[R] subset of matrix vs data frame

[Peter Dalgaard BSA]

Prof Brian D Ripley <ripley at stats.ox.ac.uk> writes:

> On Wed, 31 May 2000, Denis White wrote:
> 
> > In Splus this works,
> > 
> > > a <- data.frame(matrix(round(runif(10),0),nrow=2,ncol=5))
> > > a[a == 1] <- 2
> > 
> > but in R only if a is matrix,
> > 
> > > a[a == 1] <- 2
> > Error in [<-.data.frame(*tmp*, a == 1, value = 2) : 
> >         matrix subscripts not allowed in replacement
> > 
> > Was this a design decision?  Sorry if I missed it in
> > An Introduction to R.
> 
> S-PLUS differs from the original S here, as I understand it.
> So it was an S-PLUS design decision as I understand it. S-PLUS says:
> 
>         else if(nargs() == 3) {
> # really ambiguous, but follow common use as if list, 
> # except when one subscript is a logical matrix the shape of x, then treat
> # as if x were a matrix.

Semantically it is a rather strange thing to do since elements
in different columns of a data matrix can be of different type. 
And some really weird stuff *does* happen in Splus 3.4:

>  a<-data.frame(a=1:10,b=factor(1:10),c=I(as.character(1:10)))
> a[a==5]<-"x"
Warning messages:
  replacement values not all in levels(x): NA's generated in:
> "[<-.factor"(.A0,
        i[, k, drop = T], value = .A1)
> a        
    a  b  c 
 1  1  1 1 
 2  2  2 2 
 3  3  3 3 
 4  4  4 4 
 5  x NA x 
 6  6  6 6 
 7  7  7 7 
 8  8  8 8 
 9  9  9 9 
10 10 10 10
> a[a==4]<-"y"
Warning messages:
1: Data length is not an even multiple of group length in:
> split(Value,
        factor(col(i)[i], levels = seq(len = ncol(i))))
2: replacement values not all in levels(x): NA's generated in:
> "[<-.factor"(.\
        A0, i[, k, drop = T], value = .A1)
3: replacement values not all in levels(x): NA's generated in:
> "[<-.factor"(.\
        A0, i[, k, drop = T], value = .A1)
> a
    a  b  c 
 1  1  1 1 
 2  2  2 2 
 3  3  3 3 
 4 NA NA 4 
 5  x NA x 
 6  6  6 6 
 7  7  7 7 
 8  8  8 8 
 9  9  9 9 
10 10 10 10


-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
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
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._