Error in eval(expr, envir, enclos) : object * not found when wrapping qplot()

Question

I don't understand why my oh-so-minimal wrapper function produces the subject error. The below should reproduce it. My goal is to do a bunch of plots from data in a single dataframe, each of which lives in a new window.

library(ggplot2)
library(datasets)
data(ChickWeight)
str(ChickWeight)

# This works fine:
qplot(x = weight, y = Time, data = ChickWeight, color = Diet)

myfun <- function(pred = weight, resp = Time, dat = ChickWeight) {
  windows()
  qplot(x = pred, y = resp, data = dat, color = Diet)
}

# But this returns 'Error in eval(expr, envir, enclos) : object 'weight' not found':
myfun()
# As does this
myfun(weight, Time)

Why can't R find 'weight' in my function?

I'm running R version 3.0.1, 64-bits on windows 8.1 64-bits.

Thanks!

-Roy

Answer 1

I would suggest that in the long run something like this would be a good idea:

myfun <- function(pred = "weight", resp = "Time", dat = ChickWeight) {
    dev.new()  ## more general than windows()
    ggplot(dat,aes_string(x=pred,y=resp,color="Diet"))+geom_point()
}
myfun()

qplot does a lot of fancy evaluation which will be fragile (easy to break, hard to understand) in a context where you are passing objects in and out of functions. aes_string specifies that ggplot should base its lookup on the value of a string, rather than its usual approach of evaluating a language object (i.e. using "weight" rather than weight).