[R] Data Frame Manipulations
michael watson (IAH-C)
michael.watson at bbsrc.ac.uk
Wed Dec 1 12:50:45 CET 2004
I have used merge() to merge two data frames, very much like performing
a SQL join. Now I want to do a few different SQL-style things and I
wondered if there were functions to do it...
Is there a "group by" style function? For example if I merge() two data
frames and end up with multiple rows for each "id", and want to take the
average of the values of a particular column? I know I can probably put
something together using merge() and by() and then munging the results
together myself, but is there something in R to perform this
The second thing I'd like to do is like a cross-tab query; that is when
after a merge() I end up with multiple rows for a particular "id", and
want to cross-tab the data so that the multiple values become columns
and I end up with one row for each "id" again e.g.
ID Val1 Val2
1 5 10
2 15 20
Thanks in advance!
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