[R] Passing the name of a variable to a function

[Erik Iverson]

Hello,


> I have a problem which has bitten me occasionally. I often need to
> prepare graphs for many variables in a data set, but seldom for all.
> or for any large number of sequential or sequentially named variables.
> Often I need several graphs for different subsets of the dataset
> for a given variable.  I run into similar problems with other needs
> besides graphing.
> 
>  What I would like to do is something like "write a function which
> takes the *name* of a variable, presumably a s a character string,
> from a dataframe, as one argument, and the dataframe, as a second argument".
> 
> For example, where y is to be the the name of a variable in a given
> dataframe d, and the other variables needed, T, M and so on, are
> to be found in the same dataframe :-
> 
> pf <- function (y,data,...) {
> p1 <- xyplot(y~x|T,data)
> p2 <- xyplot(y~x|T,subset(data,M == 2))
> p3 <- xyplot(y~x|T,subset(data,M == 4))
> #print(p1,p2,p3....)
> }
>  pf(Score1,data)
> pf(Score2,data)
> 
> 
> This fails, because, of course, Score 1, Score 2 etc.. are  not
> defined, or if you pass them as pf(data$Score2,data), then when you
> subset the
> data, data$Score2 is now the wrong shape.  
> I've come up with various
> inelegant hacks, (often with for loops), for getting around this over
> the
> last few years, but I can't help feeling that I'm missing something
> obvious, which I've been too dim to spot.

Depending on your needs (e.g., you use formulas, which can be trickier),
I think I often do something like:

# I prefer this, I quote the variable name...

df1 <- data.frame(x = rnorm(100),
                   score1 = rnorm(100),
                   M = sample(c(2, 4), 100, replace = TRUE))

pf <- function (y,data,...) {
   data$y <- data[[y]]
   xyplot(y~x, subset(data, M == 2))
}

pf("score1", df1)

# as an alternative, use eval/substitute, don't have to quote

pf2 <- function (y,data,...) {
   data$y <- eval(substitute(y), data)
   xyplot(y~x, subset(data, M == 2))
}

pf2(score1, df1)