What is the best way to build a one-to-many relationship? - objective-c

Question

I have an array of Videos objects with, among other things, the properties id and tags.
I want to build a dictionary whose key is a tag and whose value is an array of id's.
For example, some Video objects might look like this:
Video{ id:1, tags:[funny,political,humor] }
Video{ id:2, tags:[political,america] }
I want the result dictionary to look like this:
VideosWithTags["funny":[1]; "political":[1,2]; "humor":[1]; "america":[2]]
Is there a standard algorithm to accomplish this?
Currently I'm doing something like this:
for (NSDictionary *video in videos)
{
NSNumber *videoId = [video objectForKey:#"id"];
NSArray *tags = [video objectForKey:#"tags"];
for (NSString *tag in tags)
{
NSMutableArray *videoIdsForTag = nil;
if ([videosAndTags objectForKey:tag] != nil) //same tag with videoIds already exists
{
videoIdsForTag = [videosAndTags objectForKey:tag];
[videoIdsForTag addObject:videoId];
//add the updated array to the tag key
[videosAndTags setValue:videoIdsForTag forKey:tag];
}
else //tag doesn't exist yet, create it and add the videoId to a new array
{
NSMutableArray *videoIds = [NSMutableArray array];
[videoIds addObject:videoId];
//add the new array to the tag key
[videosAndTags setObject:videoIds forKey:tag];
}
}
}

Answer 1

You can make this look a little cleaner by using the new literal syntax.
I think you could benefit by making your if branches do less work. e.g. You would be better of trying to retrieve the videoIds array then if it doesn't exist - create it and add it to the videosAndTags object and then the code after this point can be consistent with no duplication of logic
for (NSDictionary *video in videos) {
NSNumber *videoId = video[#"id"];
NSArray *tags = video[#"tags"];
for (NSString *tag in tags) {
NSMutableArray *videoIds = videosAndTags[tag];
if (!videoIds) {
videoIds = [NSMutableArray array];
videosAndTags[tag] = videoIds;
}
// This is the only line where I manipulate the array
[videoIds addObject:videoId];
}
}

Answer 2

NSArray* videos =
#[#{ #"id" : #1, #"tags" : #[ #"funny", #"political", #"humor" ] },
#{ #"id" : #2, #"tags" : #[ #"political", #"america" ] } ];
NSMutableDictionary* videosAndTags = [NSMutableDictionary new];
// find distinct union of tags
NSArray* tags = [videos valueForKeyPath: #"#distinctUnionOfArrays.tags"];
// for each unique tag
for( NSString* tag in tags )
{
// filter array so we only have ones that have the right tag
NSPredicate* p = [NSPredicate predicateWithFormat: #"tags contains %#", tag];
videosAndTags[ tag ] = [[videos filteredArrayUsingPredicate: p] valueForKeyPath: #"id"];
}
Here is another approach using NSPredicate and valueForKeyPath.
I don't used them often, but sometimes they can prove to be useful.
(I think they call this the Functional Programming style of things, but I am not so sure)
NSPredicate reference
Key Value Coding