NSUserDefaults-Mutable method sent to immutable object

As the documentation for NSUserDefaults says: "Values returned from NSUserDefaults are immutable, even if you set a mutable object as the value." Whenever you want to change a collection you get from NSUserDefaults you have to get the imutable version, make a mutableCopy, modify that, and set it back again.

For E.g:

   NSMutableArray *placesT= nil;
   NSrray *tempArray = [[NSUserDefaults standardUserDefaults] objectForKey:@"placesT"];

if (tempArray) {
    placeT = [tempArray mutableCopy];
} else {
    placesT=[[NSMutableArray alloc] init];
}

[placesT addObject: [NSString stringWithFormat:@"%@", tagF.text] ];

NSUserDefaults *tUD=[NSUserDefaults standardUserDefaults];
[tUD setObject:placesT forKey:@"placesT"];
[tUD synchronize];