宏交换字节的BYTE源代码 [英] Macro to swap nibble of BYTE source code

问题描述

大家好，



我在接受采访时问了一个问题。



你能不能请帮我解决这个问题。



提前感谢。



我是什么尝试过：

Hi all,

I was asked by a question in a interview.

could you please help me out in solving this question.

thanks in advance.

What I have tried:

Hi all,

I was asked by a question in a interview.

could you please help me out in solving this question.

thanks in advance.

推荐答案

宏不是一个函数。阅读两者的比较，了解差异。

A macro isnt a function. Read the comparison of both to learn the differences.

#define SWAPNIBBLES(c) ( (c << 4) + (c >> 4) } 

我不喜欢宏，因为它们不可调试，编译器可以优化函数，甚至比宏更好。使用宏编程是在80年代。

I dont like macros because they arent debuggable and the compiler can optimize functions as good or even better than macros. Using macro is programming in the 80s.

它是相似的。只需使用适当的掩码（0x000F和0xF000）和shift（12）来交换16位值的最高和最低半字节。但是不要忘记让中间位不变（也返回它们）：

It is similar. Just use the appropriate masks (0x000F and 0xF000) and shifts (12) to swap the highest and lowest nibble of a 16 bit value. But don't forget to let the middle bits untouched (return them too):

#define swap(v) ((((v) & 0x000F) << 12) | (((v) & 0xF000) >> 12) | ((v) & 0x0FF0))

因为赋值是使用宏而这些不知道传递参数的类型，它还需要添加ac得到所需的短结果并为右移转换操作数以执行无符号转移：

Because the assignment is to use a macro and these don't know about the type of the passed argument, it requires also adding a cast to get the desired short result and casting the operand for the right shift to perform an unsigned shift:

#define swap(v) ((short int)((((v) & 0x000F) << 12) | (((unsigned)(v) & 0xF000) >> 12) | ((v) & 0x0FF0)))

无论是16位还是32位还是64位处理器都可以工作

Will work whether 16 bit or 32 bit or 64 bit processor

#include<stdio.h>
#define HALFWIDTH() (sizeof(short int) << 2)
#define BITSWAP(p)     (unsigned short)(p << HALFWIDTH()) | (unsigned short)(p >> HALFWIDTH())
int main() {
  unsigned short i;
  printf("half short: %d\n", HALFWIDTH());
  printf("read the number: \n");
  scanf("%hu", &i);
  printf("shift left: %hu\n", (unsigned short)(i << HALFWIDTH()));
  printf("shift right: %hu\n", (unsigned short)(i >> HALFWIDTH()));
  printf("%hu\n", BITSWAP(i));
  return 0;
}

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