice: ");
            for (x = 15; x >= 0; --x){
                if ((c = getchar()) == '1')
                    alice |= 1 << x;
                else
                    alice |= 0 << x;
            }
            putchar('\n');

            printf("Put 16 bits for Betty: ");
            for (x = 15; x >= 0; --x){
                if ((c = getchar()) == '1')
                    betty |= 1 << x;
                else
                    betty |= 0 << x;
            }
            putchar('\n');

            printf("Put 16 bits for Carol: ");
            for (x = 15; x >= 0; --x){
                if ((c = getchar()) == '1')
                    carol |= 1 << x;
                else
                    carol |= 0 << x;
            }
            putchar('\n');

            bit_print(alice);
            putchar('\n');
            bit_print(betty);
            putchar('\n');
            bit_print(carol);
            putchar('\n');

            return 0;
        }

出于某种原因，如果我输入 1111111111111111 三次，执行程序我得到以下输出：

Put 16 bits for Alice: 1111111111111111

Put 16 bits for Betty: 1111111111111111

Put 16 bits for Carol: 1111111111111111

11111111 11111111
01111111 11111111
10111111 11111111

如您所见，最后两个变量 betty 和 carol 的最高有效位在不应该为零的地方有零。为什么？

Answer 1

当使用 getchar() 时，你在输入结束时按下的换行符会被拾取，但你没有考虑它。

您需要阅读并丢弃每个循环末尾的换行符以说明这一点：

    #include <stdio.h>
    #include <limits.h>

    void bit_print(short a)
    {
        int   i;
        int   n = sizeof(short) * CHAR_BIT;       /* in limits.h */
        short   mask = 1 << (n - 1);              /* mask = 100...0 */

        for (i = 1; i <= n; ++i) {
            putchar(((a & mask) == 0) ? '0' : '1');
            a <<= 1;
            if (i % CHAR_BIT == 0 && i < n)
                putchar(' ');
        }
    }

    int main(void){
        short alice = 0, betty = 0, carol = 0;
        int x;
        char c;

        printf("Put 16 bits for Alice: ");
        for (x = 15; x >= 0; --x){
            if ((c = getchar()) == '1')
                alice |= 1 << x;
            else
                alice |= 0 << x;
        }
        putchar('\n');
        getchar();

        printf("Put 16 bits for Betty: ");
        for (x = 15; x >= 0; --x){
            if ((c = getchar()) == '1')
                betty |= 1 << x;
            else
                betty |= 0 << x;
        }
        putchar('\n');
        getchar();

        printf("Put 16 bits for Carol: ");
        for (x = 15; x >= 0; --x){
            if ((c = getchar()) == '1')
                carol |= 1 << x;
            else
                carol |= 0 << x;
        }
        putchar('\n');
        getchar();

        bit_print(alice);
        putchar('\n');
        bit_print(betty);
        putchar('\n');
        bit_print(carol);
        putchar('\n');

        return 0;
    }

一点一点地初始化错误 - 为什么不正确地设置最后两个变量的最高有效位？

wrong initialization of a short bit by bit - why set incorrectly the most significant bits of the last two variables?

c

bit-manipulation

bitwise-operators