mutating method sent to immutable object' in nsmutablearray - ios

Question

I want to remove objects from NSmutableArray can one tell me the Best way to remove from NSMutableArray
.h
#property(nonatomic,retain)NSMutableArray *arr_property;
.m
_arr_property=[[NSMutableArray alloc]init];
MTPop *lplv = [[MTPop alloc] initWithTitle:SelectProperty(APP_SHARE.language)
options:[_arr_property valueForKeyPath:#"property_list.property_type_name"]
handler:^(NSInteger anIndex) {
txt_Property.text=[[_arr_property valueForKeyPath:#"property_list.property_type_name"] objectAtIndex:anIndex];
NSLog(#"index number %ld",(long)anIndex);
remove object--->>>
NSLog(#"index number %#",[_arr_property valueForKey:#"property_list"]);
[[_arr_property valueForKeyPath:#"property_list.property_type_name"] removeObjectAtIndex:anIndex]; ////hear the app is crashing
app is crashing error iam getting is
2015-06-09 13:21:31.104 Estater[2170:62264] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '-[__NSCFArray removeObjectAtIndex:]: mutating method sent to immutable object'

Answer 1

Think about your code:
_arr_property=[[NSMutableArray alloc]init];
You now have an empty NSMutableArray. It has no elements.
[... removeObjectAtIndex:0];
What did we just say? The array has no elements. It has no element 0 - to have an element 0 it would need to have one element at least, but it doesn't. There is nothing to remove.
[_arr_property valueForKeyPath:#"property_list.property_type_name"]
That part is the weirdest, but let's carry on. When called on an array, valueForKeyPath: results in an NSArray - not an NSMutableArray. So this gives you an empty NSArray. But you cannot say removeObjectAtIndex: to an NSArray, even if it empty - it is not mutable. That's the crash you are experiencing.

Answer 2

The real error is that you are calling removeObject on an element of your NSMutableArray:
[-->[_arr_property valueForKeyPath:#"property_list.property_type_name"]<-- removeObjectAtIndex:0];
The array looks empty, but if filled with something, to remove the first element you should do instead:
[_arr_property removeObjectAtIndex:0];

Answer 3

Firstly, you cannot work with NSMutableArray for key value coding it does not support. You must better use NSMutableDictionary for it.
As dictionaries store objects based on a key, whereas arrays store objects based on an index.
You can use NSMutableDictionary like this:
NSMutableDictionary *dict = [NSMutableDictionary dictionary];
[dict setObject:something forKey:#"Some Key"];
// ... and later ...
id something = [dict objectForKey:#"Some Key"];
Secondly, valueForKeyPath: returns a value not array and valueForKey: returns array of value for the key and also that array is not mutable.
Edit:
Thirdly, after researching more on valueForKeyPath:, found its use in collection operation and syntax for using is. So, do it by changing
[_arr_property valueForKeyPath:#"property_list.property_type_name"]
To
[_arr_property valueForKeyPath:#"#property_list.property_type_name"]