how to update NSMutableDictionary - iphone

Question

I have
NSMutableDictionary *mutDic;
its loaded with some values from other NSMutableDictionary
from an alert i am trying to update its value
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex{
[self.mutDic setValue:[[alertView textFieldAtIndex:0] text] forKey:#"lname"];
}
but i am getting this exception
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFDictionary setObject:forKey:]: mutating method sent to immutable object'
how we can update dictionary ?

Answer 1

I had the same exception before even though my dictionary was mutable. Let me explain my scenario to you, may be it will help :
I had NSMutableArray of NSMutableDictionary,
NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
dict = [array objectAtIndex:0];
[dict setObject:#"" forKey:#""]; <-- it was crashing on this line...
so I changed my code as below,
NSMutableDictionary *dict = [[NSMutableDictionary alloc] initWithDictionary:[array objectAtIndex:0]];
it worked fine :)

Answer 2

found out the exception problem, but not solved fully
on load i need to take values from other dictionary for that the method i used was wrong, i just assign oldDic to mutDic, i changed to
self.mutDic = [[[NSMutableDictionary alloc] initWithDictionary:manager.oldDic] retain];
their initialized it by
self.oldDic = [[[NSMutableDictionary alloc]initWithObjectsAndKeys:#"F Name",#"fname",#"L Name",#"lname", nil ]retain];
that solved the exception