Why does order matter when using "data" and "formula" keyword arguments?

Question

In R, why is it that the order of the data and formula keywords matters when plotting? I thought that with named arguments order isn't supposed to matter...

For an example of what I mean, check out this code:

library(MASS)
data(menarche)

# Correct formulation (apparently):
plot(formula=Menarche/Total ~ Age, data=menarche)

# In contrast, note how the following returns an error:
plot(data=menarche, formula=Menarche/Total ~ Age)  

Is this just a quirk of the plot function or is this behavior exhibited in other functions as well?

Answer 1

It is related to S3 methods for the S3 generic plot(). S3 dispatches methods based on the first argument however the exact functioning is complicated because formula is allowed as a special exception from the usual generic arguments of plot(), which are x and y plus ...:

> args(plot)
function (x, y, ...) 
NULL

Hence what happens in the first case is that the plot.formula() method is run because the first argument supplied is a formula and this matches the arguments of plot.formula()

> args(graphics:::plot.formula)
function (formula, data = parent.frame(), ..., subset, ylab = varnames[response], 
    ask = dev.interactive()) 
NULL

for example:

> debugonce(graphics:::plot.formula)
> plot(formula=Menarche/Total ~ Age, data=menarche)
debugging in: plot.formula(formula = Menarche/Total ~ Age, data = menarche)
debug: {
    m <- match.call(expand.dots = FALSE)
[...omitted...]

In contrast, when you call plot(data=menarche, formula=Menarche/Total ~ Age), the first argument is a data frame and hence the graphics:::plot.data.frame method is called:

> plot(data=menarche, formula=Menarche/Total ~ Age)
Error in is.data.frame(x) : argument "x" is missing, with no default
> traceback()
3: is.data.frame(x)
2: plot.data.frame(data = menarche, formula = Menarche/Total ~ Age)
1: plot(data = menarche, formula = Menarche/Total ~ Age)

but because that method expects an argument x, which you didn't supply, you get the error about missing x.

So in a sense, the ordering of named arguments doesn't and shouldn't matter but when S3 generics are in play method dispatch kicks in first to decide which method to pass the arguments on to and then the arguments supplied - not the ordering - is what will often catch you out, especially when mixing the formula methods with other non-formula methods.