CODEWITHSUNDEEP
scaladictionarypattern-matching
oshai
oshai

Reputation: 15365

how can i find a key in a map based on a pattern matching in scala

I want to find a key in a map that is similar to a specific text. should I use for loop or is there a more elegant way?

Upvotes: 4

Views: 3746

Answers (4)

Alan Burlison
Alan Burlison

Reputation: 1062

It really depends on what you mean by 'similar to' and 'text'. If you are using English words you could use the Soundex algorithm (http://en.wikipedia.org/wiki/Soundex) to give you a code for each word and use that as a hash key, collecting all the words with the same Soundex value together in a list. If you want to do full text matching then it's a lot more complicated and you need to use techniques such as Inverted Indexes (http://en.wikipedia.org/wiki/Inverted_index). In that case you'd be best looking at something pre-written such as Apache Lucene (http://lucene.apache.org/java/docs/) which gives you much more besides simple inverted indexes. Lucene is written in Java, so it is directly usable from Scala.

Upvotes: 0

huynhjl
huynhjl

Reputation: 41646

The direct translation of your question is map.keys.find(_.matches(pattern)) which given a map, gets they keys and finds the first key that matches the regexp pattern.

val map = Map("abc" -> 1, "aaa" -> 2, "cba" -> 3)
map.keys.find(_.matches("abc.*"))
// Some(abc)
map.keys.find(_.matches("zyx"))
// None

A loop may be counter productive if you don't want to scan all keys.

Upvotes: 9

oxbow_lakes
oxbow_lakes

Reputation: 134270

Suppose you have some regular expression which you wish to match on:

val RMatch = """\d+:(\w*)""".r

And a function, which takes the matched group for the regex, and the value in the map

def f(s: String, v: V): A

Then you can match on the regex and collect the value of the function:

map collectFirst { case (RMatch(_), v) => f(txt, v) }

If you just wanted the value...

map collectFirst { case (RMatch(txt), v) => v }

Note: the implementation of this method effects a traversal on the map, in case that is not what you were desiring

Upvotes: 8

Diomidis Spinellis
Diomidis Spinellis

Reputation: 19345

It depends on the pattern and the map's underlying implementation. If the map is a HashMap, then it can only give you an exact match against a key, so you can't do anything better than loop over its keys. If the map is a SortedMap and you know the beginning of the text you're looking for, then you can use the range method to obtain a part of the map based on the pattern, and loop over that range.

Upvotes: 0

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