？留下一前pression空当三元条件运算符的行为： [英] ?: ternary conditional operator behaviour when leaving one expression empty

问题描述

我在写一个控制台应用程序，将尝试通过试错猜一个数字，它工作得很好，所有，但它留给我觉得这件事我心不在焉地写了某一部分，

I was writing a console application that would try to "guess" a number by trial and error, it worked fine and all but it left me wondering about a certain part that I wrote absentmindedly,

在code是：

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int x,i,a,cc;
    for(;;){
    scanf("%d",&x);
    a=50;
    i=100/a;
for(cc=0;;cc++)
{
    if(x<a)
    {
        printf("%d was too big\n",a);
        a=a-((100/(i<<=1))?:1);

    }
    else if (x>a)
    {
        printf("%d was too small\n",a);
        a=a+((100/(i<<=1))?:1);

    }
    else
    {
        printf("%d was the right number\n-----------------%d---------------------\n",a,cc);
        break;
    }
}
}
return 0;
}

更具体地说是困惑我的部分是

More specifically the part that confused me is

a=a+((100/(i<<=1))?:1); 
//Code, code
a=a-((100/(i<<=1))?:1);

我用（（100 /（I＆LT;＆LT; = 1））？：1）来确保，如果 100 /（I＆LT ;＆LT; = 1）返回0（或假的）整个前pression将评估为1 （（100 /（I＆LT;＆LT; = 1））？ ：*** *** 1），和我离开的条件的一部分，如果它是真正的空（（100 /（I＆LT，将工作;＆LT; = 1 ））_this space_：？1），它似乎正常工作，但那里留下空条件的那部分的风险

I used ((100/(i<<=1))?:1) to make sure that if 100/(i<<=1) returned 0 (or false) the whole expression would evaluate to 1 ((100/(i<<=1))?:***1***), and I left the part of the conditional that would work if it was true empty ((100/(i<<=1))? _this space_ :1), it seems to work correctly but is there any risk in leaving that part of the conditional empty?

推荐答案

这是一个GNU C扩展（见的？：维基百科条目），因此便携性，你应该明确说明第二个操作数。

This is a GNU C extension (see ?: wikipedia entry), so for portability you should explicitly state the second operand.

在真的情况下，它返回条件的结果。

In the 'true' case, it is returning the result of the conditional.

下面的语句是几乎等同：

The following statements are almost equivalent:

a = x ?: y;
a = x ? x : y;

唯一的区别是在第一个声​​明中， X 总是被计算一次，而在第二个， X 将两次评估，如果这是真的。所以，唯一的区别是，当评估 X 有副作用。

The only difference is in the first statement, x is always evaluated once, whereas in the second, x will be evaluated twice if it is true. So the only difference is when evaluating x has side effects.

无论哪种方式，我认为这是一个巧妙运用语法的......如果您有任何同情那些维护您的code，你应该明确说明操作数。 ：）

Either way, I'd consider this a subtle use of the syntax... and if you have any empathy for those maintaining your code, you should explicitly state the operand. :)

在另一方面，它是一个常见的​​情况一个可爱的小把戏。

On the other hand, it's a nice little trick for a common use case.

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