iOS frame change one property (eg width)

To answer your original question: yes, it’s possible to change just one member of a CGRect structure. This code throws no errors:

myRect.size.width = 50;

What is not possible, however, is to change a single member of a CGRect that is itself a property of another object. In that very common case, you would have to use a temporary local variable:

CGRect frameRect = self.frame;
frameRect.size.width = 50;
self.frame = frameRect;

The reason for this is that using the property accessor self.frame = ... is equivalent to [self setFrame:...] and this accessor always expects an entire CGRect. Mixing C-style struct access with Objective-C property dot notation does not work well in this case.