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为什么m_array.count在for循环中用作条件时会失败？(Why does m_array.count fail when used as the conditional in a for loop? [duplicate])

这个问题在这里已有答案：

FOR命令中的表达式（for（int i = 0; i <（[arr count] -1）; i ++）{}） 3个答案

这有效：

int i, start, end; start = m_cardArr.count - 1; end = m_cardArr.count - 4; NSLog(@"%i %i", m_cardArr.count - 1, m_cardArr.count - 4); for(i = start; i > end; i--) { LabyrinthCard* labyCard = (LabyrinthCard*)[m_cardArr objectAtIndex:i]; if(labyCard.m_type != cardType || labyCard.m_usedForDoor) { return false; } }

这根本不会进入for循环：

int i; NSLog(@"%i %i", m_cardArr.count - 1, m_cardArr.count - 4); for(i = m_cardArr.count - 1; i > m_cardArr.count - 4; i--) { LabyrinthCard* labyCard = (LabyrinthCard*)[m_cardArr objectAtIndex:i]; if(labyCard.m_type != cardType || labyCard.m_usedForDoor) { return false; } }

我没有改变里面数组的大小，所以使用m_cardArr.count作为条件应该可行。 在这两种情况下，当数组中有3个元素时，NSLog会打印2，-1。

我无法弄清楚我在这里缺少什么。

This question already has an answer here:

Expression in FOR command (for (int i=0; i < ([arr count]-1);i++){}) 3 answers

This works:

int i, start, end; start = m_cardArr.count - 1; end = m_cardArr.count - 4; NSLog(@"%i %i", m_cardArr.count - 1, m_cardArr.count - 4); for(i = start; i > end; i--) { LabyrinthCard* labyCard = (LabyrinthCard*)[m_cardArr objectAtIndex:i]; if(labyCard.m_type != cardType || labyCard.m_usedForDoor) { return false; } }

This doesn't enter the for loop at all:

int i; NSLog(@"%i %i", m_cardArr.count - 1, m_cardArr.count - 4); for(i = m_cardArr.count - 1; i > m_cardArr.count - 4; i--) { LabyrinthCard* labyCard = (LabyrinthCard*)[m_cardArr objectAtIndex:i]; if(labyCard.m_type != cardType || labyCard.m_usedForDoor) { return false; } }

I am not changing the size of the array inside, so using m_cardArr.count as the conditional should work. In both cases, the NSLog prints 2, -1 when there are 3 elements in the array.

I can't figure out what I'm missing here.

最满意答案

这是因为count属性的类型为NSUInteger ，即无符号数。 当你从中减去4并且结果变为负数时，它会被解释为一个非常大的正数。

将结果分配给int ，再次将其重新解释为负数，从而解决问题。

解决这个问题的最佳方法是将-4移动到表达式的另一侧，将符号更改为+ ，如下所示：

for(i = m_cardArr.count - 1; i+4 > m_cardArr.count; i--) { ... }

表达式背后的逻辑将保持不变，但您将不再看到减法导致的有符号/无符号副作用。

This is because the count property is of type NSUInteger, i.e. an unsigned number. When you subtract 4 from it and the result becomes negative, it gets interpreted as a very large positive number.

When you assign that result to an int, you re-interpret it as a negative again, fixing the problem.

The best way of addressing this would be moving -4 to the other side of the expression, changing the sign to a +, like this:

for(i = m_cardArr.count - 1; i+4 > m_cardArr.count; i--) { ... }

The logic behind the expression would remain the same, but you would no longer see the signed/unsigned side effects caused by subtraction.