Got exception when i try to replace a Mutable Array item - objective-c

Question

i got this exception:
[__NSArrayI replaceObjectAtIndex:withObject:]: unrecognized selector sent to instance
when i try to replace a specific element by another:
EDIT:
this is my whole code:
//declaring an AppDelegate instance
AppDelegate *myAppDelegate=(AppDelegate *)[[UIApplication sharedApplication]delegate];
//get the array in which we have stored all the choosed themes
NSMutableArray *aMutableArray=myAppDelegate.themesChoosed;
for (int i=0; i<[aMutableArray count]; i++) {
NSString *str=[NSString stringWithString:[aMutableArray objectAtIndex:i]];
if ([str isEqualToString:#"B1"]) {
[aMutableArray replaceObjectAtIndex:i withObject:#"B2"];
}
}
I maked sure that the B1 element does exist in the array.

Answer 1

What is happening to your NSMuatbleArray before you get into the for loop?
Is it a property? If so, what is the property declaration? Did you use copy?
If you implement a property like this:
#property (nonatomic, copy) NSMutableArray *myArray;
...then you can run into problems like this because the synthesized setter sends copy to the array, which results in an immutable copy. If this is the case, you need to implement your own setter that calls mutableCopy on the array (or just use retain instead and design your code a little differently).
EDIT:
Based on your comments below and the updated code, I'm sure the problem must be something to do with the array on the app delegate not being mutable.
Try this:
NSMutableArray *mutableThemeseChoosed = [NSMutableArray arrayWithArray:myAppDelegate.themesChoosed];

Answer 2

I just tried your code and it works fine.
NSMutableArray *aMutableArray = [NSMutableArray arrayWithObjects:[NSString stringWithString:#"A1"],[NSString stringWithString:#"B1"],[NSString stringWithString:#"B2"],[NSString stringWithString:#"A1"],[NSString stringWithString:#"A2"],[NSString stringWithString:#"A1"],[NSString stringWithString:#"A1"], nil];
NSLog(#"%#",aMutableArray);
for (int i=0; i<[aMutableArray count]; i++) {
NSString *str=[NSString stringWithString:[aMutableArray objectAtIndex:i]];
if ([str isEqualToString:#"B1"]) {
[aMutableArray replaceObjectAtIndex:i withObject:#"B2"];
}
}
NSLog(#"%#",aMutableArray);

Answer 3

Your problem is - as the error tells - that your mutable array is a NSArray (which is not mutable)
What you get out of myAppDelegate.themesChoosed; is likely a NSArray. Try the following: NSMutableArray *aMutableArray= [NSMutableArray arrayWithArray:myAppDelegate.themesChoosed];