Extract rows for the first occurrence of a variable in a data frame

Question

I have a data frame with two variables, Date and Taxa and want to get the date for the first time each taxa occurs. There are 9 different dates and 40 different taxa in the data frame consisting of 172 rows, but my answer should only have 40 rows.

Taxa is a factor and Date is a date.

For example, my data frame (called 'species') is set up like this:

Date          Taxa 2013-07-12    A 2011-08-31    B 2012-09-06    C 2012-05-17    A 2013-07-12    C 2012-09-07    B 

and I would be looking for an answer like this:

Date          Taxa 2012-05-17    A 2011-08-31    B 2012-09-06    C 

I tried using:

t.first <-  species[unique(species$Taxa),] 

and it gave me the correct number of rows but there were Taxa repeated. If I just use unique(species$Taxa) it appears to give me the right answer, but then I don't know the date when it first occurred.

Thanks for any help.

Answer 1

t.first <- species[match(unique(species$Taxa), species$Taxa),] 

should give you what you're looking for. match returns indices of the first match in the compared vectors, which give you the rows you need.