Objective-C IOS Xcode: Unable to modify object in NSMutableArray

Question

My problem is that I have a class property that is of type NSMutableArray, as defined in my header file, yet when I attempt to modify one of the array elements (an NSDictionary) I receive the following runtime error:

2013-01-16 14:17:20.993 debtaculous[5674:c07] * Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFArray replaceObjectAtIndex:withObject:]: mutating method sent to immutable object'

Header declaration:

//  BudgetViewController.h

#import <UIKit/UIKit.h>

@interface BudgetViewController : UIViewController <UITableViewDataSource, UITableViewDelegate>
- (IBAction)afterTaxIncomeEditingDidEnd:(id)sender;
@property (strong, nonatomic) NSMutableArray *budgetArray;
@property (strong, nonatomic) IBOutlet UITextField *afterTaxIncome;
@property (strong, nonatomic) IBOutlet UITableView *budgetTableView;

@end

Method that generates the error:

-(void)applyCCCSWeights
{
    NSMutableDictionary *valueDict;
    NSString *newAmount;

    for (id budgetElement in [self budgetArray]) {
        valueDict = [[NSMutableDictionary alloc] initWithDictionary:budgetElement];
        newAmount = [NSString stringWithFormat:@"%0.2f", [[self afterTaxIncome].text floatValue] * [[budgetElement objectForKey:@"cccs_weight"] floatValue]];
        [valueDict setValue:newAmount forKeyPath:@"amount"];

        [[self budgetArray] replaceObjectAtIndex:0 withObject:valueDict];
        NSLog(@"%0.2f (%0.2f)", [[budgetElement objectForKey:@"amount"] floatValue], [[self afterTaxIncome].text floatValue] * [[budgetElement objectForKey:@"cccs_weight"] floatValue]);
    }

    [self.budgetTableView reloadData];
}

// Note the replaceObjectAtIndex:0 above is just a placeholder. This will be replaced with the correct index.

Main parts: [self setUrlData:[NSURLConnection sendSynchronousRequest:request returningResponse:&response error:&error]]; [self setBudgetArray:[NSJSONSerialization JSONObjectWithData:[self urlData] options:kNilOptions error:&error]]; — user1984468
– user1984468, Commented Jan 16, 2013 at 20:17
There you go... Look at the options in NSJSONSerialization. Use one that creates mutable arrays and dictionaries. — gnasher729
– gnasher729, Commented Mar 17, 2014 at 0:05

Ramy Al Zuhouri · Accepted Answer · 2013-01-16 19:33:16Z

4

budgetArray is surely immutable, you have to create it mutable.

Probably you're doing something like this:

budgetArray= [NSArray arraWithObjects; obj1, obj2, nil];

And ignoring the compiler warning. Make it mutable:

budgetArray= [[NSMutableArray alloc]init];

answered Jan 16, 2013 at 19:33

Ramy Al Zuhouri

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1 Comment

user1984468 Over a year ago

I found a related Question that may be the root of the issue: stackoverflow.com/questions/9912707/…

Community · Accepted Answer · 2017-05-23 12:27:42Z

3

I'm fairly certain you cannot change a mutable object during enumeration.

This SO question may help: Setting an object during fast enumeration problems

edited May 23, 2017 at 12:27

CommunityBot

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answered Jan 16, 2013 at 19:54

sbonami

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1 Comment

Ryan Poolos Over a year ago

You're right but that isn't the exception he is receiving. So +1 for predicting his next SO question after he listens to @Ramy

ahuth · Accepted Answer · 2013-01-17 01:57:12Z

0

In your init method, put this:

budgetArray = [[NSMutableArray alloc] init];

Also, why not use dictionary and array literal syntax?

-(void)applyCCCSWeights {
    NSMutableDictionary *valueDict;
    NSString *newAmount;

    for (NSDictionary *budgetElement in [self budgetArray]) {
        valueDict = [budgetElement mutableCopy];
        newAmount = [NSString stringWithFormat:@"%0.2f", [[self afterTaxIncome].text floatValue] * [budgetElement[@"cccs_weight"] floatValue]];
        valueDict[@"amount"] = newAmount;

        _budgetArray[0] = valueDict;
        NSLog(@"%0.2f (%0.2f)", [budgetElement[@"amount"] floatValue], [[self afterTaxIncome].text floatValue] * [budgetElement[@"cccs_weight"] floatValue]);
    }

    [self.budgetTableView reloadData];
}

Notice that [[self budgetArray] replaceObjectAtIndex:0 withObject:valueDict];

becomes: _budgetArray[0] = valueDict;

answered Jan 17, 2013 at 1:57

ahuth

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2 Comments

user1984468 Over a year ago

Thanks all for the help. My final solution was to use the above method and a local "NSMutableArray *tempArray" to hold the new values and finally: self.budgetArray = [[NSMutableArray alloc] initWithArray:tempArray]; [self.budgetTableView reloadData];

gnasher729 Over a year ago

See comment above. Read the NSJSONSerialization documentation.

gnasher729 · Accepted Answer · 2014-03-17 00:03:55Z

0

You can't change an array while doing a fast iteration over the array. On the other hand, it's entirely unnecessary; the code is absolutely inefficient: Just make the elements of the array NSMutableDictionaries, then change the dictionaries directly, instead of creating a copy and then changing an element in the copy.

Noticed later that you use NSJSONSerialization; look at the flags and don't pass 0 blindly.

answered Mar 17, 2014 at 0:03

gnasher729

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