[R] Creating a factor from a combination of vectors

[Eric Lecoutre]

Hi Yves,

Using your objects, here is a way:


 > cascombo=do.call("paste",c(cas,sep="."))
 > factor(do.call("paste",c(df,sep=".")),levels=cascombo,labels=rownames(cas))
[1] 
Low    <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA> 
  <NA>   Medium <NA>   <NA>
[16] <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   High
Levels: Low Medium High


It uses:
? paste (sep=.) to create the combinations ie 0.0, 10.50, etc.
? do.call to invoke the paste on the columns of the data.frames
? factor specifying existing levels (only those defined by cas data.frame) 
anbd labels

Eric



At 10:12 30/11/2004, Yves Brostaux wrote:
>Dear list,
>
>Here's a little problem I already solved with my own coding style, but I 
>feel there is a more efficient and cleaner way to write it, but had no 
>success finding the "clever" solution.
>
>I want to produce a factor from a subset of the combination of two 
>vectors. I have the vectors a et b in a data-frame :
>
> > df <- expand.grid(a=c(0, 5, 10, 25, 50), b=c(0, 25, 50, 100, 200))
> > fac.df
>    a   b
>1   0   0
>2   5   0
>3  10   0
>4  25   0
>5  50   0
>6   0  25
>7   5  25
><snip>
>
>and want to create a factor which levels correspond to particular 
>combinations of a and b (let's say Low for a=0 & b=0, Medium for a=10 & 
>b=50, High for a=50 & b=200, others levels set to NA), reading them from a 
>data-frame which describes the desired subset and corresponding levels.
>
>Here's my own solution (inputs are data-frames df and cas, output is the 
>sub factor):
>
> > cas <- as.data.frame(matrix(c(0, 10,50, 0, 50, 200), 3, 
> 2,dimnames=list(c("Low", "Medium", "High"), c("a", "b"))))
> > cas
>        a   b
>Low     0   0
>Medium 10  50
>High   50 200
>
> > sub <- character(length(df$a))
> > for (i in 1:length(df$a)) {
>+   temp <- rownames(cas)[cas$a==df$a[i] & cas$b==df$b[i]]
>+   sub[i] <- ifelse(length(temp)>0, temp, NA)
>+ }
> > sub <- ordered(sub, levels=c("Low", "Medium", "High"))
> > sub
>[1] Low    <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>
><NA>   <NA>   <NA>   Medium <NA>   <NA>   <NA>   <NA> [18] 
><NA>   <NA>   <NA>   <NA>   <NA>   <NA>   <NA>   High Levels: Low < Medium 
>< High
>
>I was looking for a vectorized solution (apply style) binding data-frames 
>df and cas, but didn't succeed avoiding the for loop. Could anybody bring 
>me the ligths over the darkness of my ignorance ? Thank you very much in 
>advance.
>
>--
>Ir. Yves BROSTAUX
>Unité de Statistique et Informatique
>Faculté universitaire des Sciences agronomiques de Gembloux (FUSAGx)
>8, avenue de la Faculté
>B-5030 Gembloux
>Belgique
>Tél: +32 81 62 24 69
>Email: brostaux.y at fsagx.ac.be
>
>______________________________________________
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Eric Lecoutre
UCL /  Institut de Statistique
Voie du Roman Pays, 20
1348 Louvain-la-Neuve
Belgium

tel: (+32)(0)10473050
lecoutre at stat.ucl.ac.be
http://www.stat.ucl.ac.be/ISpersonnel/lecoutre

If the statistics are boring, then you've got the wrong numbers. -Edward 
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