Question

What does the expression -= do?

I spent the last hour trying to fix a bug which ended up including -= in itself:

[self.myArray objectAtIndex:myButton.tag -= 1];

I’m not sure what that expression does, is it supposed to be similar to +=?

The truly mystifying part is if I NSLog() it, it borks my results:

NSLog(@"data with -=: %@", [self.myArray objectAtIndex:((UIButton *)sender).tag -= 1]);

When I comment just that line out, it works like it should. if I un-comment that line, I can’t get the index location I want. I’m not 100% sure what it does to my array, but there is no reason that I can think of that when I just log it, that it would effect other parts of the code.

Answer 1

Explained:

[self.myArray objectAtIndex:myButton.tag -= 1];

The -= operator lvalue -= rvalue is interpreted as lvalue = lvalue - rvalue. So, here your code can be written as:

[self.myArray objectAtIndex:myButton.tag = myButton.tag - 1];

An assignment statement (=), in turn, evaluates to its left side, so after decreasing myButton.tag by one, it will be passed to objectAtIndex: as if it was:

myButton.tag = myButton.tag - 1;
[self.myArray objectAtIndex:myButton.tag]; // here myButton.tag is already decreased by one