R: Generates index arrays for upper triangular storage

trind.generator {mgcv} | R Documentation

## Generates index arrays for upper triangular storage

### Description

Generates index arrays for upper triangular storage up to order four. Useful when working with higher order derivatives, which generate symmetric arrays. Mainly intended for internal use.

### Usage

    trind.generator(K = 2)

### Arguments

`K` | positive integer determining the size of the array.

### Details

Suppose that `m=1` and you fill an array using code like `for(i in 1:K) for(j in i:K) for(k in j:K) for(l in k:K) {a[,m] <- something; m <- m+1 }` and do this because actually the same "something" would be stored for any permutation of the indices i,j,k,l. Clearly in storage we have the restriction l>=k>=j>=i, but for access we want no restriction on the indices. `i4[i,j,k,l]` produces the appropriate `m` for unrestricted indices. `i3` and i2 do the same for 3d and 2d arrays.

### Value

A list where the entries `i1` to `i4` are arrays in up to four dimensions, containing K indexes along each dimension.

### Author(s)

Simon N. Wood <simon.wood@r-project.org>.

### Examples

    library(mgcv)
    A <- trind.generator(3)

    # All permutations of c(1, 2, 3) point to the same index (5)
    A$i3[1, 2, 3]
    A$i3[2, 1, 3]
    A$i3[2, 3, 1]
    A$i3[3, 1, 2]
    A$i3[1, 3, 2]

[Package mgcv version 1.8-17 Index]