Add object to sorted NSMutable array and answer index path - iphone

Question

I have a sorted mutable array of a class called Topic. The topics represent a an array of Publications. I present the topics in a table, and periodically fetch new publications from a web service. When a new publication arrives, I'd like to add to the table with an animation.
What's bothering me is the computational work I need to do to add into this array, and answer the correct index path. Can someone suggest a more direct way than this:
// add a publication to the topic model. if the publication has a new topic, answer
// the index path of the new topic
- (NSIndexPath *)addPublication:(Publication *)pub {
// first a search to fit into an existing topic
NSNumber *topicId = [pub valueForKey:#"topic_id"];
for (Topic *topic in self.topics) {
if ([topicId isEqualToNumber:[topic valueForKey:"id"]]) {
// this publication is part of an existing topic, no new index path
[topic addPublication:pub];
return nil;
}
}
// the publication must have a new topic, add a new topic (and therefore a new row)
Topic *topic = [[Topic alloc] initWithPublication:publication];
[self.topics addObject:topic];
// sort it into position
[self.topics sortUsingSelector:#selector(compareToTopic:)];
// oh no, we want to return an index path, but where did it sort to?
// yikes, another search!
NSInteger row = [self.topics indexOfObject:topic];
return [NSIndexPath indexPathForRow:row inSection:0];
}
// call this in a loop for all the publications I fetch from the server,
// collect the index paths for table animations
// so much computation, poor user's phone is going to melt!
There's no getting around the first search, I guess. But is there some more efficient way to add a new thing to an array, maintaining a sort and remembering where it got placed?

Answer 1

It's pretty straightforward to insert a value into a sorted list. Think about how you would insert the number "3" into the list "1, 2, 7, 9", for instance. You want to do exactly the same thing.
Loop through the array by index, using a for loop.
For each object, use compareToTopic: to compare it to the object you want to insert.
When you find the appropriate index to insert at, use -[NSArray insertObject:atIndex:] to insert it.
Then return an NSIndexPath with that index.
Edit: and, as the other answers point out, a binary search would be faster -- but definitely trickier to get right.

Answer 2

This is almost certainly not an issue; NSArrays are actually hashes, and search is a lot faster than it would be for a true array. How many topics can you possibly have anyways?
Still, if you measure the performance and find it poor, you could look into using a B-tree; Kurt Revis commented below with a link to a similar structure (a binary heap) in Core Foundation: CFBinaryHeap.
Another option (which would also need to be measured) might be to do the comparison as you walk the array the first time; you can mark the spot and do the insertion directly:
NSUInteger insertIndex = 0;
NSComparisonResult prevOrder = NSOrderedDescending;
for (Topic *topic in self.topics) {
NSComparisonResult order = [topicId compareToTopic:topic];
if (NSOrderedSame == order) {
// this publication is part of an existing topic, no new index path
[topic addPublication:pub];
return nil;
}
else if( prevOrder == NSOrderedDescending &&
order == NSOrderedAscending )
{
break;
}
insertIndex++;
prevOrder = order;
}
Please note that I haven't tested this, sorry.
I'm not sure this is actually better or faster than the way you've written it, though.
Don't worry about the work the computer is doing unless it's demonstrably doing it too slowly.

Answer 3

What you have done is correct I guess. There's another way. You can write your own binary search implementation method. (Which has only few lines of code). And you can retrieve the index where the new object should fit in. And add the new object to the required index using insertObject:atIndex: method.