Measure distance between current and previous state in terms of fish abundances

Calculates the sum squared difference between log(N) in current and previous state. This function can be used in projectUntilSettled() to decide when sufficient convergence to steady state has been achieved.

Only the size classes that hold fish are measured. A class whose density is zero in either state has no log to take, and one holding a negligible share of its species' biomass has a log that never stops moving: above a size where growth stops the density decays exponentially, so log(n) falls by the same amount between every pair of states, for ever. Left in the sum, a single such class holding \(10^{-92}\) g of fish can hold the distance above any tolerance indefinitely and stop projectUntilSettled() from ever converging, while the biomass drift correctly reports a fixed point. Excluding it changes nothing for a model that does not have one, because the classes that hold the fish are exactly the classes that are kept.

Usage

distanceSSLogN(
  params,
  current,
  previous,
  biomass_share_cutoff = steady_share_cutoff(),
  ...
)

Arguments

params

MizerParams

current

A named list with entries n, n_pp and n_other describing the current state

previous

A named list with entries n, n_pp and n_other describing the previous state

biomass_share_cutoff

A finite number between 0 and 1, inclusive, giving the share of a species' biomass that a size class must hold in the current state to be measured. 0 measures every class with a nonzero density in both states, which is what this function did before mizer 3.3.

...

Unused. Accepted because projectUntilSettled() forwards its own ... to both the rate functions and the distance function, so an argument meant for one arrives at the other.

Value

The sum of squares of the difference in the logs of the fish abundances n, over the size classes that hold fish in both states: sum((log(current$n) - log(previous$n))^2)

See also

Other distance functions: distanceMaxRelRDI()