R: Expand a Vector into a Symmetric Matrix

xpnd {MCMCpack} | R Documentation

## Expand a Vector into a Symmetric Matrix

### Description

This function takes a vector of appropriate length (typically created using `vech`) and creates a symmetric matrix.

### Usage

    xpnd(x, nrow = NULL)

### Arguments

`x` | A list of elements to expand into symmetric matrix.
`nrow` | The number of rows (and columns) in the returned matrix. Look into the details.

### Details

This function is particularly useful when dealing with variance covariance matrices. Note that R stores matrices in column major order, and that the items in `x` will be recycled to fill the matrix if need be.

The number of rows can be specified or automatically computed from the number of elements in a given object via (-1 + √{(1 + 8 * length(x))}) / 2.

### Value

An (nrows \times nrows) symmetric matrix.

### See Also

`vech`

### Examples

      xpnd(c(1,2,3,4,4,5,6,7,8,9),4)
      xpnd(c(1,2,3,4,4,5,6,7,8,9))

[Package MCMCpack version 1.4-0 Index]