[R] Listing Variables

[Uwe Ligges]

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)

vnames <- paste("Height", 1:20, sep=".")

for(vn in vnames){
	doSomethingWith(Data[[vn]])
}

Uwe Ligges





> 
> This is such a core capability. I would like to see it in the R-Wiki but 
> could not find it there.
>