What is the best way to search strings for something like forum? i seen horrible string search and typically get worse the more strings you use rather then better.
I also may implement a title search so if the way to search title that is better then a body of string i'd love to hear that too
Take a look at using Lucene (Java) (or Lucene.Net) for full text search. Lucene is a text-mining API that allows you to index and search documents by title, text, author, etc. I've used a Ruby port of Lucene (Ferret or acts_as_ferret) to index a specialized mailing list and found that it works very well.
I would in most cases suggest using the "Boyer Moore" string search algorithm.
You can read about it on wikipedia:
http://en.wikipedia.org/wiki/Boyer%E2%80%93Moore_string_search_algorithm
There's also a lot of example code around to look at, if you're not interested in trying to implement it yourself.
If performance isn't important at all, I would agree with using regular expressions.
Not sure what you mean exactly by "Forum", but Regular Expressions may be a good place to start.
Too general?
Related
I'm writing a help system and I would like to implement the search in modern way, like e.g. it works now in Google search: when user only starts to type in query letters he immediately gets the result and each next letter makes the result set more specific.
What are the algorithms for indexing text and searching it this way?
Wikipedia or/and other links will be fine, no need to waste your time to detailed explanation.
As a practical approach, I think you may use some library like lucene which will help you with most of the things about full text indexing.
I have to build a search facility capable of searching members by their first name/last name and may be some other search parameters (i.e. address).
The search should provide a list of match candidates so that the user can select whatever he/she seems the "correct" match.
The search should be smart enough so that the "correct" result would be among the first few items on the list. The search should also be tolerant to typos and misspellings and, may be, even be aware of name shortcuts i.e. Bob vs. Robert or Bill vs. William.
I started investigating Lucene and the family (like elastic search) as a tool for the job. While it has an impressive array of features addressing similar problems for the full text search, I am not so sure how to use them for my task - up to the point that maybe Lucene is not the right tool here at all.
What do you guys think - how can I harness Elastic Search to solve my problem? Or should I look elsewhere?
Lucene supports edit distance queries so that your search query will tolerate some typos, you define this as the allowed edit distance for a term.
for instance:
name:johnni~0.8
would return "johnny"
Also Solr provides a wide array of ready made search filters and analyzers you can use for search.
In your case I would probably chain several filter factories together:
TrimFilterFactory - trim the query
LowerCaseFilterFactory - to get rid of case differences
ISOLatin1AccentFilterFactory - to remove accents from letters (most people don't search with the accent anyway)
PhoneticFilterFactory - for matching sounds like queries like: kris -> chris
look at the documentation under the link it is pretty straight forward how to set up a new solr instance with an Analyzer that uses all the above filters. I used something similar for searching city names and it worked fairly well.
Lucene can be made tolerant of typos and misspellings, and can use synonyms. As for
The search should be smart enough so that the "correct" result would be among the first few items on the list
Are there any search engines which don't try to do this?
As far as Bob/Robert goes, that can be done with synonyms, but you need to get the synonym data from some reliable source.
In addition to what #Asaf mentioned, you might try to use N-gram indexing to deal with spelling variants. See the CJKAnalyzer for an example of how to do that.
Google/GMail/etc. doesn't offer partial or prefix search (e.g. stuff*) though it could be very useful. Often I don't find a mail in GMail, because I don't remember the exact expression.
I know there is stemming and such, but it's not the same, especially if we talk about languages other than English.
Why doesn't Google add such a feature? Is it because the index would explode? But databases offer partial search, so surely there are good algorithms to tackle this problem.
What is the problem here?
Google doesn't actually store the text that it searches. It stores search terms, links to the page, and where in the page the term exists. That data structure is indexed in the traditional database sense. I'd bet using wildcards would make the index of the index pretty slow and as Developer Art says, not very useful.
Google does search partial words. Gmail does not though. Since you ask what's the problem here, my answer is lack of effort. This problem has a solution that enables to search in constant time and linear space but not very cache friendly: Suffix Trees. Suffix Arrays is another option that is more cache-friendly and still time efficient.
It is possible via the Google Docs - follow this article:
http://www.labnol.org/internet/advanced-gmail-search/21623/
Google Code Search can search based on regular expressions, so they do know how to do it. Of course, the amount of data Code Search has to index is tiny compared to the web search. Using regex or wildcard search in the web search would increase index size and decrease performance to impractical levels.
