Error when adding NSString object to my NSMutableArray

Question

I am getting this error when trying to add the NSString object named object to my array :

erminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSArrayI addObject:]: unrecognized selector sent to instance 0x7fc48ae52ba0'

Not sure why this is so. I was originally using NSArray to changed it to NSMutableArray but I am still having problems with it. Code is below:

-(NSMutableArray *)getReplyArrayForMessage: (NSString *)message {
        //get the array
        NSMutableArray *replies = [self.messageDictionary objectForKey:message];
        NSLog(@"%@",replies);
        //if no replies we init the base set
        if([replies count]==0) {
            //get the base array
            //this also works if a key just isn't in the dictonary
            return replies=[self getBaseArray];
        }
        else {
            //add the other message
            NSString *object = @"Send a different message";
            [replies addObject:object];
            return  replies;
        }
    }

If anyone could give me a pointer to why this is happening I would appreciate it. Noob here.

How did you make it mutable?

Wain
– Wain

2015-03-03 09:22:17 +00:00
Commented Mar 3, 2015 at 9:22 — Wain
– Wain, Commented Mar 3, 2015 at 9:22

trojanfoe · Accepted Answer · 2015-03-03 09:41:59Z

2

The array is immutable (NSArray), not mutable (NSMutableArray). You can create a mutable array from an immutable array using mutableCopy:

NSMutableArray *replies = [[self.messageDictionary objectForKey:message] mutableCopy];

I know this from the exception text:

-[__NSArrayI addObject:]: unrecognized selector sent to instance ...
  ^^^^^^^^^^

EDIT I've missed off some important information from my original answer.

Having used mutableCopy you now have a copy of the array and the original array (from self.messageDictionary objectForKey:message) will remain unchanged after you add your element. This is almost certainly not what you intended.

As I've mentioned in the comments below, you probably want to create a custom object to hold these details (or an array of custom objects) that should be created from the message dictionary as soon as you receive it. The effort required to create a custom object will pay for itself tenfold in the long term.

edited Mar 3, 2015 at 9:41

answered Mar 3, 2015 at 9:23

trojanfoe

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3 Comments

nburk Over a year ago

using mutableCopy is a bit more elegant than my suggestion... +1 from me :)

Kex Over a year ago

Thanks for that mate. I assumed that because replies was a mutable array that even when I assigned it above it would still be mutable. If I am pointing any NSMutableArray to another array it will always change to an immutable unless I use mutableCopy? (works now by the way)

trojanfoe Over a year ago

@Kex You cannot tell without knowing beforehand, and casting will only keep the compiler happy, not the runtime. If you are running into these kinds of issues then it's a sign that something is wrong. In your case I would guess that you are getting the dictionary from a message (JSON, I assume) and simply keeping hold of it, when what you should be doing is converting the dictionary to a custom class instance (or array of custom class instances) where you have full control. This additional effort will pay dividends in the long run.

nburk · Accepted Answer · 2015-03-03 09:26:51Z

You are trying to add an NSString to an NSArray (which looks like an NSMutableArray) since you initialized it as one. This means that the object that you store in your messageDictionary is actually of type NSArray, and not NSMutabelArray. So, assigning it to an object of type NSMutableArray won't actually change its type and make it mutable.

It's an easy fix:

NSMutableArray *replies = [[NSMutableArray alloc] initWithArray:(NSArray *) [self.messageDictionary objectForKey:message]];

Or you can go with mutableCopy (as suggested by @trojanfoe) which is a shortcut but will lead to the same result.

I can tell because the error message says -[__NSArrayI addObject:]: unrecognized selector sent to instance 0x7fc48ae52ba0'. Notice the I at the end of __NSArrayI, this means that this array is actually immutable, so you can't add objects to it.

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