R help - Oct 2019 - Retaining attributes of columns of a data frame when subsetting.

I am writing a function that involves a data frame "X" some columns of
which have attributes.  I replace X by a data frame with a subset of the 
rows of X:

     X <- X[ok,]

where "ok" is a logical vector.  When I do this the attributes of the 
columns (which I need to retain) are lost (except for the "class" and 
"levels" attributes of columns which are factors).

Is there any sexy way to retain the attributes of the columns?

So far the only approach that I can work out is to extract the 
attributes prior to subsetting and put them back after subsetting.

Like unto:

     SaveAt <- lapply(X,attributes)
     X <- X[ok,]
     lX <- lapply(names(X),function(nm,x,Sat){
                                attributes(x[[nm]]) <- Sat[[nm]]
                                x[[nm]]},x=X,Sat=SaveAt)
     names(lX) <- names(X)
     X <- as.data.frame(lX)

This seems to work, but is rather kludgy.  Is there a better way?

Thanks for any pointers.

cheers,

Rolf Turner

-- 
Honorary Research Fellow
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276

Look at
methods(as.data.frame)
Define your specialized columns to have a newly defined class, say
"myclass".
Then write as.data.frame.myclass
It will be similar to the function you already have in the lapply statement.
Now your statement
X <- X[ok,]
should work.

Rich

On Sat, Oct 19, 2019 at 8:20 PM Rolf Turner <r.turner at auckland.ac.nz>
wrote:
>
>
> I am writing a function that involves a data frame "X" some
columns of
> which have attributes.  I replace X by a data frame with a subset of the
> rows of X:
>
>      X <- X[ok,]
>
> where "ok" is a logical vector.  When I do this the attributes of
the
> columns (which I need to retain) are lost (except for the "class"
and
> "levels" attributes of columns which are factors).
>
> Is there any sexy way to retain the attributes of the columns?
>
> So far the only approach that I can work out is to extract the
> attributes prior to subsetting and put them back after subsetting.
>
> Like unto:
>
>      SaveAt <- lapply(X,attributes)
>      X <- X[ok,]
>      lX <- lapply(names(X),function(nm,x,Sat){
>                                 attributes(x[[nm]]) <- Sat[[nm]]
>                                 x[[nm]]},x=X,Sat=SaveAt)
>      names(lX) <- names(X)
>      X <- as.data.frame(lX)
>
> This seems to work, but is rather kludgy.  Is there a better way?
>
> Thanks for any pointers.
>
> cheers,
>
> Rolf Turner
>
> --
> Honorary Research Fellow
> Department of Statistics
> University of Auckland
> Phone: +64-9-373-7599 ext. 88276
>
> ______________________________________________
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On 20/10/19 3:00 PM, Richard M. Heiberger wrote:
> Look at
> methods(as.data.frame)
> Define your specialized columns to have a newly defined class, say
"myclass".
> Then write as.data.frame.myclass
> It will be similar to the function you already have in the lapply
statement.
> Now your statement
> X <- X[ok,]
> should work.
Yes.  That idea does indeed look promising.  I'll check it out.
Thanks.

cheers,

Rolf

-- 
Honorary Research Fellow
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276