无需重写条件语句或运营商件C code的？ [英] Rewriting a piece of C code without conditional statements or operators?

问题描述

我是在技术面试，其中面试官给了我一张code这样

I was in a technical interview where the interviewer gave me a piece of code like this

  int a=1;
  a++;
  ...something..
  ...something.
  if(a==25)
   a=0;

他说，如果其他人或三元运营商如果条件我重写这个code，而不使用开关。如何可以做到？

he said me to rewrite this code without using switch,if else or ternary operator for the if condition. How it can be done?

推荐答案

这是很简单的实际上是：

It's quite simple actually:

a *= (a != 25);

这将乘以0，如果等于25，否则它将被1乘以。

This will multiply a by 0 if a is equal to 25, else it will multiply a by 1.

如果你也不许测试相等那么这里是一个完全的算术方式：

If you're also not allowed to test for equality then here is a fully arithmetic way:

unsigned int v = a - 25;

/* test for bits set and put result in lsb of v */
v |= v >> 16;
v |= v >> 8;
v |= v >> 4;
v |= v >> 2;
v |= v >> 1;

/* set a to a if lsb of v is 1, else set to 0 */
a *= v & 1;

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