R help - Oct 2014 - grep won't work finding one column

I'm having an issue with grep:

I have numerous columns that end with .at... when I use grep like so:

df[,grep(".at",colnames(df))]

it works fine.  When I have one column that ends with .at, it does not
work.  Why is that?  As this is loop with varying number of columns
ending in .at I would like some code that would work with 1 to n
number of columns.

Is there something more optimal than grep?

Thanks!

On Tue, Oct 14, 2014 at 9:23 AM, Kate Ignatius <kate.ignatius at
gmail.com> wrote:
> I'm having an issue with grep:
>
> I have numerous columns that end with .at... when I use grep like so:
>
> df[,grep(".at",colnames(df))]
>
> it works fine.  When I have one column that ends with .at, it does not
> work.  Why is that?  As this is loop with varying number of columns
> ending in .at I would like some code that would work with 1 to n
> number of columns.
>
> Is there something more optimal than grep?
>
> Thanks!
I can't answer your direct question. But do you realize that your code
does not match your words? The grep show does not _only_ match columns
who name end with the characters '.at'. It matches all column names
which contain any character followed by the characters "at". To do the
match with only columns whose names end with the characters ".at", you
need: grep("\.at$",colnames(df)).

You might want to post an example which fails. Just to be complete, be
sure to use the dput() function so that it is easy for members of the
group to cut'n'paste to get your data into our own R workspace.

-- 
There is nothing more pleasant than traveling and meeting new people!
Genghis Khan

Maranatha! <><
John McKown

Your question is missing a reproducible example, and you don't say how it
does not work, so we cannot tell what is going on.

Two things do come to mind, though.

A) Data frame subsets with only one column by default return a vector, which is
a different type of object than a single-column data frame. You would need to
read ?"[.data.frame" about the "drop" argument if you wanted
to consistently get a data frame from this expression.

B) The period is a wildcard in regular expressions. If you expect to limit your
search to literal ".at" at the end of the name then you should use the
search pattern  "\\.at$" instead (the first slash allows the second
one to be stored by R in the string, and the second one is the only one seen by
grep, which it reads as making the period not act like a wildcard). You really
should read about regular expressions before using them. There are many
tutorials on the web about this topic.

---------------------------------------------------------------------------
Jeff Newmiller                        The     .....       .....  Go Live...
DCN:<jdnewmil at dcn.davis.ca.us>        Basics: ##.#.       ##.#.  Live
Go...
                                      Live:   OO#.. Dead: OO#..  Playing
Research Engineer (Solar/Batteries            O.O#.       #.O#.  with
/Software/Embedded Controllers)               .OO#.       .OO#.  rocks...1k
--------------------------------------------------------------------------- 
Sent from my phone. Please excuse my brevity.

On October 14, 2014 7:23:55 AM PDT, Kate Ignatius <kate.ignatius at
gmail.com> wrote:
>I'm having an issue with grep:
>
>I have numerous columns that end with .at... when I use grep like so:
>
>df[,grep(".at",colnames(df))]
>
>it works fine.  When I have one column that ends with .at, it does not
>work.  Why is that?  As this is loop with varying number of columns
>ending in .at I would like some code that would work with 1 to n
>number of columns.
>
>Is there something more optimal than grep?
>
>Thanks!
>
>______________________________________________
>R-help at r-project.org mailing list
>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.