Cannot change mutable array?

Question

Ok so I have been stuck on this for a while even though it's a simple problem, I am trying to add an NSDictionary to an array however when calling the addObject method on the array the program crashes claiming I am sending a mutating method to an immutable object.

my code looks like:

    - (IBAction)btnSaveMessage:(id)sender {
        //Save The Message and Clear Text Fields

        NSMutableDictionary *newMessageDictionary = [[NSMutableDictionary alloc] init];
        [newMessageDictionary setObject:@"plist test title" forKey:@"Title"];
        [newMessageDictionary setObject:@"plist subtitle" forKey:@"Subtitle"];
        [newMessageDictionary setObject:@"-3.892119" forKey:@"Longitude"];
        [newMessageDictionary setObject:@"54.191707" forKey:@"Lattitude"];

        NSMutableArray *messagesArray =[[NSMutableArray alloc]init];

        //Load Plist into array
        NSString *messagesPath = [[NSBundle mainBundle] pathForResource:@"Messages"                 
        ofType:@"plist"];
        messagesArray = [NSArray arrayWithContentsOfFile:messagesPath];

        [messagesArray addObject:newMessageDictionary]; //this causes crash

        //write messagesarray to file

        NSString *plistPath = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,                 
            NSUserDomainMask, YES) lastObject];
        plistPath = [plistPath stringByAppendingPathComponent:@"usersMessages.plist"];
        [messagesArray writeToFile:plistPath atomically:YES];

So I understand that I am trying to add to what the compiler see's as an immutable array but I declared it as Mutable?

Whats going on? :(

digidigo · Accepted Answer · 2013-05-02 16:27:32Z

1

You are re-initializing messagesArray with

[NSArray arrayWithContentsOfFile:messagesPath]

Making it not mutable. Try:

[NSMutableArray arrayWithContentsOfFile:messagesPath];

answered May 2, 2013 at 16:27

digidigo

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1 Comment

digidigo Over a year ago

You just needed someone to be your cocker spaniel.

Pfitz · Accepted Answer · 2013-05-02 16:27:04Z

1

You are overwriting your NSMutableArray with an NSArray in this line

messagesArray = [NSArray arrayWithContentsOfFile:messagesPath];

The class method arrayWithContentsOfFile: is returning just an NSArray and not an NSMutableArray.

If you want the content of the file mutable you can do this:

NSMutableArray *messagesArray = [[NSArray arrayWithContentsOfFile:messagesPath] mutableCopy];

and you can remove the previous declaration

 NSMutableArray *messagesArray =[[NSMutableArray alloc]init];

now.

answered May 2, 2013 at 16:27

Pfitz

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