[R] Listing Variables

[Farrel Buchinsky]

bogdan romocea <br44114 <at> gmail.com> writes:

> 
> Here's an example.
> dfr <- data.frame(A1=1:10,A2=21:30,B1=31:40,B2=41:50)
> vars <- colnames(dfr)
> for (v in vars[grep("B",vars)]) print(mean(dfr[,v]))

At first I did not know why you had bothered with the (dfr[,v])) part of the 
command. I thought you should have just as easily entered ... print(mean(v)). 
However, with a little help from David Clayton I discovered that one would 
simply be passing a string to the function rather than the underlying vector 
whose name is the string. Setting it up as you did makes sure that the vector 
is passed to the function.

Armed with what I learnt from your response, I reasoned that I could roll 
everyting into an sapply and get everything coming out as a dataframe meanting 
that I could do the next step of the analysis.
data.frame(sapply(dfr[,grep("B",names(dfr))],mean))

   sapply.dfr...grep..B...names.dfr.....mean.
B1                                       35.5
B2                                       45.5

Farrel Buchinsky