正确的方法采取INT_MIN绝对值 [英] Correct way to take absolute value of INT_MIN

问题描述

我要在无符号执行一些算术和需要采取消极为int的绝对值，像

  do_some_arithmetic_in_unsigned_mode（INT some_signed_value）
{
   无符号整型幅度;
   INT阴性;
   如果（some_signed_value℃，）{
       幅度= 0  -  some_signed_value;
       负= 1;
    }其他{
       幅度= some_signed_value;
       负= 0;
    }
   ...略...
}
 

但INT_MIN可能是有问题的，0 - INT_MIN是UB如果符号算术执行。
什么是标准/强健/安全/高效的C ++做的这种方式？

编辑：

如果我们知道我们是在2补，也许隐式转换和显位OPS是标准？如果可能的话，我想避免这种假设。

  do_some_arithmetic_in_unsigned_mode（INT some_signed_value）
{
   无符号整型幅度= some_signed_value;
   诠释负= some_signed_value℃的;
   如果（负）{
       幅度=（〜幅度）+ 1;
    }
   ...略...
}
 

解决方案

转换从符号到无符号是明确的：你得到相应的再presentative模2 ^{的 N 的}。因此，下面会给你正确的绝对值 N ：

  INT N = / * ... * /;unsigned int类型abs_n = N＆LT; 0？ UINT_MAX  - （（无符号整数）（N））+ 1U
                           （无符号整数）（N）;
 

的更新：的作为@ aka.nice表明，我们实际上可以替换 UINT_MAX + 1U 按 0 U ：

  unsigned int类型abs_n = N＆LT; 0： - （（无符号整数）（N））：（unsigned int类型）（N）;
 

I want to perform some arithmetic in unsigned, and need to take absolute value of negative int, something like

do_some_arithmetic_in_unsigned_mode(int some_signed_value)
{
   unsigned int magnitude;
   int negative;
   if(some_signed_value<0) {
       magnitude = 0 - some_signed_value;
       negative = 1;
    } else {
       magnitude = some_signed_value;
       negative = 0;
    }
   ...snip...
}

But INT_MIN might be problematic, 0 - INT_MIN is UB if performed in signed arithmetic. What is a standard/robust/safe/efficient way to do this in C?

EDIT:

If we know we are in 2-complement, maybe implicit cast and explicit bit ops would be standard? if possible, I'd like to avoid this assumption.

do_some_arithmetic_in_unsigned_mode(int some_signed_value)
{
   unsigned int magnitude=some_signed_value;
   int negative=some_signed_value<0;
   if (negative) {
       magnitude = (~magnitude) + 1;
    }
   ...snip...
}

解决方案

Conversion from signed to unsigned is well-defined: You get the corresponding representative modulo 2^N. Therefore, the following will give you the correct absolute value of n:

int n = /* ... */;

unsigned int abs_n = n < 0 ? UINT_MAX - ((unsigned int)(n)) + 1U
                           : (unsigned int)(n);

Update: As @aka.nice suggests, we can actually replace UINT_MAX + 1U by 0U:

unsigned int abs_n = n < 0 : -((unsigned int)(n)) : (unsigned int)(n);

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