NCSFDictionary, Mutating method sent to immutable object

Question

I have just started to jump into the realm of Objective-C and am slowly getting it all. I have been working on unarchiving a file that was a NSMutableArray and then initializing in my model with that array. The array is filled with various NSMutableDicationary's. From what I have seen it will add those dictionaries as non-mutable, so I went ahead and copied the regular and put them in a mutable and remove the old one. This solution seems to work for every instance except the very first.

I am at a loss as to why it would work for all but the first.

Here is how I am initializing it all

-(id) initWithList:(NSMutableArray *)savedList
{
    self = [super init];
    if (self)
    {
        int size=0;
        serverList=[[NSMutableArray alloc] initWithArray:savedList copyItems:YES];
        size=[serverList count];
        for(int i=0;i<size;i++)
        {
            loginList=[NSMutableDictionary dictionaryWithDictionary:[serverList objectAtIndex:i]];
            [serverList addObject:loginList];
            [serverList removeObjectAtIndex:i];
        }
    }
    return self;
}

Here is the code that is throwing the error, The value is being read off of a checkbox in a tableview and passed here to change the value.

-(void)setMount:(int)row value:(NSNumber*)boolAsNumber
{
    [[serverList objectAtIndex:row] setObject:boolAsNumber forKey:@"mountshare"];
}

Here is the error that it shows when I try and change the first element

2010-12-01 13:38:54.445 Network Share[35992:a0f] *** -[NSCFDictionary setObject:forKey:]: mutating method sent to immutable object

Thanks for your help. If there is a better way please let me know.

score 3 · Accepted Answer · answered Dec 01 '10 at 22:04

This loop code is wrong:

    size=[serverList count];
    for(int i=0;i<size;i++)
    {

        loginList=[NSMutableDictionary dictionaryWithDictionary:[serverList objectAtIndex:i]];
        [serverList addObject:loginList];
        [serverList removeObjectAtIndex:i];
    }

When you remove an object, the array is renumbered. After you've processed the 1st object at index 0, the original 2nd object is becoming the 1st object at index 0, but i is now set to index 1, which is where the original 3rd object is! This means you're only processing alternate items from the original array, and the 2nd, 4th, etc items never get swapped, and that's why you get the errors you're seeing.

One way to solve this would be to replace the "i" in the objectAtIndex: and removeObjectAtIndex: calls with "0", so you're always taking items off the front of the array.

The alternate solution would be to create a separate newServerList array and insert your new objects into that. At the end of the loop, release the old serverList and set the variable to point to newServerList.

and i answered too.. using your answer.. :) http://stackoverflow.com/questions/6100271/nsinternalinconsistencyexception-with-nsuserdefaults-when-using-method-setvalue/13952939#13952939 — Rajneesh071, Dec 19 '12 at 13:59

score 1 · Answer 2 · edited Jan 06 '14 at 03:26

Your indexes are messed up. As soon as you remove the object at index 0, the next one will take it's place and you will never replace that, because you then carry on with index 1.

{immutable0, immutable1}

i = 0: 

addObject:
{immutable0, immutable1, mutable0}

removeObjectAtIndex:
{immutable1, mutable0}

i = 1:

addObject:
{immutable0, mutable0, mutable02}

removeObjectAtIndex:
{immutable0, mutable02}

--> still got the immutable there. Remember to never remove objects from a mutable array you are looping through at the same time.

You could condense the code a bit:

NSMutableArray *serverList = [NSMutableArray arrayWithCapacity:[savedList count]];
for (NSDictionary *dictionary in savedList)
{
  mutable = [dictionary mutableCopy];
  [serverList addObject:mutable];
  [mutable release];
}

Unrelated to your problem: the argument is obviously wrong (NSMutableArray), if you expect an immutable array there; and if you create your serverList that way, there is no need for a deep copy (copyItems:YES).

NCSFDictionary, Mutating method sent to immutable object

2 Answers2