[__NSArrayI replaceObjectAtIndex:withObject:]: unrecognized selector sent to instance

Question

I have a NSMutableArray, which I need to chance its values, but I have this error:
[__NSArrayI replaceObjectAtIndex:withObject:]: unrecognized selector sent to instance 0x5291db0
This is the declaration of my NSMutableArray:

NSMutableArray *selectedOptions = [NSArray arrayWithObjects:[NSNumber numberWithInteger:0], nil]; 

Then, I'm using replaceObjectAtIndex method, of this way:

[self.selectedOptions replaceObjectAtIndex:0 withObject:[NSNumber numberWithInteger:1]];

But I get, that error, and I'm using NSMutableArray.
Thanks

Answer 1

You are creating a regular non-mutable NSArray. Your code should be

NSMutableArray *selectedOptions = [NSMutableArray arrayWithObjects:[NSNumber numberWithInteger:0], nil]; 

Objective C is very dynamic, so it does not catch this mistake at compile time.

Answer 2

You need to initialize your NSMutableArray by doing

NSMutableArray *selectedOptions = [NSMutableArray alloc] init];

By initializing it with NSArray, you can no longer use the repalceObjectAtIndex:withObject: method and that's the cause of your problem.

After initializing your NSMutableArray with the line above, simply add objects to it with the addObject method.