The secret to finding anything in Google is to enter a combination of search terms (or quoted phrases) that are very likely to be in the content you are looking for, but unlikely to appear together in unrelated content. A wildcard expression does the opposite of this. Just enter the terms you expect the wildcard to match, keeping in mind that Google will do stemming for you. Back in the days when computers ran on steam, Lycos (iirc) had pattern matching, but they turned it off several years ago. I presume it was putting too much load on their servers.
Because you can't sensibly derive what is meant with car*:
Cars?
Carpets?
Carrots?
Google's algorithms compare document texts, also external inbound links to determine what a document is about. With these wildcards all these algorithms go into junk
Can someone please let me know how do I implement "Did you mean" feature in Lucene.net?
Thanks!
You should look into the SpellChecker module in the contrib dir. It's a port of Java lucene's SpellChecker module, so its documentation should be helpful.
(From the javadocs:)
Example Usage:
import org.apache.lucene.search.spell.SpellChecker;
SpellChecker spellchecker = new SpellChecker(spellIndexDirectory);
// To index a field of a user index:
spellchecker.indexDictionary(new LuceneDictionary(my_lucene_reader, a_field));
// To index a file containing words:
spellchecker.indexDictionary(new PlainTextDictionary(new File("myfile.txt")));
String[] suggestions = spellchecker.suggestSimilar("misspelt", 5);
AFAIK Lucene supports proximity-search, meaning that if you use something like:
field:stirng~0.5
(it s a tilde-sign)
will match "string". the float is how "tolerant" the search would be, where 1.0 is exact match and 0.0 is match everything (sort of).
Different parsers will however implement this differently.
A proximity-search is much slower than a fuzzy-search (stri*) so use it with caution. In your case, one would assume that if you find no matches on a regular search, you try a proximity-search to see what you find, and present "did you mean" based on the result somehow.
Might be useful to cache this sort of lookups for very common mispellings, for performance reasons.
Google's "Did you mean?" is (probably; they're secretive, of course) implemented by consulting their query log. Look to see if people who searched for the query you're processing searched for something very similar soon after; if so, it indicates they made a mistake, and realized what they ought to be searching for.
Since you probably don't have a huge query log, you could approximate it. Take the query, split up the terms, see if there are any similar terms in the database (by edit distance, whatever); replace your terms with those nearby terms, and rerun the query. If you get more hits, that was probably a better query. Suggest it to the user. (And since you've already got the hits, and most people only look at the top 2 results, show them those.)
Take a look at google code project called semanticvectors.
There's a decent amount of discussion on the Lucene mailing lists for doing functionality like what you're after using it - however it is written in java.
You will probably have to parse and use some machine learning algorithms on your search logs to build a feature like this!
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How does the Google “Did you mean?” Algorithm work?
Suppose you have a search system already in your website. How can you implement the "Did you mean:<spell_checked_word>" like Google does in some search queries?
Actually what Google does is very much non-trivial and also at first counter-intuitive. They don't do anything like check against a dictionary, but rather they make use of statistics to identify "similar" queries that returned more results than your query, the exact algorithm is of course not known.
There are different sub-problems to solve here, as a fundamental basis for all Natural Language Processing statistics related there is one must have book: Foundation of Statistical Natural Language Processing.
Concretely to solve the problem of word/query similarity I have had good results with using Edit Distance, a mathematical measure of string similarity that works surprisingly well. I used to use Levenshtein but the others may be worth looking into.
Soundex - in my experience - is crap.
Actually efficiently storing and searching a large dictionary of misspelled words and having sub second retrieval is again non-trivial, your best bet is to make use of existing full text indexing and retrieval engines (i.e. not your database's one), of which Lucene is currently one of the best and coincidentally ported to many many platforms.
Google's Dr Norvig has outlined how it works; he even gives a 20ish line Python implementation:
http://googlesystem.blogspot.com/2007/04/simplified-version-of-googles-spell.html
http://www.norvig.com/spell-correct.html
Dr Norvig also discusses the "did you mean" in this excellent talk. Dr Norvig is head of research at Google - when asked how "did you mean" is implemented, his answer is authoritive.
