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

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.