Update: Currently looking into NSSET's minusSet
links: Comparing Two Arrays
Hi guys,
Could benefit from your wisdom here..
I'm using Coredata in my app, on first launch I download a data file and insert over 500 objects (each with 60 attributes) - fast, no problem.
Each subsequent launch I download an updated version of the file, from which I need to update all existing objects' attributes (except maybe 5 attributes) and create new ones for items which have been added to the downloaded file.
So, first launch I get 500 objects.. say a week later my file now contains 507 items..
I create two arrays, one for existing and one for downloaded.
NSArray *peopleArrayDownloaded = [CoreDataHelper getObjectsFromContext:#"person" :#"person_id" :YES :managedObjectContextPeopleTemp];
NSArray *peopleArrayExisting = [CoreDataHelper getObjectsFromContext:#"person" :#"person_id" :YES :managedObjectContextPeople];
If the count of each array is equal then I just do this:
NSUInteger index = 0;
if ([peopleArrayExisting count] == [peopleArrayDownloaded count]) {
NSLog(#"Number of people downloaded is same as the number of people existing");
for (person *existingPerson in peopleArrayExisting) {
person *tempPerson = [peopleArrayDownloaded objectAtIndex:index];
// NSLog(#"Updating id: %# with id: %#",existingPerson.person_id,tempPerson.person_id);
// I have 60 attributes which I to update on each object, is there a quicker way other than overwriting existing?
index++;
}
} else {
NSLog(#"Number of people downloaded is different to number of players existing");
So now comes the slow part.
I end up using this (which is tooooo slow):
NSLog(#"Need people added to the league");
for (person *tempPerson in peopeArrayDownloaded) {
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"person_id = %#",tempPerson.person_id];
// NSLog(#"Searching for existing person, person_id: %#",existingPerson.person_id);
NSArray *filteredArray = [peopleArrayExisting filteredArrayUsingPredicate:predicate];
if ([filteredArray count] == 0) {
NSLog(#"Couldn't find an existing person in the downloaded file. Adding..");
person *newPerson = [NSEntityDescription insertNewObjectForEntityForName:#"person" inManagedObjectContext:managedObjectContextPeople];
Is there a way to generate a new array of index items referring to the additional items in my downloaded file?
Incidentally, on my tableViews I'm using NSFetchedResultsController so updating attributes will call [cell setNeedsDisplay];
.. about 60 times per cell, not a good thing and it can crash the app.
Thanks for reading :)
I'll begin by saying that I'm still new to using the Core Data framework, but my guess is that your problem lies in the for loop you've posted.
If you look at your loop, each time it executes it creates a new NSPredicate object and then filters your existing array looking for matches. On a small data set this technique would work with seemingly small performance losses; however, with your large data set you will end up spending a lot of time creating NSPredicate objects that only differ in the name you've provided. I would suggest that you look at how to create a single predicate and then use variable substitution to perform the search. For information about variable use in predicates check out: http://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/CoreData/Articles/cdImporting.html#//apple_ref/doc/uid/TP40003174
As a side note, you may also consider how you've sorted your data and how you are performing the search operation. And another thing I noticed is that you don't release your NSPredicate object, so you're just tossing memory away too.
Related
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?
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.
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.
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.
I have recently started programming for the iOS Platform but now I need some help figuring out how to do 'something':
For my application I fetch some JSON data and put this data as objects into an Array
This Array is written to my own PLIST file (in the docs directory)
Now when the users starts a sync action I:
Fetch the data from the PLIST
Get the timestamp for a certain object in the Array that came from the PLIST
Use timestamp in new JSON request (for the new data)
So far so good.
Now for my (current) problem -> After receiving the new data (JSON req) I wish to update the timestamp of this 'certain' object in the array (and write this to the Plist).
Using an NSPredicate I am able to find the right set of data within the main Array (stampArr).