So its spell-checking, presumably with a dynamic dictionary build from other searches or even actual internet phrases and such. But that's still spell checking.
SOUNDEX and other guesses don't get a look in, people!
Check this article on wikipedia about the Levenshtein distance. Make sure you take a good look at Possible improvements.
I was pleasantly surprised that someone has asked how to create a state-of-the-art spelling suggestion system for search engines. I have been working on this subject for more than a year for a search engine company and I can point to information on the public domain on the subject.
As was mentioned in a previous post, Google (and Microsoft and Yahoo!) do not use any predefined dictionary nor do they employ hordes of linguists that ponder over the possible misspellings of queries. That would be impossible due to the scale of the problem but also because it is not clear that people could actually correctly identify when and if a query is misspelled.
Instead there is a simple and rather effective principle that is also valid for all European languages. Get all the unique queries on your search logs, calculate the edit distance between all pairs of queries, assuming that the reference query is the one that has the highest count.
This simple algorithm will work great for many types of queries. If you want to take it to the next level then I suggest you read the paper by Microsoft Research on that subject. You can find it here
The paper has a great introduction but after that you will need to be knowledgeable with concepts such as the Hidden Markov Model.
I would suggest looking at SOUNDEX to find similar words in your database.
You can also access google own dictionary by using the Google API spelling suggestion request.
You may want to look at Peter Norvig's "How to Write a Spelling Corrector" article.
I believe Google logs all queries and identifies when someone makes a spelling correction. This correction may then be suggested when others supply the same first query. This will work for any language, in fact any string of any characters.
http://en.wikipedia.org/wiki/N-gram#Google_use_of_N-gram
I think this depends on how big your website it. On our local Intranet which is used by about 500 member of staff, I simply look at the search phrases that returned zero results and enter that search phrase with the new suggested search phrase into a SQL table.
I them call on that table if no search results has been returned, however, this only works if the site is relatively small and I only do it for search phrases which are the most common.
You might also want to look at my answer to a similar question:
"Similar Posts" like functionality using MS SQL Server?
If you have industry specific translations, you will likely need a thesaurus. For example, I worked in the jewelry industry and there were abbreviate in our descriptions such as kt - karat, rd - round, cwt - carat weight... Endeca (the search engine at that job) has a thesaurus that will translate from common misspellings, but it does require manual intervention.
I do it with Lucene's Spell Checker.
Soundex is good for phonetic matches, but works best with peoples' names (it was originally developed for census data)
Also check out Full-Text-Indexing, the syntax is different from Google logic, but it's very quick and can deal with similar language elements.
Soundex and "Porter stemming" (soundex is trivial, not sure about porter stemming).
There's something called aspell that might help:
http://blog.evanweaver.com/files/doc/fauna/raspell/classes/Aspell.html
There's a ruby gem for it, but I don't know how to talk to it from python
http://blog.evanweaver.com/files/doc/fauna/raspell/files/README.html
Here's a quote from the ruby implementation
Usage
Aspell lets you check words and suggest corrections. For example:
string = "my haert wil go on"
string.gsub(/[\w\']+/) do |word|
if !speller.check(word)
# word is wrong
puts "Possible correction for #{word}:"
puts speller.suggest(word).first
end
end
This outputs:
Possible correction for haert:
heart
Possible correction for wil:
Will
Implementing spelling correction for search engines in an effective way is not trivial (you can't just compute the edit/levenshtein distance to every possible word). A solution based on k-gram indexes is described in Introduction to Information Retrieval (full text available online).
U could use ngram for the comparisment: http://en.wikipedia.org/wiki/N-gram
Using python ngram module: http://packages.python.org/ngram/index.html
import ngram
G2 = ngram.NGram([ "iis7 configure ftp 7.5",
"ubunto configre 8.5",
"mac configure ftp"])
print "String", "\t", "Similarity"
for i in G2.search("iis7 configurftp 7.5", threshold=0.1):
print i[1], "\t", i[0]
U get:
>>>
String Similarity
0.76 "iis7 configure ftp 7.5"
0.24 "mac configure ftp"
0.19 "ubunto configre 8.5"
Why not use google's did you mean in your code.For how see here
http://narenonit.blogspot.com/2012/08/trick-for-using-googles-did-you-mean.html