使用问号？设置变量时

Question

  

可能重复：
  What does the question mark and the colon (?: ternary operator) mean in objective-c?

我知道我们将oldRow设置为等于某个索引路径。我从未见过这种语法，也无法在我正在使用的书中找到解释。以下代码中?的目的是什么？这段代码到底是做什么的？

int oldRow = (lastIndexPath != nil) ? [lastIndexPath row] : -1;

Answer 1

int oldRow = (lastIndexPath != nil) ? [lastIndexPath row] : -1;

相当于：

int oldrow = 0;
if (lastIndexPath != nil)
    oldRow = [lastIndexPath row];
else 
    oldRow = -1;

该语法称为三元运算符，遵循以下语法：

condition ? trueValue : falseValue;

i.e oldRow = (if lastIndexPath is not nil ? do this : if it isn't do this);

Answer 2

这是if语句的简写。基本上它与：

相同

int oldRow;

if(lastIndexPath != nil)
{
    oldRow = [lastIndexPath row];
}
else
{
     oldRow = -1;
}

条件分配非常方便

Answer 3

此代码等于此代码

int oldRow;

if (lastIndexPath != nil)
   oldRow = [lastIndexPaht row];
else
   oldRow = -1;