英文:
How does "num" become changed in this boolean so that it becomes 6?
问题
int num = 5;
if (num != 5 & num++ != 6 | (num = num--) == 6)
System.out.println("true " + num);
else
System.out.println("false " + num);
这段代码的输出是："true, 6"。我需要帮助理解 num 如何通过布尔语句计算出数字 6。
英文:
int num = 5;
if (num != 5 & num++ != 6 | (num = num--) == 6)
System.out.println("true " + num);
else
System.out.println("false " + num);
The output of this code is "true, 6". I need help on understanding how num evaluates to the number 6 through the boolean statement.
答案1
得分: 1
正如Colin在这里所提到的，实际上在这里发生了很多事情！
首先，让我来看看if条件中表达式的一半；
num != 5 & num++ != 6
这段代码首先评估num是否不等于5，即false。
接下来，评估num是否不等于6，即true（后增量）。
然后，评估按位与运算符，即false & true
使得这半个表达式的结果为false
接着，将num的值递增，从5变为6
现在来看剩余的表达式；
(num = num--) == 6
这部分表达式首先评估括号内的内容。
在这里，num--会将num减少并返回旧值，而当前值为6。
然后，将这个值再次赋给num，这是一个经典的后增量/赋值混淆（详见https://stackoverflow.com/a/24564625/11226302，有详细的解释）。
其次，评估num是否等于6，即true。
这就是num在表达式结束时被评估为6的方式。
这使得表达式的第二部分为true
在此之后，|位包含或运算符起作用，评估整个表达式，即
false | true
使其变为true。
英文:
As Colin here has put, there is actually a lot going on here!
Let me take one half of the expression in the if condition first;
num != 5 & num++ != 6
Now what this does is first evaluates that num is not equal to 5 ,ie, false
Second, evaluates that num is not equal to 6 ,ie, true (postincrement)
Third, evaluates for the bitwise AND operator ,ie, false & true
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Making the result false for this half of the expression
Forth, increments the value of num ,ie, from 5 to 6
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Now for the remaining expression;
(num = num--) == 6
This part of the expression first evaluates the bracket.
Here num-- decrements num and returns the old value which is 6 currently.
Then this value is assigned to num again it's a classic postincrement/assignment confusion (Do see https://stackoverflow.com/a/24564625/11226302 for detailed explanation)
Second, it evaluates if num is equal to 6 ,ie, true
That is how the value of num evaluates to 6 at the end of the expression.
This makes the second half of the expression true
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After this the | bitwise inclusive OR operator takes precedence and evaluates the overall expression, which is
false | true
Making it true.
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