与逗号运算符优先使用？ ：operator [英] Comma operator precedence while used with ? : operator

问题描述

我不知道为什么下面两个子程序的结果是不同的：

I have no idea why the result of the two sub programs below are different:

    int a , b;

    a = 13, b=12;
    (a > b)? (a++,b--):(a--,b++); // Now a is 14 and b is 11

    a = 13, b=12;
    (a > b)? a++,b-- : a--,b++;   // Now a is 14 but b is 12

但是对于这些情况，结果是相同的： / p>

However for these cases, the results are identical:

    a = 13, b=12;
    (a < b) ? a++,b-- : a--,b++; // Now a is 12 and b is 13

    a = 13, b=12;
    (a < b) ? (a++,b--) : (a--,b++); // Again a is 12 and b is 13

为什么括号对？，但对：后的语句没有区别？

Why parentheses make difference for the statement after "?", but make no difference for the statement after ":"? Do you have any idea?

推荐答案

这一个：

(a > b)? a++,b-- : a--,b++; 

等效于：

((a > b) ? (a++, b--) : a--), b++;

所以 b 总是递增，递减。没有办法解析？和：之间的逗号运算符，而不是等效于表达式中的括号。但是在：之后，未加阴影的逗号终止三元？：运算符，并保持无条件执行增量。逗号运算符的优先级非常低，非常低。

so b is always incremented and only sometimes decremented. There is no way to parse the comma operator between ? and : other than as parenthesized in the 'equivalent to' expression. But after the :, the unparenthesized comma terminates the ternary ?: operator and leaves the increment as unconditionally executed. The precedence of the comma operator is very, very low.

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