The C programing language K&R 笔记

《C程序设计语言（第2版·新版）》

2.1

编写一个程序以确定分别由signed及unsigned限定的char、short、int及long类型变量的取值范围。采用打印标准头文件中的相应值以及直接计算两种方式实现：

#include <stdio.h>
int main()
{
printf("采用直接计算的方式：\n");
//signed types
printf("signed char min = %d\n", -(char)((unsigned char)~0>>1)-1);
printf("signed char max = %d\n", (char)((unsigned char)~0>>1));
printf("signed short min = %d\n", -(short)((unsigned short)~0>>1)-1);
printf("signed short max = %d\n", (short)((unsigned short)~0>>1));
printf("signed int min = %d\n", -(int)((unsigned int)~0>>1)-1);
printf("signed int max = %d\n", (int)((unsigned int)~0>>1));
printf("signed long min = %ld\n", -(long)((unsigned long)~0>>1)-1);
printf("signed long max = %ld\n", (long)((unsigned long)~0>>1));
//unsigned types
printf("unsigned char max = %u\n", (unsigned char)~0);
printf("unsigned short max = %u\n", (unsigned short)~0);
printf("unsigned int max = %u\n", (unsigned int)~0);
printf("unsigned long max = %lu\n", (unsigned long)~0);
system("pause");
return 0;
}

小结：

　　利用按位运算符进行计算。表达式：(char)((unsigned char)~0>>1)

　　先把数字0的各个二进制位全部转换为1：取反，~0 0默认为int型，占用4个字节，存储为0x00000000 ~0的存储值为0xffffffff

　　然后，将结果值转换为unsigned char类型：(unsigned char)~0 转换为unsigned char后存储为0xff，其值为255

　　再把这个unsigned char类型值右移一位以清除符号位：(unsigned char)~0 >> 1 >>1就是除以2，存储为0111 1111, 所以为255/2 = 127

　　最后，把它转换为char类型：(char)((unsigned char)~0>>1)

　　这一系列操作的最终结果就得到了signed类型字符的最大值。