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 t

Answer 1

The package binhf has the function shift:

library(binhf)

shift(1:5, places = 2)
#[1] 4 5 1 2 3

places can be positive or negative

Answer 2

You can also use the permute package:

require(permute)

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

shuffleSeries(a, start = 2)

output:

[1] 3 4 5 1 2

Answer 3

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 4

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