NSString *documentsDir = [NSHomeDirectory()stringByAppendingPathComponent:#"Documents"];
NSString *plistPath = [documentsDir stringByAppendingPathComponent:#"stamps.plist"];
NSMutableArray *stampArr = [[NSMutableArray alloc] initWithContentsOfFile:plistPath];
NSPredicate *filter = [NSPredicate predicateWithFormat:#"eventid = 1"];
NSMutableArray *filteredStampArr = [stampArr filteredArrayUsingPredicate:filter];
But now, after I update the filteredStampArr, I want to update the main Array with the data from the filtered Array.
In other words, I need to update the object from the Array with the new 'timestamp' (field of object).
I could off course use something like [stampArr addObject: [filteredStampArr copy]] after changing the filterd array but that would just create a duplicate of the information. I wish to overwrite the original object.
Somehow (I think) I need a 'pointer' that tells me the location of the data in the original array so that I can change the data directly in the main array?
(I hope my questions is clear - If not please say so)
Get the item, find it's index in stampArr and replace it with the newItem.
NSArray *filteredStampArr = [stampArr filteredArrayUsingPredicate:filter];
id item = [filteredStampArr objectAtIndex:0]; // id because the type of the item is not known
NSUInteger itemIndex = [stampArr indexOfObject:item];
[stampArr replaceObjectAtIndex:itemIndex withObject:newItem];
When you get filteredArray, you can directly update objects in it (not replace) and thay willbe uopdated in main array.
Read the API carefully!
try:
[stampArr filterUsingPredicate:];
My app uses Core Data and has an attribute called 'beenSeen'. When a user refreshes the app, all 'beenSeen' values of 1 are changed to 0. On an iPod Touch 2nd gen with over 2000 objects, refreshing takes over a minute. My code looks like this:
for (Deck *deck in self.deckArray) {
if ([deck.beenSeen isEqualToNumber:[NSNumber numberWithInt:1]]) {
[deck setBeenSeen:[NSNumber numberWithInt:0]];
[self.managedObjectContext save:&error];
}
}
I'm also considering deleting the sqlite file and having an alert ask the user to restart the app themselves. Doing that sure is a whole lot quicker than what I have now. Is there a quicker way to refresh an entity? Could I have a 'backup' entity and copy it over? Thanks for any help.
Hm. The first optimization I'd suggest would be
for (Deck *deck in self.deckArray) {
if ([deck.beenSeen isEqualToNumber:[NSNumber numberWithInt:1]]) {
[deck setBeenSeen:[NSNumber numberWithInt:0]];
}
}
[self.managedObjectContext save:&error];
I suspect it might speed things up to do one big context save, instead of 2,000 little ones.
The second suggestion would be to try getting rid of the if test – if the majority of your beenSeen values are changing from 1 to 0, and the others are already 0, then you might as well just set all of them to 0 and save the time of checking each one individually. (On the other hand, if there are 10,000 objects and you're resetting 2,000 of them, then getting rid of the test might not be optimal.)
for (Deck *deck in self.deckArray) {
[deck setBeenSeen:[NSNumber numberWithInt:0]];
}
[self.managedObjectContext save:&error];
}
The third suggestion would be to think about implementing this another way – for instance, your deck object could implement a lastSeen attribute, storing the date and time when the deck was last seen, and then instead of doing a mass reset (and writing 2,000 Core Data rows) you could just test each deck's lastSeen date and time against the timestamp of the last user refresh.
