Question

This piece of code causes an error but it should be correct.

(void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    NSNumber *rowNumber = [[NSNumber alloc] initWithInt:indexPath.row];
    [genreArray addObject:rowNumber];
    [self.tableView reloadData];
}

genreArray is a NSMutableArray

But on touch on a cell I get this error

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFArray insertObject:atIndex:]: mutating method sent to immutable object'
*** First throw call stack:
(0x1c9c012 0x10d9e7e 0x1c9bdeb 0x1d1c9a5 0x1d1c8b0 0x7b4c 0xcd285 0xcd4ed 0xad75b3 0x1c5b376 0x1c5ae06 0x1c42a82 0x1c41f44 0x1c41e1b 0x1bf67e3 0x1bf6668 0x1dffc 0x271d 0x2645)
libc++abi.dylib: terminate called throwing an exception

Has anyone got an idea why this isn’t working?

Answer 1

Because your array is immutable…
Check the code where you instantiated genreArray : both type declaration, AND instantiation must be done with NSMutableArray

//valid declaration + instantiation
NSMutableArray *genreArray = [NSArray array]; 

//valid at compile-time : genreArray is supposed to be mutable
//however will result in runtime error : genreArray is actually immutable
[genreArray addObject:anObject];