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Grouping and Computing

3 messages · alexander.hener@gmx.de, Kaspar Pflugshaupt, Joerg Maeder

Original
alexander.hener@gmx.de
# 
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
Kaspar Pflugshaupt
#
On 7.2.2002 15:03 Uhr, alexander.hener at gmx.de wrote:

            

      
      
      
    

        
It's probably easiest with a dataframe, but you can also use a matrix

            

      
      
      
    

        
Do it directly, by all means. You can use any of tapply(), by() or
aggregate():

            

      
      
      
    

        
a           b
1  a  0.23158790
2  a -0.38852120
    [snip]
9  b -1.81645407
10 b -0.44034004

            

      
      
      
    

        
a        b 
1.282057 1.511260

            

      
      
      
    

        
INDICES: a
[1] 1.282057
------------------------------------------------------------
INDICES: b
[1] 1.51126

            

      
      
      
    

        
Group.1        x
1       a 1.282057
2       b 1.511260


See the functions' help texts and examples for further information. For me,
tapply() does all I need.

Cheers

Kaspar Pflugshaupt

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Joerg Maeder
    

    

      
      #
    

  


  

      
Hello Alexander,

the function you are looking for is tapply and it works with dataframes
and matrixs.

here a small example
d <- data.frame(group=c(1,4,5,4,5,2,1),value=c(1,54,2,6,87,4,6))
tapply(d$value,d$group,mean)#arguments: the datas, the groups, the
function

will produce
   1    2    4    5 
 3.5  4.0 30.0 44.5 

you can also use factors to define the group, this has the advantage,
that you can give them a real name
d <-
data.frame(group=c('John','Jack','Joan','Jack','Jack','Joan','John'),value=c(1,54,2,6,87,4,6))
tapply(d$value,d$group,mean)

will produce
Jack Joan John 
49.0  3.0  3.5

        
alexander.hener at gmx.de wrote: