Why does the default S3 method gets called when I pass a vector to the method and the vector S3 method is implemented?

Question

I have defined the following in R:

plotWaterfall         <- function(x, ...) UseMethod("plotWaterfall")
plotWaterfall.default <- function(x, ...) {print("Default method does nothing")}
plotWaterfall.vector  <- function(x, ...) {print("Vector method does something")}

Now if I test the following example:

x<-c(1,2,3)
plotWaterfall(x)

it will print "Default method does nothing" indicating that the S3 framework matches the default method instead of the vector method. Now why is that?

Answer 1

It's because the class of your vector is numeric. So you have to do this:

plotWaterfall.numeric  <- function(x, ...) {print("Numeric vector")}

plotWaterfall(x)
[1] "Numeric vector"

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You can determine the class of an object with the class() function:

class(x)
[1] "numeric"

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This behaviour is described in the help for ?UseMethod:

Method dispatch takes place based on the class(es) of the first argument to the generic function or of the object supplied as an argument to UseMethod