Re: [R] Listing Variables

From: Marc Schwartz (via MN) <mschwartz_at_mn.rr.com>
Date: Thu 04 May 2006 - 01:18:16 EST

On Wed, 2006-05-03 at 10:46 -0400, Farrel Buchinsky wrote:
> How does one create a vector whose contents is the list of variables in a
> dataframe pertaining to a particular pattern?
> This is so simple but I cannot find a straightforward answer.
> I want to be able to pass the contents of that list to a "for" loop.
>
> So let us assume that one has a dataframe whose name is Data. And let us
> assume one had the height of a group of people measured at various ages.
>
> It could be made up of vectors Data$PersonalID, Data$FirstName,
> Data$LastName, Data$Height.1, Data$Height.5, Data$Height.9,
> Data$Height.10,Data$Height.12,Data$Height.20....many many more variables.
>
> How would one create a vector of all the Height variable names.
>
> The simple workaround is to not bother creating the vector "Data$Height.1"
> "Data$Height.5" "Data$Height.9" "Data$Height.10"
> "Data$Height.12""Data$Height.20"...but rather just to use the sapply
> function. However with some functions the sapply will not work and it is
> necessary to supply each variable name to a function (see thread at
> Repeating tdt function on thousands of variables)
>
>
> This is such a core capability. I would like to see it in the R-Wiki but
> could not find it there.

I may be misunderstanding what you want to do, but to simply get the names of the columns in Data that contain "Height", you can do this:

> grep("Height", names(Data), value = TRUE)
[1] "Height.1" "Height.5" "Height.9" "Height.10" "Height.12" [6] "Height.20"

Now you could use something like the following:

  for (i in grep("Height", names(Data), value = TRUE))     YourFunctionHere(Data[[i]])

If it makes for easier reading, you could first assign the subset of the column names to a vector and then use that in the for() loop, rather than the above.

HTH, Marc Schwartz



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