The Art of R Programming

Testing Vector Equality

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 ...