[R] Aggregate: assembling the "by" part on the fly

[Petr Pikal]

On 23 Jul 2002 at 17:11, Andrew C. Ward wrote:

> Dear R users,
> 
> I'm having trouble using aggregate() and would
> greatly appreciate your advice. I am using R 1.5.1
> on Windows 2000.
> 
> I want to call my function in the following way
>  extract.data(x=dat[, "Age", "Year"])
> where extract.data() uses aggregate() to count the number
> of cases for each combination of "Age" and "Year".
> 
> I've defined extract.data() in the following way
>  extract.data <- function(x) {
>   tmp <- aggregate(x[,1], by=list(dimnames(x)[[2]]), length)
>   dimnames(tmp)[[2]] <- c(dimnames(x)[[2]], "N")
>   invisible(tmp)
>  }
> The call to aggregate obviously won't work (and doesn't).

Well, I think you are not doing what you exactly want (and what aggregate 
requires).

if your data frame looks for instance:

> daf
   ID Age Year
1   1   5 1995
2   2   6 1995
3   3   8 1995
4   4   5 1995
5   5   6 1996
6   6   7 1996
7   7   5 1996
8   8   6 1996
9   9   7 1996
10 10   5 1996

aggregate will do what you want smoothly

> aggregate(daf[,1],list(daf$Age,daf$Year),length)
  Group.1 Group.2 x
1       5    1995 2
2       6    1995 1
3       8    1995 1
4       5    1996 2
5       6    1996 2
6       7    1996 2

look at the difference in output

> list(daf$Age,daf$Year)
[[1]]
 [1] 5 6 8 5 6 7 5 6 7 5

[[2]]
 [1] 1995 1995 1995 1995 1996 1996 1996 1996 1996 1996

> list(dimnames(daf)[[2]])
[[1]]
[1] "ID"   "Age"  "Year"



> 
> I'm wondering how to assemble the "by" argument inside
> extract.data() when I wish aggregate() to use all the
> columns in my data frame.

Using all columns seems to me a liitle bit surprising. Do you mean to use also a 
column on which the aggregation is performed?

> 
> Thank you very much for your help.
> 
> 
> Regards,
> 
> Andrew C. Ward
> 
> CAPE Centre
> Department of Chemical Engineering
> The University of Queensland
> Brisbane Qld 4072 Australia
> andreww at cheque.uq.edu.au


Best regardsPetr Pikal
petr.pikal at precheza.cz
p.pik at volny.cz


-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._