What does this mean: unable to find an inherited method for function ‘A’ for signature ‘“B”’

Question

I am new to R and keep getting errors with the following message:

unable to find an inherited method for function ‘A’ for signature ‘\"B\"’

Answer 1

That is the type of message you will get when attempting to apply an S4 generic function to an object of a class for which no defined S4 method exists (or at least has been attached to the current R session).

Here's an example using the raster package (for spatial raster data), which is chock full of S4 functions.

library(raster)

## raster::rotate() is an S4 function with just one method, for "Raster" class objects
isS4(rotate)
# [1] TRUE
showMethods(rotate)
# Function: rotate (package raster)
# x="Raster"

## Lets see what happens when we pass it an object that's *not* of class "Raster"
x <- 1:10
class(x)
# [1] "integer"
rotate(x)
# Error in (function (classes, fdef, mtable)  : 
#   unable to find an inherited method for function ‘rotate’ for signature ‘"integer"’

Answer 2

I have seen this message numerous times, as a result of namespace conflicts.

Here is a MRE: Both the hash and data.table libraries have copy functions.

In a new R session:

> library(data.table)
> library(hash)

causes copy from data.table to be masked:

> DT = data.table(x=rep(c("b","a","c"),each=3), y=c(1,3,6), v=1:9)
> copy(DT)
Error in (function (classes, fdef, mtable)  : 
  unable to find an inherited method for function ‘copy’ for signature ‘"data.table"’

The solution is to specify the namespace:

> data.table::copy(DT)
   x y v
1: b 1 1
2: b 3 2
3: b 6 3
4: a 1 4
5: a 3 5
6: a 6 6
7: c 1 7
8: c 3 8
9: c 6 9