Objective-C Find the most commonly used words in an NSString

Question

I am trying to write a method:

- (NSDictionary *)wordFrequencyFromString:(NSString *)string {}

where the dictionary returned will have the words and how often they were used in the string provided. Unfortunately, I can't seem to find a way to iterate through words in a string to analyze each one - only each character which seems like a bit more work than necessary. Any suggestions?

Answer 1

NSString has -enumerateSubstringsInRange: method which allows to enumerate all words directly, letting standard api to do all necessary stuff to define word boundaries etc:

[s enumerateSubstringsInRange:NSMakeRange(0, [s length])
                      options:NSStringEnumerationByWords
                   usingBlock:^(NSString *substring, NSRange substringRange, NSRange enclosingRange, BOOL *stop) {
                       NSLog(@"%@", substring);
                   }];

In the enumeration block you can use either NSDictionary with words as keys and NSNumber as their counts, or use NSCountedSet that provides required functionality for counts.

Answer 2

You can use componentsSeparatedByCharactersInSet: to split the string and NSCountedSet will count the words for you.

1) Split the string into words using a combination of the punctuation, whitespace and new line character sets:

NSMutableCharacterSet *separators = [NSMutableCharacterSet punctuationCharacterSet];
[separators formUnionWithCharacterSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]];

NSArray *words = [myString componentsSeparatedByCharactersInSet:separators];

2) Count the occurrences of the words (if you want to disregard capitalization, you can do NSString *myString = [originalString lowercaseString]; before splitting the string into components):

NSCountedSet *frequencies = [NSCountedSet setWithArray:words];
NSUInteger aWordCount = [frequencies countForObject:@"word"]);

If you are willing to change your method signature, you can just return the counted set.