Try this, First, filter the array using a predicate:
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"beenSeen == %#",
[NSNumber numberWithInt:1]];
NSArray* filtered = [self.deckArray filteredArrayUsingPredicate:predicate];
Now set the new value:
[filtered setValue:[NSNumber numberWithInt:0] forKeyPath:#"beenSeen"];
Finally save the context:
[self.managedObjectContext save:&error];
Hope this helps :)
I´m using Core Data as local storage in my app. I´ve set it up properly and made subclasses of NSManagedObject for each entity. However, when I´m trying to insert values into my store, it only inserts the last object from my JSON feed.
res = [JSONHandler requestJSONResponse:jsonString];
shows = [res valueForKeyPath:#"Show.Name"];
NSUInteger showIndex = 0;
for(NSString *showName in shows){
showObject = [NSEntityDescription insertNewObjectForEntityForName:#"Show" inManagedObjectContext:managedObjectContext_];
showObject.name = showName;
showObject.iD = [[res valueForKeyPath:#"Show.Id"]objectAtIndex:showIndex];
showObject.desc = [[res valueForKeyPath:#"Show.Description"]objectAtIndex:showIndex];
showObject.activityType = [[res valueForKeyPath:#"Show.ActivityType"]objectAtIndex:showIndex];
showIndex++;
}
This only stores the last object from my JSON feed. Any idea why?
EDIT: It works fine when I do this:
res = [JSONHandler requestJSONResponse:jsonString];
shows = [res valueForKeyPath:#"Show.Name"];
NSUInteger index = 0;
for(NSString *showName in shows){
show = [NSEntityDescription insertNewObjectForEntityForName:#"Show" inManagedObjectContext:managedObjectContext_];
[show setValue:showName forKey:#"name"];
[show setValue:[[res valueForKeyPath:#"Show.Id"]objectAtIndex:index] forKey:#"iD"];
[show setValue:[[res valueForKeyPath:#"Show.Description"]objectAtIndex:index] forKey:#"desc"];
[show setValue:[[res valueForKeyPath:#"Show.ActivityType"]objectAtIndex:index] forKey:#"activityType"];
index++;
}
It´s basically the same thing, isn´t it? But I want to use subclasses of NSManagedObject instead of doing like I did above. Because in the snippet above show is NSManagedObject *show instead of what it should be: Show *show.
How many shows are there? You can find this by doing: NSLog(#"Number of shows: %d.", shows.count);, assuming that shows is an NSArray. It could be that your Core Data code is fine and the JSON parsing itself is at fault.
EDIT: Also, are you correctly saving the changes to the persistent store?
Usually when you see just one of several objects being saved like this, the problem is that a relationship that should be to-many is improperly set as to-one. No matter how many objects you try to add to the relationship, only the last one is set because the relationship can hold only one value.
I think in this circumstance the problem is most likely in the code of the custom subclass instead of the data model itself given that the data model works with generic NSManagedObjects.
I am working on my first iPhone application and I've hit a wall. I'm trying to develop a 'statistics' page for a three entity relationship. My entities are the following:
Department - Name, Address, Building, etc.
People - Name, Gender (BOOL), Phone, etc
If I have fetched a specific department how do I filter those results and only return people that are Male (Gender == 0)?
If I do
NSLog(#"%d", [department.people count]);
I get the correct number of people in that department so I know I'm in the neighborhood. I know I could re-fetch and modify the predicate each time but with 20+ stats in my app that seems inefficient. Thanks for any advice!
You don't need to refetch:
NSPredicate* pred = [NSPredicate predicateWithFormat:#"gender == NO"];
NSUInteger count = [[department.people filteredArrayUsingPredicate:pred] count];
NSLog(#"%lu", (unsigned long)count);
Somehow gender==NO still looks strange though ;)
If copying is too expensive, you could use enumerators instead. E.g.:
NSUInteger CountIf(NSEnumerator* en, NSPredicate* pred) {
NSUInteger count = 0;
id obj;
while (obj = [en nextObject]) {
if([pred evaluateWithObject:obj])
++count;
}
return count;
}
NSUInteger count = CountIf([department.people objectEnumerator], predicate));
... though this would be ideally moved to a suitable category as say countOfObjectsMatchingPredicate:.
You could create NSPredicates representing your different filters and use NSSet's filteredSetWithPredicate: method. The count method will give you the number of entities matching the predicate. This isn't terribly efficient because you're creating a new set for each calculation, but it may be significantly faster than fetching each time.