Insertion in one array effects other array? - ios

Question

I have 2 NSMutableArray.I am adding custom class objects into them. I have initialized them as below.
arr_post=[[NSMutableArray alloc]init];
copy_arr_user_post=[[NSMutableArray alloc]init];
I am adding the objects into them like this.
for(i=0;i<[arr_main count];i++)
{
Post *obj=[[Post alloc]init];
obj.name=#"abc";
obj.gender=#"male";
[arr_post addObject:obj];
[copy_arr_user_post addObject:obj];
}
Now when i remove object from arr_post & insert again then it is also effected in another array.
Post *post=[arr_post objectAtIndex:indexPath.row];
[arr_post removeObjectAtIndex:indexPath.row];
post.name=#"def";
[arr_post addObjectAtIndex:indexPath.row];
then def is also added in second array.

Answer 1

You misunderstand how arrays work.
Arrays store references to objects, not the objects themselves. Its a bit like keeping people's phone numbers. If I have a list of 10 phone numbers and you have the same list of phone numbers, and the person at index 1 in my list moves to a new address, when you call your person at index one and ask for their address they will give you the changed address too.
Both arrays point to the same objects.
If you change settings on one of your objects, you will see those changes if you look up the object in either array.
If you want your arrays to contain independent objects then implement NSCopying in your custom class and use code like this:
for(i=0;i<[arr_main count];i++)
{
Post *obj=[[Post alloc]init];
obj.name=#"abc";
obj.gender=#"male";
[arr_post addObject:obj];
Post objectCopy = [obj copy]; //create a copy of our object
[copy_arr_user_post addObject: objectCopy]; //insert copy into other array
}
But again, to do that you need to teach your custom Post class to conform to the NSCopying protocol so it understands the copy message. Take a look at the docs on the NSCopying protocol for more information.

Answer 2

Of course it does affect the other array because both arrays contain the same (identical) object.
In Objective-C objects are reference types. That means a pointer is stored in the arrays respectively which points to the same object.
If you want to have different objects you have to copy them for example
[copy_arr_user_post addObject:[obj copy]];

Answer 3

This happens because arrays store pointers to the data not the actual data.
if you want to have two different copies of an object you can use this method:
for(i=0;i<[arr_main count];i++)
{
Post *obj=[[Post alloc]init];
obj.name=#"abc";
obj.gender=#"male";
NSData* objectData = [NSKeyedArchiver archivedDataWithRootObject: obj];
Post* obj2= [NSKeyedUnarchiver unarchiveObjectWithData: objectData];
[arr_post addObject:obj];
[copy_arr_user_post addObject:obj2];
}
OR you can simply declare two objects:
for(i=0;i<[arr_main count];i++)
{
Post *obj1=[[Post alloc]init];
Post *obj2=[[Post alloc]init];
obj1.name=#"abc";
obj1.gender=#"male";
obj2.name=#"abc";
obj2.gender=#"male";
[arr_post addObject:obj1];
[copy_arr_user_post addObject:obj2];
}