Best way to remove from NSMutableArray while iterating?

by

For clarity I like to make an initial loop where I collect the items to delete. Then I delete them. Here’s a sample using Objective-C 2.0 syntax:

NSMutableArray *discardedItems = [NSMutableArray array];

for (SomeObjectClass *item in originalArrayOfItems) {
    if ([item shouldBeDiscarded])
        [discardedItems addObject:item];
}

[originalArrayOfItems removeObjectsInArray:discardedItems];

Then there is no question about whether indices are being updated correctly, or other little bookkeeping details.

Edited to add:

It’s been noted in other answers that the inverse formulation should be faster. i.e. If you iterate through the array and compose a new array of objects to keep, instead of objects to discard. That may be true (although what about the memory and processing cost of allocating a new array, and discarding the old one?) but even if it’s faster it may not be as big a deal as it would be for a naive implementation, because NSArrays do not behave like “normal” arrays. They talk the talk but they walk a different walk. See a good analysis here:

The inverse formulation may be faster, but I’ve never needed to care whether it is, because the above formulation has always been fast enough for my needs.

For me the take-home message is to use whatever formulation is clearest to you. Optimize only if necessary. I personally find the above formulation clearest, which is why I use it. But if the inverse formulation is clearer to you, go for it.

More Related Contents:

  1. @class vs. #import
  2. How do I use NSTimer?
  3. How to read data from NSFileHandle line by line?
  4. How to Check if an NSDate occurs between two other NSDates
  5. NSInvocation for Dummies?
  6. Number of occurrences of a substring in an NSString?
  7. Hidden Features of Xcode
  8. @synthesize vs @dynamic, what are the differences?
  9. What exactly is a so called “Class Cluster” in Objective-C?
  10. Why are umbrella frameworks discouraged? [closed]
  11. Regular expressions in an Objective-C Cocoa application
  12. How to Write OS X Finder plugin
  13. View-based NSTableView with rows that have dynamic heights
  14. NSMutableArray – force the array to hold specific object type only
  15. filtering NSArray into a new NSArray in Objective-C
  16. Parsing XML in Cocoa
  17. Get CPU percent usage on macOS
  18. How to perform Callbacks in Objective-C
  19. Can I use NSURLCredentialStorage for HTTP Basic Authentication?
  20. Calculating Connection/Download Speed
  21. Using Swift CFunctionPointer to pass a callback to CoreMIDI API
  22. using objc_msgSend to call a Objective C function with named arguments
  23. Call instance method from class method
  24. What happens if two ObjC categories override the same method?
  25. Mac Mountain Lion send notification from CLI app
  26. What does Apple mean when they say that a NSManagedObjectContext is owned by the thread or queue that created it?
  27. NSString instance reports its class as NSCFString
  28. Xcode linker error: file too small for architecture x86_64
  29. viewDidLoad in NSViewController?
  30. Listen to a value change of my text field

Leave a Comment