Function `as.function.permutation()` in the permutations package: group actions

In permutations: The Symmetric Group: Permutations of a Finite Set

knitr::opts_chunk$set(echo = TRUE)
library("permutations")
set.seed(0)

knitr::include_graphics(system.file("help/figures/permutations.png", package = "permutations"))

as.function.permutation

To cite the permutations package in publications, please use @hankin2020. The permutations package was intended to manipulate and combine permutations, but often one wants to consider the effect of a permutation on the underlying set, taken to be $\left[n\right]=\left\lbrace 1,2,\ldots,n\right\rbrace$. In other words, we wish to consider a permutation as a function. In package idiom, coercing a permutation to a function is straightforward:

g <- as.cycle("(45)(127)")
as.function(g)(4)

Above we see that permutation $(45)(127)$ maps 4 to 5. We can see from the function body, at the top of the page, that permutations are coerced to word form. Function as.function.permutation() uses as.matrix() to stop "x[a,]" dispatching to [.word() and use matrix extraction instead. It might be argued that unclass() would be better coding.

Coercion is vectorized:

as.function(g)(1:7)
as.function(allperms(4))(3)
as.function(rperm(7,8))(1:7)

The second and third forms use the alist(a = , x[cbind(seq_len(nrow(x)),a)]) construction. We now discuss the extent to which the underlying permutation group is represented in package idiom. Consider the following construction:

(p <- cyc_len(2))
as.function(p)(3)

On the one hand, object p is a permutation on the set $[2]=\left\lbrace 1,2\right\rbrace$. The action of this permutation on 3 is not defined, and the package returns an error. Above we effectively see

t(1:2)[,3]

which is the origin of the error. On the other hand, one might reasonably hold that the action of $(12)$ on 3 should be 3, on the grounds that $(12)$ transposes elements 1 and 2 and leaves all other elements unchanged. To realise this interpretation we need to ensure that p has underlying set including 3, in this case $\left\lbrace 1,2,3\right\rbrace$. This is straightforward with as.word():

as.function(as.word(p,n=3))(3)

Note on identity permutation

The ever-problematic identity permutation acts on the empty set so its functionalization always returns an error:

as.function(id)(4)

Again the resolution is to coerce to word form with explicit n:

as.function(as.word(id,n=4))(4)

References {-}

permutations documentation built on April 3, 2025, 7:09 p.m.