[R] subsetting matrix replacement oddities

[Spencer Graves]

A subset of a data.frame is still a data.frame.  A data.frame is 
actually a list with additional attributes.  As far as I know, your 
solution, "as.matrix", is an appropriate tool to convert a data.frame to 
a matrix.  Caution may be appropropriate, however, because if the 
data.frame includes anything not numeric, the result may not be numeric, 
and the result is inconsistent between S-Plus 6.1 and R 1.7.1.  Consider 
the following: 

S-PLUS 6.1: 

 > DF <- data.frame(a = 1:2, b = letters[1:2])
 > DF2 <- data.frame(c = TRUE)
 > DF3 <- data.frame(c = TRUE, d = 2)
 > sapply(DF, class)
         a        b
 "integer" "factor"
 > as.matrix(DF)
    a   b
1 "1" "a"
2 "2" "b"
 > as.matrix(DF2)
  c
1 T
 > as.matrix(DF3)
  c d
1 1 2

##################
R 1.7.1: 

 > DF <- data.frame(a=1:2, b=letters[1:2])
 > DF2 <- data.frame(c=TRUE)
 > DF3 <- data.frame(c=TRUE, d=2)
 > sapply(DF, class)
        a         b
"integer"  "factor"
 > as.matrix(DF)
  a   b 
1 "1" "a"
2 "2" "b"
 > as.matrix(DF2)
  c    
1 "TRUE"
 > as.matrix(DF3)
  c      d 
1 "TRUE" "2"
#####################
Hope this helps.  spencer graves

Ross Boylan wrote:

>When assigning a dataframe to a subset of a matrix I got a very odd
>result.  Am I missing something, or is this a bug?  Details are below.
>
>Also, if m is defined outside of the current function, is
>m[...] <<- foo
>necessary to update it, or does regular replacement
>m[....] <- foo
>work (that is, does it update the "global" rather than creating a
>"local" that conceals it)?  I got conflicting results, which were
>tangled up with the oddity below.
>--------------------------------------------------------
>
>Browse[1]> covs
>    epilepsy other.cancer
>680        0            0
>681        0            0
>682        0            0
>683        0            1
>684        0            0
>Browse[1]> m <- matrix(NA, 5,2)
>Browse[1]> m[,1:2] <- covs
>Browse[1]> m
>[[1]]
>[1] 0 0 0 0 0
>
>[[2]]
>[1] 0 0 0 1 0
>
>[[3]]
>[1] 0 0 0 0 0
>
>[[4]]
>[1] 0 0 0 1 0
>
>[[5]]
>[1] 0 0 0 0 0
>
>[[6]]
>[1] 0 0 0 1 0
>
>[[7]]
>[1] 0 0 0 0 0
>
>[[8]]
>[1] 0 0 0 1 0
>
>[[9]]
>[1] 0 0 0 0 0
>
>[[10]]
>[1] 0 0 0 1 0
>
>Browse[1]> dim(covs)
>[1] 5 2
>Browse[1]> class(covs)
>[1] "data.frame"
>Browse[1]> class(m)
>[1] "list"
>Browse[1]> length(m)
>[1] 10
>
>Fortunately, the following seems to work as expected:
>Browse[1]> m[,1:2] <- as.matrix(covs)
>
>
>Ross Boylan                                      wk:  (415) 502-4031
>530 Parnassus Avenue (Library) rm 115-4          ross at biostat.ucsf.edu
>Dept of Epidemiology and Biostatistics           fax: (415) 476-9856
>University of California, San Francisco
>San Francisco, CA 94143-0840                     hm:  (415) 550-1062
>
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>  
>