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

[Joshua Wiley]

Hi Anthony,

I don't know if this will help you.  A similar method should work for
any function that accepts a formula, which is your main issue I think.
 Outside of formulae, you should just be able to pass the arguments
directly (as with the data argument of xyplot).

The only other thought that comes to mind involves a combination of
eval() and parse(), but its obvious messiness and fortune(106),
suggest that that is not worth sharing.

#Some data to work with
mydata <- data.frame(x = 1:10, Score1 = 1:10,
                     T = rep(c(1,2), 5), M = rep(c(2,4), each = 5))

#Function
pf <- function (y, data, ...) {
  #Save the old par() settings, and restore at the end
  old.par <- par(no.readonly = TRUE)
  on.exit(par(old.par))
  #Set it so it asks before changing graphs
  par(ask = TRUE)
  #Create a text string of your formula
  my.f <- paste(y, " ~ x|T", sep = "")

  #Initialize a list and calculate plots
  p <- vector("list", length = 3)
  p[[1]] <- xyplot(x = formula(my.f),
                   data = data)
  p[[2]] <- xyplot(x = formula(my.f),
                   data = subset(x = data, subset = M == 2))
  p[[3]] <- xyplot(x = formula(my.f),
                   data = subset(x = data, subset = M == 4))

  #Graph the plots
  for(i in 1:length(p)) {
    print(p[[i]])
  }
}

#Graphs!
pf(y = "Score1", data = mydata)

HTH,

Josh

On Wed, Aug 4, 2010 at 6:56 AM, Anthony Staines
<anthony.staines at gmail.com> wrote:
> Dear colleagues,
>
> 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.
>
> Please note the answer to my requirements is *not* a clever use of
> lattice. This question goes beyond graphs.
> All suggestions gratefully received!
>
> Best wishes,
> Anthony Staines
> --
> Anthony Staines,  Professor of Health Systems Research,
> School  of Nursing, Dublin City University, Dublin 9, Ireland.
> Tel:- +353 1 7007807. Mobile:- +353 86 606 9713
>
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> and provide commented, minimal, self-contained, reproducible code.
>



-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/