[R] Choosing columns by number

[David Winsemius]

On Aug 25, 2015, at 8:17 AM, Sam Albers 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)]

The request to "identify column numbers" seems a bit vague at the moment because it misses any criterion for such "identification". If your goal is to construct a vector that "identified" (by number) the names of the columns that contained the text "Width" it would be:

grep("Width",   names(iris) )

You do need some rule ... which you never articulated.

> 
> 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
> 
> 	[[alternative HTML version deleted]]

Still posting in HTML? Having trouble finding the Posting Guide? Can't find the mechanism in gmail to send plain text? What is the problem?

-- 


David Winsemius
Alameda, CA, USA