a method for changing objects inside deeply nested dictionaries

editNestedDict

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {

    //create a example nested dictionary with just 4 levels
    NSMutableDictionary *parent = [[[NSMutableDictionary alloc] init] autorelease];
    NSDictionary *thirdChild = [NSDictionary dictionaryWithObjectsAndKeys:@"bi",@"l4",@"nar",@"l4_", nil];     
    NSDictionary *secondChild = [NSDictionary dictionaryWithObjectsAndKeys:thirdChild,@"l3",@"bar",@"l3_", nil];     
    NSDictionary *firstChild = [NSDictionary dictionaryWithObjectsAndKeys:secondChild,@"l2",secondChild,@"l2_", nil];       
    [parent setObject:firstChild forKey:@"l1"];
    [parent setObject:firstChild forKey:@"l1_"];
    
    //NSLog(@"%@",[parent description]);
    //NSLog(@"Was: %@",[[[parent objectForKey:@"l1_"] objectForKey:@"l2_"] objectForKey:@"l3_"]);
    NSLog(@"Was: %@",[parent valueForKeyPath:@"l1_.l2_.l3_"]);    
    
    //let's change the value of l1_/l2_/l3_ to baz, the goal is a one line call to a method with minimal overhead
    
    //manual method for reference, gets out of hand fast when dealing with deeply nested structures and has no type checking
    if (1 == 2) {     
        NSMutableDictionary *firstChild = [[parent objectForKey:@"l1_"] mutableCopy];
        NSMutableDictionary *secondChild = [[firstChild objectForKey:@"l2_"] mutableCopy];    
        [secondChild setObject:@"baz" forKey:@"l3_"];
        [firstChild setObject:secondChild forKey:@"l2_"];
        [secondChild release];
        [parent setObject:firstChild forKey:@"l1_"];
        [firstChild release];
    }    
 
    //proposed method    
    NSDictionary *changed_parent = [self editNestedDict:parent setObject:@"baz" forKeyHierarchy:[NSArray arrayWithObjects:@"l1_",@"l2_",@"l3_", nil]];
    
    //NSLog(@"%@",[changed_parent description]);    
    //NSLog(@"Is: %@",[[[changed_parent objectForKey:@"l1_"] objectForKey:@"l2_"] objectForKey:@"l3_"]); 
    NSLog(@"Is: %@",[changed_parent valueForKeyPath:@"l1_.l2_.l3_"]);    
	
}

-(NSDictionary*)editNestedDict:(NSDictionary*)dict setObject:(id)object forKeyHierarchy:(NSArray*)hierarchy{
    if (dict == nil) return dict;
    if (![dict isKindOfClass:[NSDictionary class]]) return dict;    
    NSMutableDictionary *parent = [[dict mutableCopy] autorelease];

    //drill down mutating each dict along the way
    NSMutableArray *structure = [NSMutableArray arrayWithCapacity:1];    
    NSMutableDictionary *prev = parent;
    for (id key in hierarchy) {
        if (key != [hierarchy lastObject]) {
            prev = [[[prev objectForKey:key] mutableCopy] autorelease];                            
            if (![prev isKindOfClass:[NSDictionary class]]) return dict;              
            [structure addObject:prev];
            NSLog(@"loading %@",key); 
        }else{
            NSLog(@"changing %@",key);
        }
    }   
        
    //do the change
    [[structure lastObject] setObject:object forKey:[hierarchy lastObject]];    
    
    //drill back up saving the changes each step along the way   
    for (int c = [structure count]-1; c >= 0; c--) {
        if (c == 0) {
            [parent setObject:[structure objectAtIndex:c] forKey:[hierarchy objectAtIndex:c]];                                
        }else{
            [[structure objectAtIndex:c-1] setObject:[structure objectAtIndex:c] forKey:[hierarchy objectAtIndex:c]];                                
        }
        NSLog(@"saving %@",[hierarchy objectAtIndex:c]);        
    }
    
    return parent;
}

A recursive version would look something like this:

-(NSDictionary*)editNestedDict:(NSDictionary*)dict setObject:(id)object forKeyHierarchy:(NSArray*)hierarchy
{
    NSMutableDictionary *newDict = [NSMutableDictionary dictionaryWithDictionary: dict];

    if([hierarchy count] > 1)
    {
        NSString *key = [hierarchy objectAtIndex: 0];
        NSString *remainder = [hierarchy subArrayWithRange: NSMakeRange(1, [hierarchy count] - 1)];

        NSDictionary *subDict = [newDict objectForKey: key];
        subDict = [self editNestedDict: subDict setObject: object forKeyHierarchy: remainder];
        [newDict setObject: subDict forKey: key];
    }
    else
        [newDict setObject: object forKey: [hierarchy lastObject]];

    return newDict;
}

NSString *remainder = [hierarchy subArrayWithRange: NSMakeRange(1, [hierarchy count] - 1)];
should probably be:
NSArray *remainder = [hierarchy subArrayWithRange: NSMakeRange(1, [hierarchy count] - 1)];