Relevance the search result in Lucene - c#-4.0

Question

What I want is :
In the search method i will add an extra parameter say relevance param of type float to setup the cuttoff relevance. So lets say if the cutoff is 60% I want items that are higher than 60% relevance.
Here is current code of search :
say the search text is a
and in lucene file system i have following description:
1) abcdef
2)abc
3)abcd
for now it will fetch all the above three docuements , i want to fetch those which are that are higher than 60% relevance.
//for now i am not using the relevanceparam anywhere in the method :
public static string[] Search(string searchText,float relevanceparam)
{
//List of ID
List<string> searchResultID = new List<string>();
IndexSearcher searcher = new IndexSearcher(reader);
Term searchTerm = new Term("Text", searchText);
Query query = new TermQuery(searchTerm);
Hits hits = searcher.Search(query);
for (int i = 0; i < hits.Length(); i++)
{
float r = hits.Score(i);
Document doc = hits.Doc(i);
searchResultID.Add(doc.Get("ID"));
}
return searchResultID.ToArray();
}
Edit :
what if i set boost to my query
say : query.SetBoost(1.6);-- is this is equivalent to 60 percent?

Answer 1

You can easily do this by ignore those hits that have less than a TopDocs.MaxScore * minRelativeRelevance where minRelativeRelevance should be a value between 0 and 1.
I've modified your code to match the 3.0.3 release of Lucene.Net, and added a FieldSelector to your call to IndexSearcher.Doc to avoid loading non-required fields.
Calling Query.SetBoost(1.6) would only mean that the score calculated by that query would be boosted by 60% (multiplied with 1.6). It may change the ordering of the result if there were other queries involved (in a BooleanQuery, for example), but it wont change which results are returned.
public static String[] Search(IndexReader reader, String searchText,
Single minRelativeRelevance) {
var resultIds = new List<String>();
var searcher = new IndexSearcher(reader);
var searchTerm = new Term("Text", searchText);
var query = new TermQuery(searchTerm);
var hits = searcher.Search(query, 100);
var minScore = hits.MaxScore * minRelativeRelevance;
var fieldSelector = new MapFieldSelector("ID");
foreach (var hit in hits.ScoreDocs) {
if (hit.Score >= minScore) {
var document = searcher.Doc(hit.Doc, fieldSelector);
var hitId = document.Get("ID");
resultIds.Add(hitId);
}
}
return resultIds.ToArray();
}