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

Recycling works in your example:

> x <- seq(5)
> y <- seq(11)
> x+y
 [1]  2  4  6  8 10  7  9 11 13 15 12
Warning message:
In x + y : longer object length is not a multiple of shorter object length
> v <- 2*x +y +1 
Warning message:
In 2 * x + y :
  longer object length is not a multiple of shorter object length
> v
 [1]  4  7 10 13 16  9 12 15 18 21 14

The "error" that you reported is in fact a "warning" which means that R is notifying you that it is recycling but recycles anyway. You may have options(warn=2) turned on, which converts warnings into error messages.

In general, avoid relying on recycling. If you get in the habit of ignoring the warnings, some day it will bite you and your code will fail in some very hard to diagnose way.

Answer 2

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