Circular shift of vector (equivalent to numpy.roll)

Question

I have a vector:

a <- c(1,2,3,4,5) 

And I'd like to do something like:

b <- roll(a, 2) # 4,5,1,2,3 

Is there a function like that in R? I've been googling around, but "R Roll" mostly gives me pages about Spanish pronunciation.

Answer 1

How about using head and tail...

roll <- function( x , n ){   if( n == 0 )     return( x )   c( tail(x,n) , head(x,-n) ) }  roll(1:5,2) #[1] 4 5 1 2 3  #  For the situation where you supply 0 [ this would be kinda silly! :) ] roll(1:5,0) #[1] 1 2 3 4 5 

One cool thing about using head and tail... you get a reverse roll with negative n, e.g.

roll(1:5,-2) [1] 3 4 5 1 2 

Answer 2

Here's an alternative which has the advantage of working even when x is "rolled" by more than one full cycle (i.e. when abs(n) > length(x)):

roll <- function(x, n) {     x[(seq_along(x) - (n+1)) %% length(x) + 1] }  roll(1:5, 2) # [1] 4 5 1 2 3 roll(1:5, 0) # [1] 1 2 3 4 5 roll(1:5, 11) # [1] 5 1 2 3 4 

FWIW (and not that it's worth much) it also works on data.frames:

head(mtcars, 1) #           mpg cyl disp  hp drat   wt  qsec vs am gear carb # Mazda RX4  21   6  160 110  3.9 2.62 16.46  0  1    4    4 head(roll(mtcars, 2), 1) #           gear carb mpg cyl disp  hp drat   wt  qsec vs am # Mazda RX4    4    4  21   6  160 110  3.9 2.62 16.46  0  1