# R: Permute Indices for Triangular Matrices

lower.to.upper.tri.inds {cluster} | R Documentation

## Permute Indices for Triangular Matrices

### Description

Compute index vectors for extracting or reordering of lower or upper triangular matrices that are stored as contiguous vectors.

### Usage

    lower.to.upper.tri.inds(n)
    upper.to.lower.tri.inds(n)

### Arguments

`n` | integer larger than 1.

### Value

integer vector containing a permutation of `1:N` where N = n(n-1)/2.

### Note

these functions are mainly for internal use in the cluster package, and may not remain available (unless we see a good reason).

### See Also

`upper.tri`, `lower.tri` with a related purpose.

### Examples

    m5 <- matrix(NA,5,5)
    m <- m5; m[lower.tri(m)] <- upper.to.lower.tri.inds(5); m
    m <- m5; m[upper.tri(m)] <- lower.to.upper.tri.inds(5); m

    stopifnot(lower.to.upper.tri.inds(2) == 1,
              lower.to.upper.tri.inds(3) == 1:3,
              upper.to.lower.tri.inds(3) == 1:3,
         sort(upper.to.lower.tri.inds(5)) == 1:10,
         sort(lower.to.upper.tri.inds(6)) == 1:15)

[Package cluster version 1.11.9 Index]