[R] Selecting obs within groups defined by 2 variables

[Peter Alspach]

Tena koe Naomi

There are lots of ways to do this.  Here are a couple (note I've made a minor modification to your example):

> naomi
  C1 C2 C3
1  1  x  1
2  1  x  2
3  1  y  1
4  1  y  2
5  2  x  1
6  2  x  2
7  2  x  3
8  2  y  1
9  2  y  2

> tapply(naomi[,3], naomi[,1:2], function(x) x[length(x)])
   C2
C1  x y
  1 2 2
  2 3 2

> aggregate(naomi[,3], naomi[,1:2], function(x) x[length(x)])
  C1 C2 x
1  1  x 2
2  2  x 3
3  1  y 2
4  2  y 2

HTH ....

Peter Alspach

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Naomi Sugie
Sent: Thursday, 5 April 2012 8:21 a.m.
To: r-help at r-project.org
Subject: [R] Selecting obs within groups defined by 2 variables

Hello,

I am relatively new to R, and I am trying to select the last  
observation within a group, where the group is defined by two  
variables.  One of the variables is a date.

In the below example, C3 varies within C2, which varies within C1. I  
need to select the last observation in C3 for 4 groups (C1*C2):  1x,  
1y, 2x, and 2y.  In my real dataset, C2 is a date (mm/dd/yy)

C1	C2	C3
1	x	1
1	x	2
1	y	1
1	y	2
2	x	1
2	x	2
2	y	1
2	y	2

I have found code (from UCLA R FAQs and this list's archives) for  
selecting the last observation when a group is defined by ONE variable  
(e.g., C1):

last <-by(mydata, mydata$C1, tail, n=1)
lastd<-do.call("rbind", as.list(last))

The by function does not seem to allow two variables in the Indices  
argument:
last <-by(mydata, mydata$C1 mydata$C2, tail, n=1) THIS DOESN'T WORK

I tried creating a new variable C1*C2, but I think this is risky since  
it may not be unique depending on my values of C1 and C2 (I have a  
very large dataset)

Thank you for the help,



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