[R] Choosing columns by number

[stephen sefick]

?grep

I think this will do what you want.

#something like
a <- data.frame(a=rnorm(10), b=rnorm(10), c=rnorm(10), d=rnorm(10))

toMatch <- c("a", "d")

grep(paste(toMatch,collapse="|"), colnames(a))

#to subset
a[,grep(paste(toMatch,collapse="|"), colnames(a))]


On Tue, Aug 25, 2015 at 10:17 AM, Sam Albers <tonightsthenight at gmail.com>
wrote:

> Hi all,
>
> This is a process question. How do folks efficiently identify column
> numbers in a dataframe without manually counting them. For example, if I
> want to choose columns from the iris dataframe I know of two options. I can
> do this:
>
> > str(iris)'data.frame':        150 obs. of  5 variables:
>  $ Sepal.Length: num  5.1 4.9 4.7 4.6 5 5.4 4.6 5 4.4 4.9 ...
>  $ Sepal.Width : num  3.5 3 3.2 3.1 3.6 3.9 3.4 3.4 2.9 3.1 ...
>  $ Petal.Length: num  1.4 1.4 1.3 1.5 1.4 1.7 1.4 1.5 1.4 1.5 ...
>  $ Petal.Width : num  0.2 0.2 0.2 0.2 0.2 0.4 0.3 0.2 0.2 0.1 ...
>  $ Species     : Factor w/ 3 levels "setosa","versicolor",..: 1 1 1 1
> 1 1 1 1 1 1 ...
>
> or this:
>
> > names(iris)[1] "Sepal.Length" "Sepal.Width"  "Petal.Length"
> "Petal.Width"  "Species"
>
> Neither option explicitly identifies the column number so that I can
> do something like this:
>
> iris[,c(2,4)]
>
> I feel like there must be a better way to do this so I wanted to ask
> the collective wisdom here what people do to accomplish this.
> Obviously this is a trivial example, but the issue really becomes
> problematic when you have a large dataframe.
>
> Thanks in advance!
>
> Sam
>
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>
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-- 
Stephen Sefick
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Auburn University
Biological Sciences
331 Funchess Hall
Auburn, Alabama
36849
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sas0025 at auburn.edu
http://www.auburn.edu/~sas0025
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