In the cases of problems where logical vector contains NA values.

a <- c(TRUE, FALSE, NA)

sum(a) # gives you NA

table(a)["TRUE"] # gives you 1

length(a[a==TRUE]) # f3lix answer, gives you 2 (because NA indexing returns values)

sum(a, na.rm=TRUE) # best way to count TRUE values #which gives 1.

You should be careful with the "table" solution, in case there are no TRUE values in the logical vector.

Suppose a <- c(NA, FALSE, NA) or simply a <- c(FALSE, FALSE)

table(a)["TRUE"] # gives you NA for both cases.