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optextras (version 2013-10.27)

bmchk: Check bounds and masks for parameter constraints used in nonlinear optimization

Description

Nonlinear optimization problems often have explicit or implicit upper and lower bounds on the parameters of the function to be miminized or maximized. These are called bounds or box constraints. Some of the parameters may be fixed for a given problem or for a temporary trial. These fixed, or masked, paramters are held at one value during a specific 'run' of the optimization.

It is possible that the bounds are inadmissible, that is, that at least one lower bound exceeds an upper bound. In this case we set the flag admissible to FALSE.

Parameters that are outside the bounds are moved to the nearest bound and the flag parchanged is set TRUE. However, we DO NOT change masked parameters, and they may be outside the bounds. This is an implementation choice, since it may be useful to test objective functions at point outside the bounds.

The package bmchk is essentially a test of the R function bmchk(), which is likely to be incorporated within optimization codes.

Usage

bmchk(par, lower=NULL, upper=NULL, bdmsk=NULL, trace=0, tol=NULL, shift2bound=TRUE)

Arguments

par
A numeric vector of starting values of the optimization function parameters.
lower
A vector of lower bounds on the parameters.
upper
A vector of upper bounds on the parameters.
bdmsk
An indicator vector, having 1 for each parameter that is "free" or unconstrained, and 0 for any parameter that is fixed or MASKED for the duration of the optimization. Partly for historical reasons, we use the same array during the progres
trace
An integer that controls whether diagnostic information is displayed. A positive value displays information, 0 (default) does not.
tol
If provided, is used to detect a MASK, that is, lower=upper for some parameter.
shift2bound
If TRUE, non-masked paramters outside bounds are adjusted to the nearest bound. We then set parchanged = TRUE which implies the original parameters were infeasible.

Value

  • A list with components:
  • bvecThe vector of parameters, possibly adjusted for bounds. Parameters outside bounds are adjusted to the nearest bound.
  • bdmskadjusted input masks
  • bcharindicator for humans -- "-","L","F","U","+","M" for out-of-bounds-low, lower bound, free, upper bound, out-of-bounds-high, masked (fixed)
  • lower(adjusted) lower bounds. If upper-lowerbmchk.R.
  • upper(adjusted) upper bounds
  • nolowerTRUE if no lower bounds, FALSE otherwise
  • noupperTRUE if no upper bounds, FALSE otherwise
  • boundsTRUE if there are any bounds, FALSE otherwise
  • admissibleTRUE if bounds are admissible, FALSE otherwise This means no lower bound exceeds an upper bound. That is the bounds themselves are sensible. This condition has nothing to do with the starting parameters.
  • maskaddedTRUE when a mask has been added because bounds are very close or equal, FALSE otherwise. See the code for the implementation.
  • parchangedTRUE if parameters are changed by bounds, FALSE otherswise. Note that parchanged = TRUE implies the input parameter values were infeasible, that is, violated the bounds constraints.
  • feasibleTRUE if parameters are within or on bounds, FALSE otherswise.
  • onboundTRUE if any parameter is on a bound, FALSE otherswise. Note that parchanged = TRUE implies onbound = TRUE, but this is not used inside the function. This output value may be important, for example, in using the optimization function nmkb from package dfoptim.

encoding

UTF-8

Details

The bmchk function will check that the bounds exist and are admissible, that is, that there are no lower bounds that exceed upper bounds.

There is a check if lower and upper bounds are very close together, in which case a mask is imposed and maskadded is set TRUE. NOTE: it is generally a VERY BAD IDEA to have bounds close together in optimization, but here we use a tolerance based on the double precision machine epsilon. Thus it is not a good idea to rely on bmchk() to test if bounds constraints are well-posed.

Examples

Run this code
#####################

cat("25-dimensional box constrained function
")
flb <- function(x)
    { p <- length(x); sum(c(1, rep(4, p-1)) * (x - c(1, x[-p])^2)^2) }

start<-rep(2, 25)
cat("start:")
print(start)
lo<-rep(2,25)
cat("lo:")
print(lo)
hi<-rep(4,25)
cat("hi:")
print(hi)
bt<-bmchk(start, lower=lo, upper=hi, trace=1)
print(bt)

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