Objective-C IOS Xcode：无法修改NSMutableArray中的对象

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.

Answer 1

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];

Answer 2

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

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

Answer 3

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;

Answer 4

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.