[Bioperl-l] What does Expect(2) mean in a blast result?

[Amir Karger]

> From: trutane at gmail.com [mailto:trutane at gmail.com] On Behalf 
> Of Steve Chervitz
> 
> The Bioperl blast parser should extract that value and you can obtain
> it from an HSP object, via the HSPI::n() method, documented here:
> 
> http://doc.bioperl.org/releases/bioperl-current/bioperl-live/B
io/Search/HSP/HSPI.html#POD23

As I mentioned in my email:

And does anyone know off-hand if Bioperl will tell me when situations
like this happen? I thought the Bio::Search::HSP::BlastHSP::n subroutine
would help, but I just get a bunch of empty strings for that, whether or
not there's a (2) in the Expect string. (hsp->n is empty, hsp->{"_n"} is
undef.)

And the docs for n() actually say, "This value is not defined with NCBI
Blast2 with gapping" although they don't say why. Which may explain why,
when I ran the following code on the blast result I included in my last
email, I got empty values for all of the n's. (Why is n() undefined for
gapped blast if I'm getting n's in my results from that blast?)

use warnings;
use strict;
use Bio::SearchIO;

my $blast_out = $ARGV[0];
my $in = new Bio::SearchIO(-format => 'blast',
                            -file   => $blast_out,
                            -report_type => 'tblastn');

print join("\t", qw(Qname Qstart Qend Strand Sname Sstart Send Frame N
Evalue)), "\n";
while(my $query = $in->next_result) {
    while(my $subject = $query->next_hit) {
        while (my $hsp = $subject->next_hsp) {
            print join("\t",
                $query->query_name,
                $hsp->start("query"),
                $hsp->end("query"),
                $hsp->strand("hit"),
                $subject->name,
                $hsp->start("hit"),
                $hsp->end("hit"),
                $subject->frame,
                $hsp->n,
                $hsp->evalue,
            ),"\n";
        }
    }
}

> Dave's basically correct in his explanation. It's a result of the
> application of sum statistics by the blast algorithm. You can read all
> about it in Korf et al's BLAST book. Here's the relevant section:

[snip]

Thanks,

-Amir