[R] Removing variables from data frame with a wile card

[Valentin Petzel]

Hello Avi,

while something like d$something <- ... may seem like you're directly modifying the data it does not actually do so. Most R objects try to be immutable, that is, the object may not change after creation. This guarantees that if you have a binding for same object the object won't change sneakily.

There is a data structure that is in fact mutable which are environments. For example compare

L <- list()
local({L$a <- 3})
L$a

with

E <- new.env()
local({E$a <- 3})
E$a

The latter will in fact work, as the same Environment is modified, while in the first one a modified copy of the list is made.

Under the hood we have a parser trick: If R sees something like

f(a) <- ...

it will look for a function f<- and call

a <- f<-(a, ...)

(this also happens for example when you do names(x) <- ...)

So in fact in our case this is equivalent to creating a copy with removed columns and rebind the symbol in the current environment to the result.

The data.table package breaks with this convention and uses C based routines that allow changing of data without copying the object. Doing

d[, (cols_to_remove) := NULL]

will actually change the data.

Regards,
Valentin

14.01.2023 18:28:33 avi.e.gross using gmail.com:

> Steven,
> 
> Just want to add a few things to what people wrote.
> 
> In base R, the methods mentioned will let you make a copy of your original DF that is missing the items you are selecting that match your pattern.
> 
> That is fine.
> 
> For some purposes, you want to keep the original data.frame and remove a column within it. You can do that in several ways but the simplest is something where you sat the column to NULL as in:
> 
> mydata$NAME <- NULL
> 
> using the mydata["NAME"] notation can do that for you by using a loop of unctional programming method that does that with all components of your grep.
> 
> R does have optimizations that make this less useful as a partial copy of a data.frame retains common parts till things change.
> 
> For those who like to use the tidyverse, it comes with lots of tools that let you select columns that start with or end with or contain some pattern and I find that way easier.
> 
> 
> 
> -----Original Message-----
> From: R-help <r-help-bounces using r-project.org> On Behalf Of Steven Yen
> Sent: Saturday, January 14, 2023 7:49 AM
> To: Andrew Simmons <akwsimmo using gmail.com>
> Cc: R-help Mailing List <r-help using r-project.org>
> Subject: Re: [R] Removing variables from data frame with a wile card
> 
> Thanks to all. Very helpful.
> 
> Steven from iPhone
> 
>> On Jan 14, 2023, at 3:08 PM, Andrew Simmons <akwsimmo using gmail.com> wrote:
>> 
>> You'll want to use grep() or grepl(). By default, grep() uses
>> extended regular expressions to find matches, but you can also use
>> perl regular expressions and globbing (after converting to a regular expression).
>> For example:
>> 
>> grepl("^yr", colnames(mydata))
>> 
>> will tell you which 'colnames' start with "yr". If you'd rather you
>> use globbing:
>> 
>> grepl(glob2rx("yr*"), colnames(mydata))
>> 
>> Then you might write something like this to remove the columns starting with yr:
>> 
>> mydata <- mydata[, !grepl("^yr", colnames(mydata)), drop = FALSE]
>> 
>>> On Sat, Jan 14, 2023 at 1:56 AM Steven T. Yen <styen using ntu.edu.tw> wrote:
>>> 
>>> I have a data frame containing variables "yr3",...,"yr28".
>>> 
>>> How do I remove them with a wild card----something similar to "del yr*"
>>> in Windows/doc? Thank you.
>>> 
>>>> colnames(mydata)
>>>   [1] "year"       "weight"     "confeduc"   "confothr" "college"
>>>   [6] ...
>>> [41] "yr3"        "yr4"        "yr5"        "yr6" "yr7"
>>> [46] "yr8"        "yr9"        "yr10"       "yr11" "yr12"
>>> [51] "yr13"       "yr14"       "yr15"       "yr16" "yr17"
>>> [56] "yr18"       "yr19"       "yr20"       "yr21" "yr22"
>>> [61] "yr23"       "yr24"       "yr25"       "yr26" "yr27"
>>> [66] "yr28"...
>>> 
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> 
> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help
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> 
> ______________________________________________
> R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.