英文:
Why here error comes in ternary Operator?
问题
The below statement gives an error "unexpected type"
int i = 3;
System.out.println((i == 3) ? i += 3 : i -= 3);
Why does this happen?
英文:
The below statement gives error "unexpected type"
int i = 3;
System.out.println( (i==3) ? i+=3 : i-=3);
why this happens ?
答案1
得分: 8
这是由于运算符优先级：+= 和 -= 赋值运算符的优先级低于? : 三元条件表达式运算符。
只有一种方式来解释 i+=3，因为它后面跟着一个 :，即作为三元条件表达式的第一个分支。但 -=3 是模棱两可的，根据优先级进行解析。因为三元条件表达式具有更高的优先级，所以表达式被解析为：
((3==3) ? (i+=3) : i) -= 3
这显然是无意义的，因为你不能赋值给表达式的结果。
如果添加额外的括号以提高赋值的优先级，则可以正常工作：
(3==3) ? (i+=3) : (i-=3)
括号可以省略，但出于可读性的考虑，建议使用（尽管这种情况下很难阅读）。
英文:
It's due to operator precedence: the += and -= assignment operators have lower precedence than the ? : ternary conditional expression operator.
There is only one way to interpret the i+=3 because it's followed by a :, namely, as the first branch of the ternary. But the -=3 is ambiguous and resolved according to precedence. Because the ternary has higher precedence, the expression is parsed like this:
((3==3) ? (i+=3) : i) -= 3
which is obviously nonsense, because you cannot assign to the result of an expression.
It works if you add extra parentheses to make the assignments take precedence:
(3==3) ? (i+=3) : (i-=3)
The parentheses around i+=3 are optional but recommended for readability (insofar as this thing is ever going to be readable).
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