title: object at index updates whole nsmutablearray
body: <p>I have an NSMutableArray with multiple products. Each product has an amount. I want to update the amount of the associated product when a stepper is clicked. 
  But all my products (whole NSMutableArray) are updated with the amount of the stepper.</p>
  
  <pre><code>    NSInteger index = stepper.tag;
  
      Product *p = [products objectAtIndex:index];
      p.amount = [NSNumber numberWithDouble:stepper.value];
     //    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:[NSString stringWithFormat:@"%d", stepper.tag] message:@"test" delegate:nil cancelButtonTitle:@"ok"otherButtonTitles:nil];
     //    [alert show];
  
     for (Product *p in products) {
         UIAlertView *alert = [[UIAlertView alloc] initWithTitle:[NSString stringWithFormat:@"%@", p.amount] message:p.name delegate:nil cancelButtonTitle:@"ok"otherButtonTitles:nil];
         [alert show];
     }
  </code></pre>
  
  <p>Does anyone have a solution?</p>