而与所用逗号操作precedence？ ：运营商 [英] Comma operator precedence while used with ? : operator

问题描述

我不知道为什么下面的两个子计划的结果是不同的：

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

然而，对于这些情况下，结果是相同的：

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

为什么括号作出声明的区别后， ，但要在的声明没有区别？：？你有什么想法？

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

这一项：

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

相当于：

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

所以 B 总是递增，只是有时递减。有没有办法来解析之间的逗号操作符 和：不是括号中的'等同于'恩等pression。但经过：中，逗号加括号终止三元：运营商和叶为无条件执行的增量。逗号操作符的precedence是非常，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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