under what circumstances does R recycle?

Question

I have two variables, x (takes in 5 values) and y (takes in 11 values). When I want to run the argument,

> v <- 2*x +y +1

R responds:

Answer 1

It doesn't work this way. You have to have vectors of the same length:

    x_samelen = c(1,2,3)
    y_samelen = c(10,20,30)
    x_samelen*y_samelen
    [1] 10 40 90

If vectors are of the same length, the result is well defined and understood. You can do "recycling", but it really is not advisable to do so.

I wrote a short script to make your two vectors of the same length, via padding the short vector. This will let you execute your code without warnings:

    x_orig <- c(1,2,3,4,5,6,7,8,9,10,11)
    y_orig <- c(21,22,23,24,25)

    if ( length(x_orig)>length(y_orig) ) {
        x <- x_orig
        y <- head(x = as.vector(t(rep(x=y_orig, times=ceiling(length(x_orig)/length(y_orig))))), n = length(x_orig) )   
        cat("padding y\r\n")
    } else {
        x <- head(x = as.vector(t(rep(x=x_orig, times=ceiling(length(y_orig)/length(x_orig))))), n = length(y_orig) )
        y <- y_orig
        cat("padding x\r\n")
    }

The results are:

    x_orig
     [1]  1  2  3  4  5  6  7  8  9 10 11
    y_orig
    [1] 21 22 23 24 25
    x
     [1]  1  2  3  4  5  6  7  8  9 10 11
    y
     [1] 21 22 23 24 25 21 22 23 24 25 21

If you reverse x_orig and y_orig:

    x_orig
    [1] 21 22 23 24 25
    y_orig
     [1]  1  2  3  4  5  6  7  8  9 10 11
    x
     [1] 21 22 23 24 25 21 22 23 24 25 21
    y
     [1]  1  2  3  4  5  6  7  8  9 10 11