Minimizes a general objective function subject to linear equality/inequality constraints.
Required Arguments
FCN — User-supplied subroutine to evaluate the function to be minimized. The usage is CALLFCN (N, X, F), where
N – Value of NVAR. (Input)
X – Vector of length N at which point the function is evaluated. (Input) X should not be changed by FCN.
F – The computed function value at the point X. (Output)
FCN must be declared EXTERNAL in the calling program.
NEQ — The number of linear equality constraints. (Input)
A — NCON by NVAR matrix. (Input) The matrix contains the equality constraint gradients in the first NEQ rows, followed by the inequality constraint gradients.
B — Vector of length NCON containing right-hand sides of the linear constraints. (Input) Specifically, the constraints on the variables X(I), I = 1, …, NVAR are A(K, 1)*X(1) + … + A(K, NVAR)*X(NVAR).EQ.B(K), K = 1, …, NEQ.A(K, 1)*X(1) + … + A(K, NVAR)*X(NVAR).LE.B(K), K = NEQ + 1, …, NCON. Note that the data that define the equality constraints come before the data of the inequalities.
XLB — Vector of length NVAR containing the lower bounds on the variables; choose a very large negative value if a component should be unbounded below or set XLB(I) = XUB(I) to freeze the I‑th variable. (Input) Specifically, these simple bounds are XLB(I).LE.X(I), I = 1, …, NVAR.
XUB — Vector of length NVAR containing the upper bounds on the variables; choose a very large positive value if a component should be unbounded above. (Input) Specifically, these simple bounds are X(I).LE.XUB(I), I = 1, …, NVAR.
SOL — Vector of length NVAR containing solution. (Output)
Optional Arguments
NVAR — The number of variables. (Input) Default: NVAR = SIZE (A,2).
NCON — The number of linear constraints (excluding simple bounds). (Input) Default: NCON = SIZE (A,1).
LDA — Leading dimension of A exactly as specified in the dimension statement of the calling program. (Input) Default: LDA = SIZE (A,1).
XGUESS — Vector of length NVAR containing the initial guess of the minimum. (Input) Default: XGUESS = 0.0.
ACC — The nonnegative tolerance on the first order conditions at the calculated solution. (Input) Default: ACC = 1.e-4 for single precision and 1.d-8 for double precision.
MAXFCN — On input, maximum number of function evaluations allowed. (Input/ Output) On output, actual number of function evaluations needed. Default: MAXFCN = 400.
OBJ — Value of the objective function. (Output)
NACT — Final number of active constraints. (Output)
IACT — Vector containing the indices of the final active constraints in the first NACT positions. (Output) Its length must be at least NCON + 2 *NVAR.
ALAMDA — Vector of length NVAR containing the Lagrange multiplier estimates of the final active constraints in the first NACT positions. (Output)
FORTRAN 90 Interface
Generic: CALLLCONF (FCN, NEQ, A, B, XLB, XUB, SOL[, …])
Specific: The specific interface names are S_LCONF and D_LCONF.
The routine LCONF is based on M.J.D. Powell’s TOLMIN, which solves linearly constrained optimization problems, i.e., problems of the form
subject to A1x = b1
A2x ≤ b2
xl ≤ x ≤ xu
given the vectors b1, b2, xl and xu and the matrices A1, and A2.
The algorithm starts by checking the equality constraints for inconsistency and redundancy. If the equality constraints are consistent, the method will revise x0, the initial guess provided by the user, to satisfy
A1x = b1
Next, x0 is adjusted to satisfy the simple bounds and inequality constraints. This is done by solving a sequence of quadratic programming subproblems to minimize the sum of the constraint or bound violations.
Now, for each iteration with a feasible xk, let Jk be the set of indices of inequality constraints that have small residuals. Here, the simple bounds are treated as inequality constraints. Let Ik be the set of indices of active constraints. The following quadratic programming problem
subject to ajd = 0 j∈Ik
ajd ≤ 0 j∈Jk
is solved to get (dk, λk) where aj is a row vector representing either a constraint in A1or A2 or a bound constraint on x. In the latter case, the aj = ei for the bound constraint xi ≤ (xu)i and aj = -ei for the constraint ‑xi≤ (-xl)i. Here, ei is a vector with a 1 as the i-th component, and zeroes elsewhere. λk are the Lagrange multipliers, and Bk is a positive definite approximation to the second derivative ∇2f(xk).
