How to choose and use Analyzers.
Try swapping out the EasyAnalyzer in our Schema for a :
c
StandardTokenizer *tokenizer = StandardTokenizer_new();
FullTextType *type = FullTextType_new((Analyzer*)tokenizer);
perl
my $tokenizer = Lucy::Analysis::StandardTokenizer->new;
my $type = Lucy::Plan::FullTextType->new(
analyzer => $tokenizer,
);
Search for senate, Senate, and Senator before and after making the
change and re-indexing.
Under EasyAnalyzer, the results are identical for all three searches, but
under StandardTokenizer, searches are case-sensitive, and the result sets for
Senate and Senator are distinct.
EasyAnalyzer
What's happening is that is performing more aggressive
processing than StandardTokenizer. In addition to tokenizing, it's also
converting all text to lower case so that searches are case-insensitive, and
using a "stemming" algorithm to reduce related words to a common stem (senat,
in this case).
EasyAnalyzer is actually multiple Analyzers wrapped up in a single package.
In this case, it's three-in-one, since specifying a EasyAnalyzer with
language => 'en' is equivalent to this snippet creating a
:
``` c Vector analyzers = Vec_new(3); Vec_Push(analyzers, (Analyzer)StandardTokenizer_new()); Vec_Push(analyzers, (Analyzer*)Normalizer_new(NULL, true, false)); Vec_Push(analyzers, (Analyzer*)SnowStemmer_new(language));
PolyAnalyzer *analyzer = PolyAnalyzer_new(NULL, analyzers);
DECREC(analyzers);
```
perl
my $tokenizer = Lucy::Analysis::StandardTokenizer->new;
my $normalizer = Lucy::Analysis::Normalizer->new;
my $stemmer = Lucy::Analysis::SnowballStemmer->new( language => 'en' );
my $polyanalyzer = Lucy::Analysis::PolyAnalyzer->new(
analyzers => [ $tokenizer, $normalizer, $stemmer ],
);
You can add or subtract Analyzers from there if you like. Try adding a fourth Analyzer, a SnowballStopFilter for suppressing "stopwords" like "the", "if", and "maybe".
c
Vec_Push(analyzers, (Analyzer*)StandardTokenizer_new());
Vec_Push(analyzers, (Analyzer*)Normalizer_new(NULL, true, false));
Vec_Push(analyzers, (Analyzer*)SnowStemmer_new(language));
Vec_Push(analyzers, (Analyzer*)SnowStop_new(language, NULL));
perl
my $stopfilter = Lucy::Analysis::SnowballStopFilter->new(
language => 'en',
);
my $polyanalyzer = Lucy::Analysis::PolyAnalyzer->new(
analyzers => [ $tokenizer, $normalizer, $stopfilter, $stemmer ],
);
Also, try removing the SnowballStemmer.
c
Vec_Push(analyzers, (Analyzer*)StandardTokenizer_new());
Vec_Push(analyzers, (Analyzer*)Normalizer_new(NULL, true, false));
perl
my $polyanalyzer = Lucy::Analysis::PolyAnalyzer->new(
analyzers => [ $tokenizer, $normalizer ],
);
The original choice of a stock English EasyAnalyzer probably still yields the best results for this document collection, but you get the idea: sometimes you want a different Analyzer.
When the best Analyzer is no Analyzer
Sometimes you don't want an Analyzer at all. That was true for our "url" field because we didn't need it to be searchable, but it's also true for certain types of searchable fields. For instance, "category" fields are often set up to match exactly or not at all, as are fields like "last_name" (because you may not want to conflate results for "Humphrey" and "Humphries").
To specify that there should be no analysis performed at all, use StringType:
c
String *name = Str_newf("category");
StringType *type = StringType_new();
Schema_Spec_Field(schema, name, (FieldType*)type);
DECREF(type);
DECREF(name);
perl
my $type = Lucy::Plan::StringType->new;
$schema->spec_field( name => 'category', type => $type );
Highlighting up next
In our next tutorial chapter, , we'll add highlighted excerpts from the "content" field to our search results.