[R] Dropping columns from data frame

[Mike Harwood]

Thank you, David.  I was merely using "head" to limit the code/
output.  My question remains, because a created data frame has the
same columns as was output from "head":

> head(orig.df,3)
  num1.10 num11.20 lc1.10 lc11.20 uc1.10 uc11.20
1       1       11      a       k      A       K
2       2       12      b       l      B       L
3       3       13      c       m      C       M
> # Illustration 1: contiguous columns at beginning of data frame
> head(orig.df[,-c(1:3)],2)
  lc11.20 uc1.10 uc11.20
1       k      A       K
2       l      B       L
> new.df <- orig.df[,-c(1:3)]
> head(new.df,2)
  lc11.20 uc1.10 uc11.20
1       k      A       K
2       l      B       L
>
> # Illustration 2: non-contiguous columns
> head(orig.df[,-c(1,3,5)],2)
  num11.20 lc11.20 uc11.20
1       11       k       K
2       12       l       L
> new.df <- orig.df[,-c(1,3,5)]
> head(new.df,2)
  num11.20 lc11.20 uc11.20
1       11       k       K
2       12       l       L




On Jan 6, 9:49 am, David Winsemius <dwinsem... at comcast.net> wrote:
> On Jan 6, 2012, at 10:00 AM, Mike Harwood wrote:
>
> > How does R do it, and should I ever be worried?  I always remove
> > columns by index, and it works exactly as I would naively expect - but
> > HOW?  The second illustration, which deletes non contiguous columns,
> > represents what I do all the time and have some trepidation about
> > because I don't know the mechanics (e.g. why doesn't the column
> > formerly-known-as-4 become 3 after column 1 is dropped: doesn't vector
> > removal from a df/list invoke a loop in C?).
>
> You are NOT "removing columns". You are returning (to `head` and then
> to `print`) an extract from the dataframe, but that does not change
> the original dataframe. To effect a change you would need to assign
> the value back to the same name as the original daatframe.
>
> --
> David
>
>
>
>
>
>
>
>
>
> >  Can I delete a named
> > list of columns, which are examples 4 and 5 and which generate the
> > "unary error' mesages, without resorting to "orig.df$num1.10 <- NULL"?
>
> > Thanks!
>
> > orig.df <- data.frame(cbind(
> >    1:10
> >    ,11:20
> >    ,letters[1:10]
> >    ,letters[11:20]
> >    ,LETTERS[1:10]
> >    ,LETTERS[11:20]
> >    ))
> > names(orig.df) <- c(
> >    'num1.10'
> >    ,'num11.20'
> >    ,'lc1.10'
> >    ,'lc11.20'
> >    ,'uc1.10'
> >    ,'uc11.20'
> >    )
> > # Illustration 1: contiguous columns at beginning of data frame
> > head(orig.df[,-c(1:3)])
>
> > # Illustration 2: non-contiguous columns
> > head(orig.df[,-c(1,3,5)])
>
> > # Illustration 3: contiguous columns at end of data frame
> > head(orig.df[,-c(4:6)])    ## as expected
>
> > # Illustrations 4-5: unary errors
> > head(orig.df[,-c(as.list('num1.10', 'lc1.10', 'uc1.10'))])
> > head(orig.df[,-c('num1.10', 'lc1.10', 'uc1.10')])
>
> > Mike
>
> > ______________________________________________
> > R-h... at r-project.org mailing list
> >https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guidehttp://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>
> David Winsemius, MD
> West Hartford, CT
>
> ______________________________________________
> R-h... at r-project.org mailing listhttps://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guidehttp://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.