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Specify how initial attractivenesses are generated

Source: R/blvim_initialise.R
blvim_initialise.Rd

blvim_initialise() creates a reusable specification for generating initial attractivenesses in blvim(). The generator is evaluated only after the dimensions, production values, and conversion factors of the model are known.

Usage

blvim_initialise(generator = blvim_simplex_initialise, ...)

Arguments

generator

A function used to generate initial attractivenesses. Its first argument must be context. The default uses sample_weighted_simplex().

...

Arguments stored in the specification and passed to generator.

Value

An object of class blvim_initialise.

Details

A generator must be a function whose first argument is context. It must return either:

  • a numeric vector representing one initial configuration;

  • a numeric matrix whose rows represent initial configurations; or

  • a list containing one of these objects in a component named Z.

The context argument is a named list containing:

  • costs: the cost matrix;

  • X: the production vector;

  • alpha and beta: the interaction-model parameters;

  • kappa: the conversion factors expanded to one value per destination;

  • C: total production, equal to sum(X);

  • bipartite: whether the model is bipartite;

  • n_origins: number of origins;

  • n_destinations: number of destinations.

Some of the parameters are easily deducible from others (C, n_origins and n_destinations) but are included to simplify the design of initialisers.

Additional arguments supplied through ... are stored in the specification and passed to generator when starting points are generated.

See also

blvim() for the use of this function, and blvim_simplex_initialise() for the default initialiser.

Examples

# Ten starting points from the default simplex generator
initialise <- blvim_initialise(n = 10)

# Emphasise proper faces of the simplex
boundary_initialise <- blvim_initialise(
  n = 100,
  face_fraction = 0.8,
  points_per_face = 5
)

# A custom generator
concentrated_initialise <- function(context, n = 20, concentration = 100) {
  centre <- rep(1 / context$n_destinations, context$n_destinations)
  U <- matrix(
    stats::rgamma(n * context$n_destinations,
      shape = concentration * centre
    ),
    nrow = n, byrow = TRUE
  )
  U <- U / rowSums(U)
  sweep(U * context$C, 2, context$kappa, "/")
}

initialise <- blvim_initialise(
  generator = concentrated_initialise,
  n = 50,
  concentration = 20
)