bmchk {optimx}

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, offset=100, 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 progress of optimization as an indicator that a parameter is at a lower bound (bdmsk element set to -3) or upper bound (-1).

trace

An integer that controls whether diagnostic information is displayed. A positive value displays information, 0 (default) does not.

offset

If provided, is used to detect equality of numbers. That is, two values a and b are taken as equal if a + offset is equal to b + offset. Default value is 100. Note that in previous versions of this code a tolerance tol was used.

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.

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.

Value

A list with components:

bvec

The vector of parameters, possibly adjusted to bounds. Parameters outside bounds are adjusted to the nearest bound. We let n be the length of this vector.

bdmsk

adjusted input masks

bchar

a set of indicators that give information about the parameters, that is, if they are out-of-bounds-low ("-"), at a lower bound ("L"), free ("F"), at an upper bound ("U"), out-of-bounds-high ("+"), masked (fixed) ("M"), of unknown characteristics ("?""), or inadmissible ("!"").

lower

(adjusted) lower bounds. If there are no lower bounds, a vector of values equal to -Inf is substituted. Similarly, a single value is expanded to a complete vector. If any upper and lower bounds are equal (as mediated by offset), we create a mask.

upper

(adjusted) upper bounds

nolower

TRUE if no lower bounds, FALSE otherwise

noupper

TRUE if no upper bounds, FALSE otherwise

bounds

TRUE if there are any bounds, FALSE otherwise

admissible

TRUE 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.

maskadded

TRUE when a mask has been added because bounds are very close or equal, FALSE otherwise. See the code for the implementation.

parchanged

TRUE 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.

feasible

TRUE if parameters are within or on bounds, FALSE otherswise.

onbound

TRUE 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.

Examples

#####################

## cat("25-dimensional box constrained function\n")
## 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("\n start:")
print(start)
lo<-rep(2,25)
cat("\n lo:")
print(lo)
hi<-rep(4,25)
cat("\n hi:")
print(hi)
bt<-bmchk(start, lower=lo, upper=hi, trace=1)
print(bt)