Writing ggplot functions in R with optional arguments - r

Question

I have a series of ggplot graphs that I'm repeating with a few small variations. I would like to wrap these qplots with their options into a function to avoid a lot of repetition in the code.
My problem is that for some of the graphs I am using the + facet_wrap() option, but for others I am not. I.e. I need the facet wrap to be an optional argument. When it is included the code needs to call the +facet_wrap() with the variable supplied in the facets argument.
So ideally my function would look like this, with facets being an optional argument:
$ qhist(variable, df, heading, facets)
I have tried googling how to add optional arguments and they suggest either passing a default value or using an if loop with the missing() function. I haven't been able to get either to work.
Here is the function that I have written, with the desired functionality of the optional facets argument included too.
$ qhist <- function(variable, df, heading, facets) {
qplot(variable, data = df, geom = "histogram", binwidth = 2000,
xlab = "Salary", ylab = "Noms") +
theme_bw() +
scale_x_continuous(limits=c(40000,250000),
breaks=c(50000,100000,150000,200000,250000),
labels=c("50k","100k","150k","200k","250k")) +
opts(title = heading, plot.title = theme_text(face = "bold",
size = 14), strip.text.x = theme_text(size = 10, face = 'bold'))
# If facets argument supplied add the following, else do not add this code
+ facet_wrap(~ facets)

Answer 1

the way to set up a default is like this:
testFunction <- function( requiredParam, optionalParam=TRUE, alsoOptional=123 ) {
print(requiredParam)
if (optionalParam==TRUE) print("you kept the default for optionalParam")
paste("for alsoOptional you entered", alsoOptional)
}
*EDIT*
Oh, ok... so I think I have a better idea of what you are asking. It looks like you're not sure how to bring the optional facet into the ggplot object. How about this:
qhist <- function(variable, df, heading, facets=NULL) {
d <- qplot(variable, data = df, geom = "histogram", binwidth = 2000,
xlab = "Salary", ylab = "Noms") +
theme_bw() +
scale_x_continuous(limits=c(40000,250000),
breaks=c(50000,100000,150000,200000,250000),
labels=c("50k","100k","150k","200k","250k")) +
opts(title = heading, plot.title = theme_text(face = "bold",
size = 14), strip.text.x = theme_text(size = 10, face = 'bold'))
# If facets argument supplied add the following, else do not add this code
if (is.null(facets)==FALSE) d <- d + facet_wrap(as.formula(paste("~", facets)))
d
return(d)
}
I have not tested this code at all. But the general idea is that the facet_wrap expects a formula, so if the facets are passed as a character string you can build a formula with as.formula() and then add it to the plot object.
If I were doing it, I would have the function accept an optional facet formula and then pass that facet formula directly into the facet_wrap. That would negate the need for the as.formula() call to convert the text into a formula.

Answer 2

Probably, the best way is to stop using such unusual variable names including commas or spaces.
As a workaround, here is an extension of #JDLong's answer. The trick is to rename the facet variable.
f <- function(dat, facet = NULL) {
if(!missing(facet)) {
names(dat)[which(names(dat) == facet)] <- ".facet."
ff <- facet_wrap(~.facet.)
} else {
ff <- list()
}
qplot(x, y, data = dat) + ff
}
d <- data.frame(x = 1:10, y = 1:10, "o,o" = gl(2,5), check.names=F)
f(d, "o,o")
f(d)

Answer 3

Note that you can also use missing(facets) to check if the facets argument was specified or not. If you use #JD Long's solution, it would look something like this:
qhist <- function(variable, df, heading, facets) {
... insert #JD Longs' solution ...
if (!missing(facets)) d <- d + facet_wrap(as.formula(paste("~", facets)))
return(d)
}
...Note that I also changed the default argument from facets=NULL to just facets.
Many R functions use missing arguments like this, but in general I tend to prefer #JD Long's variant of using a default argument value (like NULL or NA) when possible. But sometimes there is no good default value...