Question

I have a very long vector of single characters i.e. somechars<-c("A","B","C","A"...) (length is somewhere in the millions)

what is the fastest way I can count the total occurrences of say “A” and “B” in this vector?
I have tried using grep and lapply but they all take so long to execute.

My current solution is:

tmp<-table(somechars)
sum(tmp["A"],tmp["B"])

But this still takes a while to compute. Is there some faster way I can be doing this? Or are there any packages I can be using to that does this already faster? I’ve looked into the stringr package but they use a simple grep.

Answer 1

I thought that this would be fastest…

sum(somechars %in% c('A', 'B'))

And, it is faster than…

sum(c(somechars=="A",somechars=="B"))

But not faster than…

sum(somechars=="A"|somechars=="B")

But this is qualified by how many comparisons you make… which brings me back to my first guess. Once you want to sum more than 2 letters using the %in% version is the fastest.