please help on double NSMutableArray issue

Question

I found very strange issue on NSMutableArray. First I create 1 NSMutableArray (arrA) and add object to it. Then I create another NSMutableArray (arrB).

Then I add arrA to arrB. Finally when I update data inside arrB then arrA update also. Code like below

[arrA addObjectsFromArray:dataBack];

[arrB removeAllObjects];
[arrB addObjectsFromArray:arrA];

After this for loop I found data arrA change like arrB

for (int i = arrB.count-1; i >= 0; i--) {
    if (itemExLong > 0 && [[[arrB objectAtIndex:i] objectForKey:@"IM_QtyPriceEx"] doubleValue] > itemExLong)
    {
        itemExLong = [[[arrB objectAtIndex:i] objectForKey:@"IM_QtyPriceEx"] doubleValue] + itemExLong;
        itemTaxLong = [[[arrB objectAtIndex:i] objectForKey:@"IM_QtyTax"] doubleValue] - itemTaxLong;
        data = [arrB objectAtIndex:i];
        [data setValue:[NSString stringWithFormat:@"%0.2f",itemExLong] forKey:@"IM_PriceEx"];
        [data setValue:[NSString stringWithFormat:@"%0.2f",itemTaxLong] forKey:@"IM_QtyTax"];

        [arrB replaceObjectAtIndex:i withObject:data];
        break;
    }
} 

When I replaceObjectAtIndex, I found arrA data change. Please Help.

Answer 1

Both arrays contain pointers to same underlaying objects. So if you will change an object using a pointer in any array it will change object (not the pointer) and that object is same for both arrays. If you want to keep to different copies of objects make a copy of objects before manipulating them.

If you have custom class then you add copying functionality you can do that by implementing NSCopying protocol, to learn more about NSCopying