R help - Apr 2005 - Data frames

I have two questions about data frames:

(1) How can one extract a simple matrix
from a data frame? I tried

      Matrixfromdf = function (frame,without=1)
      {a=frame[colnames(frame)[-without]]
      v=unlist(a,use.names=F)
      matrix(v,ncol=ncol(a))}

but it works well only for without=1,
perhaps also because the function in (2)
gives probably a different meaning to
the first column.

(2) How does one define a void data frame
with only column names but no values?
I tried this indirect way:

     # Void df with titles from ...
     Newvoid = function (...)
     {a=c(...); m=length(a)
     titles=paste(a,collapse=' ')
     conn=textConnection(titles)
     tab=read.table(conn,header=T)
     close(conn); tab}

Thanks
Josef Eschgf??ller

-- 
Josef Eschgf??ller
Dipartimento Matematico
Universita' di Ferrara
http://felix.unife.it

> I have two questions about data frames:
> 
> (1) How can one extract a simple matrix
> from a data frame? I tried
> 
>       Matrixfromdf = function (frame,without=1)
>       {a=frame[colnames(frame)[-without]]
>       v=unlist(a,use.names=F)
>       matrix(v,ncol=ncol(a))}
> 
> but it works well only for without=1,
> perhaps also because the function in (2)
> gives probably a different meaning to
> the first column.

?as.matrix
data(iris)
as.matrix(iris[,-5]) # numeric
as.matrix(iris[,-1]) # character

> (2) How does one define a void data frame
> with only column names but no values?
> I tried this indirect way:
> 
>      # Void df with titles from ...
>      Newvoid = function (...)
>      {a=c(...); m=length(a)
>      titles=paste(a,collapse=' ')
>      conn=textConnection(titles)
>      tab=read.table(conn,header=T)
>      close(conn); tab}
Basically, you have to create the sctruture by providing one observation
This will allow you to specify the classes of the variables.
> data.frame(list(x=1,y="character"))[-1,]
[1] x y
<0 rows> (or 0-length row.names)


Eric

Perhaps this works for creating a new void dataframe:

Newvoid = function (...)
{a=c(...); m=length(a)
initial=matrix(rep(NA,m),byrow=T,ncol=m)
tab=data.frame(initial)
colnames(tab)=a; subset(tab,F)}

Josef Eschgf??ller

-- 
Josef Eschgf??ller
Dipartimento Matematico
Universita' di Ferrara
http://felix.unife.it

> From: Eric Lecoutre
> 
> > I have two questions about data frames:
> > 
> > (1) How can one extract a simple matrix
> > from a data frame? I tried
> > 
> >       Matrixfromdf = function (frame,without=1)
> >       {a=frame[colnames(frame)[-without]]
> >       v=unlist(a,use.names=F)
> >       matrix(v,ncol=ncol(a))}
> > 
> > but it works well only for without=1,
> > perhaps also because the function in (2)
> > gives probably a different meaning to
> > the first column.
> 
> 
> ?as.matrix
> data(iris)
> as.matrix(iris[,-5]) # numeric
> as.matrix(iris[,-1]) # character
data.matrix() might be a safer choice...
 
> > (2) How does one define a void data frame
> > with only column names but no values?
> > I tried this indirect way:
> > 
> >      # Void df with titles from ...
> >      Newvoid = function (...)
> >      {a=c(...); m=length(a)
> >      titles=paste(a,collapse=' ')
> >      conn=textConnection(titles)
> >      tab=read.table(conn,header=T)
> >      close(conn); tab}
> 
> Basically, you have to create the sctruture by providing one 
> observation
> This will allow you to specify the classes of the variables.
> 
> > data.frame(list(x=1,y="character"))[-1,]
> [1] x y
> <0 rows> (or 0-length row.names)
If the columns are all numerics, you can create a matrix with the appriate
column names and 0 rows, then coerce to data frame:

emptyData <- function(varNames) {
    m <- matrix(numeric(0), ncol=length(varNames),
                dimnames=list(NULL, varNames))
    as.data.frame(m)
}

Yet another possibility is to construct the 0-row data frame
directly by giving 0-length named arguments to data.frame().

Andy
> 
> Eric
> 
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