[R] cannot turn some columns in a data frame into factors

[Sam Steingold]

> * Douglas Grove <qtebir at supep.bet> [2006-05-11 09:41:07 -0700]:
>
> You need to create a new object and assign it to 'df'

why can't I modify an existing object?!

> so you'd do something like this:
>
> df <- sapply(factors, function (name) {
>              pos <- match(name,df.names)
>              factor(df[[pos]])
>              })

this cannot be right:
this will make df have the same length as factors.


> On Thu, 11 May 2006, Sam Steingold wrote:
>
>> > * jim holtman <wubygzna at tznvy.pbz> [2006-05-11 12:27:39 -0400]:
>> >
>> > try '<<-' as the assignment to make it global.
>> >
>> >      df[[pos]] <<- factor(df[[pos]])
>> 
>> nothing changed -- I observe the exact same behaviour:
>> 
>> Month ( 1 ): TRUE 
>> factors: FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE 
>> 
>> 
>> > On 5/11/06, Sam Steingold <sds at podval.org> wrote:
>> >>
>> >> Hi,
>> >> I have a data frame df and a list of names of columns that I want to
>> >> turn into factors:
>> >>
>> >> df.names <- attr(df,"names")
>> >> sapply(factors, function (name) {
>> >>    pos <- match(name,df.names)
>> >>    if (is.na(pos)) stop(paste(name,": no such column\n"))
>> >>    df[[pos]] <- factor(df[[pos]])
>> >>    cat(name,"(",pos,"):",is.factor(df[[pos]]),"\n")
>> >> })
>> >> cat("factors:",sapply(df,is.factor),"\n")
>> >>
>> >> the output is:
>> >>
>> >>
>> >> Month ( 1 ): TRUE
>> >> factors: FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>> >>
>> >>
>> >> i.e., there is a column named "Month" (the 1st column), and it is indeed
>> >> turned into a factor inside sapply(), but after that it is numerical
>> >> again!
>> >>
>> >> what am I doing wrong?
>> 
>> -- 
>> Sam Steingold (http://www.podval.org/~sds) on Fedora Core release 5 (Bordeaux)
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>> 
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>
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-- 
Sam Steingold (http://www.podval.org/~sds) on Fedora Core release 5 (Bordeaux)
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There are two ways to write error-free programs; only the third one works.