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.
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.