posterior (version 1.6.0)

rvar: Random variables of arbitrary dimension

Description

Random variables backed by arrays of arbitrary dimension

Usage

rvar(
  x = double(),
  dim = NULL,
  dimnames = NULL,
  nchains = NULL,
  with_chains = FALSE
)

Value

An object of class "rvar" representing a random variable.

Arguments

x

(multiple options) The object to convert to an rvar:

• A vector of draws from a distribution.

• An array where the first dimension represents draws from a distribution. The resulting rvar will have dimension dim(x)[-1]; that is, everything except the first dimension is used for the shape of the variable, and the first dimension is used to index draws from the distribution (see Examples). Optionally, if with_chains == TRUE, the first dimension indexes the iteration and the second dimension indexes the chain (see with_chains).

• An rvar.

dim

(integer vector) One or more integers giving the maximal indices in each dimension to override the dimensions of the rvar to be created (see dim()). If NULL (the default), dim is determined by the input. NOTE: This argument controls the dimensions of the rvar, not the underlying array, so you cannot change the number of draws using this argument.

dimnames

(list) Character vectors giving the names in each dimension to override the names of the dimensions of the rvar to be created (see dimnames()). If NULL (the default), this is determined by the input. NOTE: This argument controls the names of the dimensions of the rvar, not the underlying array.

nchains

(positive integer) The number of chains. The if NULL (the default), 1 is used unless x is already an rvar, in which case the number of chains it has is used.

with_chains

(logical) Does x include a dimension for chains? If FALSE (the default), chains are not included, the first dimension of the input array should index draws, and the nchains argument can be used to determine the number of chains. If TRUE, the nchains argument is ignored and the second dimension of x is used to index chains. Internally, the array will be converted to a format without the chain index. Ignored when x is already an rvar.

Details

The "rvar" class internally represents random variables as arrays of arbitrary dimension, where the first dimension is used to index draws from the distribution. Most mathematical operators and functions are supported, including efficient matrix multiplication and vector and array-style indexing. The intent is that an rvar works as closely as possible to how a base vector/matrix/array does, with a few differences:

• The default behavior when subsetting is not to drop extra dimensions (i.e. the default drop argument for [ is FALSE, not TRUE).

• Rather than base R-style recycling, rvars use a limited form of broadcasting: if an operation is being performed on two vectors with different size of the same dimension, the smaller vector will be recycled up to the size of the larger one along that dimension so long as it has size 1.

For functions that expect base numeric arrays and for which rvars cannot be used directly as arguments, you can use rfun() or rdo() to translate your code into code that executes across draws from one or more random variables and returns a random variable as output. Typically rdo() offers the most straightforward translation.

As rfun() and rdo() incur some performance cost, you can also operate directly on the underlying array using the draws_of() function. To re-use existing random number generator functions to efficiently create rvars, use rvar_rng().

See Also

as_rvar() to convert objects to rvars. See rdo(), rfun(), and rvar_rng() for higher-level interfaces for creating rvars.

Examples

set.seed(1234)

# To create a "scalar" `rvar`, pass a one-dimensional array or a vector
# whose length (here `4000`) is the desired number of draws:
x <- rvar(rnorm(4000, mean = 1, sd = 1))
x

# Create random vectors by adding an additional dimension:
n <- 4   # length of output vector
x <- rvar(array(rnorm(4000 * n, mean = rep(1:n, each = 4000), sd = 1), dim = c(4000, n)))
x

# Create a random matrix:
rows <- 4
cols <- 3
x <- rvar(array(rnorm(4000 * rows * cols, mean = 1, sd = 1), dim = c(4000, rows, cols)))
x

# If the input sample comes from multiple chains, we can indicate that using the
# nchains argument (here, 1000 draws each from 4 chains):
x <- rvar(rnorm(4000, mean = 1, sd = 1), nchains = 4)
x

# Or if the input sample has chain information as its second dimension, we can
# use with_chains to create the rvar
x <- rvar(array(rnorm(4000, mean = 1, sd = 1), dim = c(1000, 4)), with_chains = TRUE)
x