Suppose we wish to test whether two vectors are equal. The naive approach, using ==, won’t work.
> x <- 1:3 > y <- c(1,3,4) > x == y [1] TRUE FALSE FALSE
What happened? The key point is that we are dealing with vectorization. Just like almost anything else in R, == is a function.
> "=="(3,2) [1] FALSE > i <- 2 > "=="(i,2) [1] TRUE
In fact, == is a vectorized function. The expression x == y applies the function ==() to the elements of x and y. yielding a vector of Boolean values.
What can be done instead? One option is to work with the vectorized nature of ==, applying the function all():
> x <- 1:3 > y <- c(1,3,4) > x == y [1] TRUE FALSE FALSE > all(x == y) [1] FALSE
Applying all() to the result of == asks whether all of the elements of the latter are true, which is the same as asking whether x and y are identical.
Or even better, we can simply use the identical function, like this:
> identical(x,y) [1] FALSE
Be careful, though because the word identical really means what it says. Consider this little R session:
> x <- 1:2 > y <- c(1,2) > x [1] 1 2 > y [1] 1 2 > identical(x,y) [1] FALSE > typeof(x) [1] "integer" > typeof(y) [1] "double"
So, : produces integers while c() produces floating-point numbers. Who knew?