[myLabel setText:] only works if I use an intermediate variable

Question

I am trying to set the text of a UILabel to be equal to the name of the day of the week represented in an NSDateComponents. I am using the following code:

NSDateComponents *dateComponents = [[[NSDateComponents alloc] init] autorelease];
dateComponents.weekday = 1; //Sunday
NSString * const weekdayNames[8] = {@"Array starts at 1", @"Sunday", @"Monday", @"Tuesday", @"Wednesday", @"Thursday", @"Friday", @"Saturday"};

UILabel *myLabel = [[[UILabel alloc] init] autorelease];
myLabel.text = weekdayNames[dateComponents.weekday]; //compiler error: Assignment of read-only variable 'prop.49'

I can make the code work in any of three ways:

1. Make weekdayNames not be const
2. Assign dateComponents.weekday to an intermediate int variable before using it as an array index
3. Assign weekday[dateComponents.weekday] to an intermediate NSString * variable before calling setText:

But I want to know why my code, as originally written, fails.

Answer 1

You are not initializing NSDateComponents properly, thus weekday does not return the value you expect. See the documentation.

This is now you would initialize it using the current date:

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *weekdayComponents = [gregorian components:NSWeekdayCalendarUnit fromDate:[NSDate date]];
int weekday = [weekdayComponents weekday];

Later edit: Initialization is part of the problem, to fix the compiler error change:

NSString * const weekdayNames[8] =

to

NSString const * weekdayNames[8] =

(pointer to a constant NSString).

However, when you do that you will hit a warning "passing argument 1 of 'setText:' discards qualifiers from pointer target type" because you're passing a constant pointer to a function that expects a pointer. To fix the warning you can cast the argument to setText to (NSString *).

It probably makes sense to remove the const qualifier altogether. NSStrings are already immutable in Objective-C, so they're already constants.