[R] how to find number of unique rows for combination of r columns

[Ana Marija]

Hello,

I have a data frame like this:

> head(dt,20)
     chr    pos         gene_id pval_nominal  pval_ret       wl      wr
 1: chr1  54490 ENSG00000227232    0.6084950 0.7837780 31.62278 21.2838
 2: chr1  58814 ENSG00000227232    0.2952110 0.8975820 31.62278 21.2838
 3: chr1  60351 ENSG00000227232    0.4397880 0.8679590 31.62278 21.2838
 4: chr1  61920 ENSG00000227232    0.3195280 0.6018090 31.62278 21.2838
 5: chr1  63671 ENSG00000227232    0.2377390 0.9880390 31.62278 21.2838
 6: chr1  64931 ENSG00000227232    0.2766790 0.9070370 31.62278 21.2838
 7: chr1  81587 ENSG00000227232    0.6057930 0.6167630 31.62278 21.2838
 8: chr1 115746 ENSG00000227232    0.4078770 0.7799110 31.62278 21.2838
 9: chr1 135203 ENSG00000227232    0.4078770 0.9299130 31.62278 21.2838
10: chr1 138593 ENSG00000227232    0.8464560 0.5696060 31.62278 21.2838

it is very big,
> dim(dt)
[1] 73719122        8

To count number of unique rows for all 3 columns: chr, pos and gene_id
I could just join those 3 columns and than count. But how would I find
unique number of rows for these 4 columns without joining them?

Thanks
Ana