[R] grep won't work finding one column

[Kate Ignatius]

In the sense - it does not work.  it works when there are 50 samples
in the file, but it does not work when there is one.

The usual headings are:  sample1.at sample1.dp
sample1.fg sample2.at sample2.dp sample2.fg.... and so on to a max of
sample50.at sample50.dp sample50.fg

using this greps out all the .at columns perfectly:

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

When I come across a file when there is one sample:

sample1.at sample1.dp sample1.fg

Using this:

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

returns nothing.

Oh - AT/at was just an example... thats not my problem...



On Tue, Oct 14, 2014 at 10:57 AM, Jeff Newmiller
<jdnewmil at dcn.davis.ca.us> wrote:
> 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.
>
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> 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!
>>
>>______________________________________________
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>>https://stat.ethz.ch/mailman/listinfo/r-help
>>PLEASE do read the posting guide
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>>and provide commented, minimal, self-contained, reproducible code.
>