What is the best way to sort an array of strings by frequency in objective c? Also, if there is a tie between frequency, I would like to sort by alphabetical order. And I want to remove duplicates.
I can think of ways to do this, just not sure what the most optimal way is.
["b","a","b","c","b"] -> ["b","a","c"]
It sounds like what you want is NSOrderedSet. That will enforce zero duplicates, and will maintain an order - this means that you could sort it and it will function similarly to NSArray.
Of course, you would need NSMutableOrderedSet to sort in-place. Here is a link to the docs.
https://developer.apple.com/library//ios/documentation/Foundation/Reference/NSMutableOrderedSet_Class/index.html#//apple_ref/swift/cl/c:objc(cs)NSMutableOrderedSet
Edit: All that would be required for you is to implement - (void)sortUsingComparator:(NSComparator)cmptr and give it a way to determine order.
Your proposed solution is ok, but rather than iterate the dictionary yourself you can use one of the built in sort methods and use the counts in the dictionary to decide on the sort result. If the counts of the two items your function is provided with match then you can apply your secondary sorting logic.
Ok, this is my solution to solve this problem:
-(NSMutableArray *)sortByFreq : (NSArray *)inArray { //first calculate counts NSMutableDictionary *d = [NSMutableDictionary new]; for (NSString *s in inArray) { if (![d objectForKey:s]) { [d setValue:[NSNumber numberWithInt:1] forKey:s]; }else{ [d setValue:[NSNumber numberWithInt:[[d objectForKey:s] intValue]+1] forKey:s]; } } //create help array with keys to sort NSMutableDictionary *d2 = [NSMutableDictionary new]; for (NSString *key in d) { [d2 setValue:key forKey:[NSString stringWithFormat:@"%@_%@", [d objectForKey:key], key]]; } //sort keys NSArray *sortedArray = [[d2 allKeys] sortedArrayUsingComparator: ^NSComparisonResult(id obj1, id obj2){ NSArray *a1 = [obj1 componentsSeparatedByString:@"_"]; NSArray *a2 = [obj2 componentsSeparatedByString:@"_"]; //compare if ([[a1 objectAtIndex:0] isEqualToString:[a2 objectAtIndex:0]]) { return [[a1 objectAtIndex:1] compare:[a2 objectAtIndex:1]]; } return [obj2 compare:obj1]; }]; //create array to output NSMutableArray *outputArray = [NSMutableArray new]; for (NSString *key in sortedArray) { [outputArray addObject:[d2 objectForKey:key]]; } return outputArray; } i hope this help.
You can call it like this:
NSLog(@"%@", [self sortByFreq:[[NSArray alloc] initWithObjects:@"c", @"a", @"b", @"c", @"b", nil]]); NSLog(@"%@", [self sortByFreq:[[NSArray alloc] initWithObjects:@"c", @"c", @"c", @"a", @"b", nil]]); NSLog(@"%@", [self sortByFreq:[[NSArray alloc] initWithObjects:@"b", @"a", @"a", @"a", @"c", nil]]); NSLog(@"%@", [self sortByFreq:[[NSArray alloc] initWithObjects:@"c", @"c", @"b", @"b", @"a", nil]]); output will be:
(b,c,a) (c,a,b) (a,b,c) (b,c,a) 