[BioC] rtrackleyer/GenomicRanges: How can I group GRanges by metadata attributes

[chris warth [guest]]

After reading in a GTF file with rtrackler::import(), I have a GRanges object.  How can I select the ranges whose gene_id (or transcript_id) match a particular value?

> library(rtracklayer)
> gtf <- import(system.file("tests", "gtf.gff", package="rtracklayer"), asRangedData=F)

I can see the metadata with 'mcols(gtf)', and I can even see the gene_id with 'mcols(gtf)$group', 

> mcols(gtf)$group
[1] gene_id "ENSMUSG00000033501.1"; transcript_id "ENSMUST00000040592.1"; exon_id "ENSMUSE00000310143.1";
[omitted]
3 Levels: gene_id "ENSMUSG00000033501.1"; transcript_id "ENSMUST00000040592.1"; exon_id "ENSMUSE00000310143.1"; ...

but I don't see any way to utilize the name=value strings in the group column.

Do I just have to parse the values group column myself?

Thanks in advance for any help.


 -- output of sessionInfo(): 

R version 3.0.2 (2013-09-25)
Platform: x86_64-apple-darwin10.8.0 (64-bit)

locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] parallel  stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] rtracklayer_1.20.4   GenomicRanges_1.12.5 IRanges_1.18.4       BiocGenerics_0.6.0   BiocInstaller_1.10.4

loaded via a namespace (and not attached):
[1] Biostrings_2.28.0 bitops_1.0-6      BSgenome_1.28.0   RCurl_1.95-4.1    Rsamtools_1.12.4  stats4_3.0.2     
[7] tools_3.0.2       XML_3.95-0.2      zlibbioc_1.6.0   


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