Grouping and Computing

Generally, the data frame is the most useful object for storing data.  It
allows each column to have a different type (factor, numeric, ...).

This is the default object type returned from read.table(), read.csv(), etc.

If you have your data in a data file named "data.txt" in the format

Group  Value1  Value2
A	  1       2
A       1       3
B       2       3
B       1       3
C       1       1
...


you can read it into R with

data <- read.table("data.txt",header=T)

Now, to get the summary you want, you should 

  1) create a function to compute the summary for a data.frame containing
only the data for one group.  Something like

	compute.summary <- function(x)
            sum( x$Value1 / x$Value2 )

  2) Use 'split' to break the data frame into one chunk per group, and
'sapply' to call your function on each chunk:

	tmp <- split( data, data$Group )

	results <- sapply( tmp, compute.summary )

You will probably want to look at the help pages for read.table, split, and
sapply.  You should also (if you haven't already) picked up the manual 'An
Introduction to R' from http://cran.r-project.org/manuals.html

-Greg

-----Original Message-----
From: alexander.hener at gmx.de [mailto:alexander.hener at gmx.de]
Sent: Thursday, February 07, 2002 9:03 AM
To: r-help at stat.math.ethz.ch
Subject: [R] Grouping and Computing


Hi group,

To mention it in advance, I am an R newbie, and most likely, 
my question is
more a mix of smaller, simpler tasks. Anyway, I got mixed up 
between by,
select, aggregate, lapply etc.
My problem is as follows : 

I have read data in and transformed them into a matrix for no 
special reason
so far. This matrix contains a column with regard to which I 
would like to
group, i.e. one realisation specifies one group. Neither the number of
occurences nor the value of these realisations is known in 
advance, which seems to
be the mayor problem. For each group separately then, I would 
like to compute
some aggregation function, namely the sum of a fraction of 
two columns. These
sums should be kept in form of another vector. 

My two questions are then

- Which object type (matrix, dataframe, list) lends itself to such a
problem?
- Do I have to create different objects for the groups, or 
can I compute the
vector of sums directly? And how?
 
Thanks in advance

Alexander Hener

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