Objective-C: weird ternary behavior

Question

I have a simple function that returns a primitive sum of NSNumber s, but doesn't count them if they're -1:

- (NSInteger)total {
    return [[self obj1] integerValue] == -1 ? 0 : [[self obj1] integerValue] + 
           [[self obj2] integerValue] == -1 ? 0 : [[self obj2] integerValue] +
           [[self obj3] integerValue] == -1 ? 0 : [[self obj3] integerValue];
}

In this case total will always return 0.

But if I write it like this:

- (NSInteger)total {
    NSInteger ret = 0;
    ret += [[self obj1] integerValue] == -1 ? 0 : [[self obj1] integerValue];
    ret += [[self obj2] integerValue] == -1 ? 0 : [[self obj2] integerValue];
    ret += [[self obj3] integerValue] == -1 ? 0 : [[self obj3] integerValue];
    return ret;
}

total will return the correct value.

I don't have a preference for writing it one way over the other, but I don't see what's wrong with the first way.

Answer 1

Operator precedence. The ternary operator has the third lowest precedence in C, so the addition is evaluated before it could get to the conditional part. Use parentheses:

return ([[self obj1] integerValue] == -1 ? 0 : [[self obj1] integerValue]) + 
       ([[self obj2] integerValue] == -1 ? 0 : [[self obj2] integerValue]) +
       ([[self obj3] integerValue] == -1 ? 0 : [[self obj3] integerValue]);