Function `getcoeffs()` in the `clifford` package
In clifford: Arbitrary Dimensional Clifford Algebras

library("clifford")
options(rmarkdown.html_vignette.check_title = FALSE)
knitr::opts_chunk$set(echo = TRUE)
knit_print.function <- function(x, ...){dput(x)}
registerS3method(
  "knit_print", "function", knit_print.function,
  envir = asNamespace("knitr")
)

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

getcoeffs

To cite the clifford package in publications please use @hankin2025_clifford_rmd. This short document discusses function getcoeffs() in the clifford R package which, as its name suggests, retrieves specific coefficients from a clifford object. It is a relatively low-level helper function that is a wrapper for a C routine. It takes as arguments a clifford object and a list of terms:

set.seed(0)
(a <- rcliff())
getcoeffs(a,list(1:2, 0, c(2,5), c(1,5,6), c(2,6), 1:2))

Note that the first and last element of the returned vector are both the coefficient of $e_{12}$, viz. -9. The coefficients are returned in the form of a numeric vector [not a disord object: the order of the elements is determined by the order of argument B]. Compare standard extraction, e.g. a[index], which returns a clifford object. Also, compare coeffs() which extracts all coefficients of a clifford object:

coeffs(a)

The index for the constant is formally list(numeric(0)), but this is a pain to type, so there is special dispensation for argument B having list elements of zero, which are translated by helper function list_modifier() to numeric(0) and listified if necessary. The upshot is that a zero list element in argument B works as expected extracting the constant. Also, passing B=0 works as expected, returning the constant (there is no need to coerce to a list: coercion is performed by list_modifier). A similar scheme is used in the square bracket extraction and replacement methods

Attempting to extract a coefficient of a term that includes a negative index will throw an error. The coefficient of a term including an index larger than indicated by maxyterm() will return zero.