[R] How to get row numbers of a subset of rows

[affy snp]

Hello list,

I read in a txt file using

<B<-read.table(file="data.snp",header=TRUE,row.names=NULL)

by specifying the row.names=NULL so that the rows are numbered.
Below is an example after how the table looks like using
<B[1:10,1:3]


      SNP                Chromosome  PhysicalPosition
1 SNP_A-1909444          1           7924293
2 SNP_A-2237149          1           8173763
3 SNP_A-4303947          1           8191853
4 SNP_A-2236359          1           8323433
5 SNP_A-2205441          1           8393263
6 SNP_A-1909445          1           7924293
7 SNP_A-2237146          2           8173763
8 SNP_A-4303946          2           8191853
9 SNP_A-2236357          2           8323433
10 SNP_A-2205442         2           8393263

I am wondering if there is a way to return the start and end row numbers
for a subset of rows.

For example, If I specify B[,2]=1, I would like to get
start=1 and end=6

if B[,2]=2, then start=7 and end=10

Is there any way in R to quickly do this?

Thanks a bunch!

Allen