# [R] For-loop

jim holtman jholtman at gmail.com
Mon Dec 20 14:30:06 CET 2010

```try this:

> # create indexing vector to select 3 adjacent columns
> indx <- sapply(seq(1, 36, 3), seq, length = 3)
> # process each row of the table
> ans <- t(apply(data, 1, function(.row){
+     # use indx to sum up the columns
+     apply(indx, 2, function(.indx){
+         sum(.row[.indx])
+     })
+ }))
> ans
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10] [,11] [,12]
[1,]   18   63  108  153  198  243  189   36   81   126   171   216
[2,]   21   66  111  156  201  246  192   39   84   129   174   219
[3,]   24   69  114  159  204  249  195   42   87   132   177   222
[4,]   27   72  117  162  207  252  198   45   90   135   180   225
[5,]   30   75  120  165  210  255  102   48   93   138   183   228

On Mon, Dec 20, 2010 at 5:48 AM, Anne-Christine Mupepele
<anne-chr.afs at web.de> wrote:
> Hi,
> I have the following problem:
>
> I have a data.frame with 36 sample sites (colums) for which I have covariates in 3 categories: Area, Month and River. Each Area consists of 3 rivers, which were sampled over 3 month. Now I want to fuse River 1-3 for one area in one month. To get a data.frame with 12 colums.
> I am trying to do a "for loop" (which may be a complicated solution, but I don't see an easier way), which is not working, apparently because a[,ij] or a[,c(i,j)] is not working as a definition of the matrix with a double condition in the colums.
> How can  I make it work or what would be an easier solution?
>
> Thank you for your help,
> Anne
>
> data=data.frame(matrix(1:99,nrow=5,ncol=36))
> colnames(data)=c(paste("plot",1:36))
> cov=data.frame(rep(1:3,12),c(rep("Jan",12),rep("Feb",12),rep("Mar",12)),rep(c(1,1,1,2,2,2,3,3,3,4,4,4),3))
> dimnames(cov)=list(colnames(data),c("River","Month","Area"))
>
> ###loop###
> a=matrix(nrow=dim(data),ncol=length(levels(factor(cov\$Month)))*length(levels(factor(cov\$Area))))
>
>  for(i in 1:length(levels(factor(cov\$Month))))
>  {
>  for(j in 1:length(levels(factor(cov\$Area))))
>  {
> a[,ij]=as.numeric(rowSums(data[,factor(cov\$Month)==levels(factor(cov\$Month))[i]&factor(cov\$Area)==levels(factor(cov\$Area))[j]]))
> }
> }
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> and provide commented, minimal, self-contained, reproducible code.
>

--
Jim Holtman
Data Munger Guru

What is the problem that you are trying to solve?

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