[R] Extracting columns with specific string in their names

[David Winsemius]

On Aug 22, 2011, at 1:45 PM, Dennis Murphy wrote:

> Hi:
>
> You need a leading ^ in your grep string. Here's a reproducible
> example to illustrate:
>
> df <- data.frame(Xyz1 = rnorm(5), Xyz2 = rnorm(5), Xyz3 = rnorm(5),
>                  Abc1 = rnorm(5), Abc2 = rnorm(5))
> df[, grep('^Xyz', names(df))]
> df[, grep('^Abc', names(df))]

The leading "^" should not be necessary to solve the problem of "no  
matches at all" reported in the followup message, since it narrows the  
search rather than expanding it. It would only be necessary if there  
were column names that you wanted to exlcude where "Xyz" was not at  
the beginning of the string. My guess is that th eOP does not realize  
the importantce of capitaliszation and that all these are different so  
would NOT be matched by grep("Xyz" ,...):    "xyz", xYz", "xyZ".

>
> HTH,
> Dennis
>
> On Mon, Aug 22, 2011 at 7:33 AM, Jay <josip.2000 at gmail.com> wrote:
>> Hi,
>>
>> Let's say that I have a set of column names that begin with the  
>> string
>> "Xyz". How do I extract these specific columns? I tried to do the
>> following:
>>
>> dataframe1[,grep("Xyz",colnames(dataframe1))]
>>
>> But it does not work. What is wrong with my expression?
>>

-- 
David Winsemius, MD
West Hartford, CT