Take the following code that generates a fractions vector:
> example<-MASS:: as.fractions(c(0.2,0.4))
> example
[1] 1/5 2/5
If we were to replace one of these fractions with a string, we get the following error:
> example[1]<-"0/1"
Error in floor(x) : non-numeric argument to mathematical function
How does this happen? As far as I was concerned, fractions vectors aren't S4 objects and shouldn't have any sort of error checking, so how is floor(x) being ran when I'm trying to replace an entry in such a a vector?
fractions has an S3 [<- method:
library(MASS)
methods(class = "fractions")
## [1] [ [<- as.character Math Ops
## [6] print Summary t
## see '?methods' for accessing help and source code
and have a look at the R source code at https://github.com/cran/MASS/blob/master/R/fractions.R
floor is called in .rat which is called in fractions which is called in [<-.fractions which is invoked in the [<- generic.
> example[1] <- "0/1"
Error in floor(x) : non-numeric argument to mathematical function
> traceback()
5: matrix(floor(x))
4: .rat(x, cycles, max.denominator)
3: fractions(NextMethod())
2: `[<-.fractions`(`*tmp*`, 1, value = "0/1")
1: `[<-`(`*tmp*`, 1, value = "0/1")