R quantile on frequency values

I'd like to get quantiles on frequency values. For example, suppose I have data like the following:

length frequency
1      13    # There are 13 length 1 items.
2      20    # There are 20 length 2 items.
8      17
10     25
...
[10000+ more entries in file]

So I'd like to get quantiles for certain values like 0.05, 0.10, 0.50, 0.90, 0.95, 0.99. Also, I'd like to get the rank of a certain length. How can I do that on R or on Python?

via Chebli Mohamed