[R] Converting a matrix to a dataframe: how to prevent conversion to factor

[Peter Dalgaard]

Dennis Fisher <fisher at plessthan.com> writes:

> Colleages
> 
> I am running R 2.1.0 on a Mac (same problem occurs in Linux).  In  
> some situations, I have mixed text/numeric data that is stored as  
> characters in a matrix.  If I convert this matrix to a dataframe, the  
> numeric data becomes factors, not what I intend.
> 
>      TEXT    <- paste("Text", 1:4, sep="")
>      NUMBERS    <- 10 + 4:1
>      MATRIX    <- cbind(TEXT, NUMBERS)
>      FRAME    <- as.data.frame(MATRIX)
> 
>  > str(FRAME)
> `data.frame':    4 obs. of  2 variables:
> $ TEXT   : Factor w/ 4 levels "Text1","Text2",..: 1 2 3 4
> $ NUMBERS: Factor w/ 4 levels "11","12","13",..: 4 3 2 1
> 
> One work-around is to write the matrix (or the dataframe) to a file,  
> then read the file back using the as.is argument.
>      write.table(MATRIX, "JUNK", row.names=F)
>      NEWFRAME    <- read.table("JUNK", as.is=T, header=T)
> 
>  > str(NEWFRAME)
> `data.frame':    4 obs. of  2 variables:
> $ TEXT   : chr  "Text1" "Text2" "Text3" "Text4"
> $ NUMBERS: int  14 13 12 11
> 
> This restores the NUMBERS to their intended mode (integers, not  
> factors).  The text column is also not read as a factor (not a  
> problem for me).
> 
> It appears that the function AsIs [I(x)] would enable me to  
> accomplish this without the write/read steps.  However, it is not  
> obvious to me how to implement I(x).  Can anyone advise?

I don't think that is going to help.... 

There are really several issues here: Your numeric column was
converted to character by the cbind, using as.data.frame(I(MATRIX))
will not split it into individual columns, and things like
apply(MATRIX,2,f) may do the right thing to begin with, but then
there's coercion due to an implicit cbind at the end.

It's a bit awkward, but this may do it:

> FRAME <- as.data.frame(lapply(split(MATRIX,col(MATRIX)),type.convert))
> names(FRAME) <- colnames(MATRIX)
> str(FRAME)
`data.frame':   4 obs. of  2 variables:
 $ TEXT   : Factor w/ 4 levels "Text1","Text2",..: 1 2 3 4
 $ NUMBERS: int  14 13 12 11

whereas this isn't right:

> str(apply(MATRIX,2,type.convert))
 int [1:4, 1:2] 1 2 3 4 14 13 12 11
 - attr(*, "dimnames")=List of 2
  ..$ : NULL
  ..$ : chr [1:2] "TEXT" "NUMBERS"



-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907