After the search direction dk is obtained, a line search is performed to locate a better point. The new point xk+1= xk + αkdk has to satisfy the conditions
and
The main idea in forming the set Jk is that, if any of the inequality constraints restricts the step-length αk, then its index is not in Jk. Therefore, small steps are likely to be avoided.
Finally, the second derivative approximation, Bk , is updated by the BFGS formula, if the condition
holds. Let xk←xk+1, and start another iteration.
The iteration repeats until the stopping criterion
is satisfied; here, is a user-supplied tolerance. For more details, see Powell (1988, 1989).
Since a finite-difference method is used to estimate the gradient for some single precision calculations, an inaccurate estimate of the gradient may cause the algorithm to terminate at a noncritical point. In such cases, high precision arithmetic is recommended. Also, whenever the exact gradient can be easily provided, routine LCONG should be used instead.
Comments
1. Workspace may be explicitly provided, if desired, by use of L2ONF/DL2ONF. The reference is:
INFO — Informational flag (see Comment 3). (Output)
WK — Real work vector of length NVAR**2 + 11 * NVAR + NCON.
2. Informational Errors
Type
Code
Description
4
4
The equality constraints are inconsistent.
4
5
The equality constraints and the bounds on the variables are found to be inconsistent.
4
6
No vector X satisfies all of the constraints. In particular, the current active constraints prevent any change in X that reduces the sum of constraint violations.
4
7
Maximum number of function evaluations exceeded.
4
9
The variables are determined by the equality constraints.
3. The following are descriptions of the arguments IPRINT and INFO:
IPRINT — This argument must be set by the user to specify the frequency of printing during the execution of the routine LCONF. There is no printed output if IPRINT = 0. Otherwise, after ensuring feasibility, information is given every IABS(IPRINT) iterations and whenever a parameter called TOL is reduced. The printing provides the values of X(.), F(.) and G(.) = GRAD(F) if IPRINT is positive. If IPRINT is negative, this information is augmented by the current values of IACT(K) K = 1, …, NACT, PAR(K) K = 1, …, NACT and RESKT(I) I = 1, …, N. The reason for returning to the calling program is also displayed when IPRINT is nonzero.
INFO — On exit from L2ONF, INFO will have one of the following integer values to indicate the reason for leaving the routine:
INFO = 1 SOL is feasible, and the condition that depends on ACC is satisfied.
INFO = 2 SOL is feasible, and rounding errors are preventing further progress.
INFO = 3 SOL is feasible, but the objective function fails to decrease although a decrease is predicted by the current gradient vector.
INFO = 4 In this case, the calculation cannot begin because LDA is less than NCON or because the lower bound on a variable is greater than the upper bound.
INFO = 5 This value indicates that the equality constraints are inconsistent. These constraints include any components of X(.) that are frozen by setting XL(I) = XU(I).
INFO = 6 In this case there is an error return because the equality constraints and the bounds on the variables are found to be inconsistent.
INFO = 7 This value indicates that there is no vector of variables that satisfies all of the constraints. Specifically, when this return or an INFO = 6 return occurs, the current active constraints (whose indices are IACT(K), K = 1, …, NACT) prevent any change in X(.) that reduces the sum of constraint violations. Bounds are only included in this sum if INFO = 6.
INFO = 8 Maximum number of function evaluations exceeded.
INFO = 9 The variables are determined by the equality constraints.
Example
The problem from Schittkowski (1987)
min f(x) = ‑x1x2x3
subject to ‑x1‑2x2‑2x3 ≤ 0
x1 +2 x2 + 2 x3 ≤ 72
0 ≤ x1 ≤ 20
0 ≤ x2 ≤ 11
0 ≤ x3 ≤ 42
is solved with an initial guess x1 = 10, x2 = 10 and x3 = 10.
USE LCONF_INT
USE UMACH_INT
IMPLICIT NONE
! Declaration of variables
INTEGER NCON, NEQ, NVAR
PARAMETER (NCON=2, NEQ=0, NVAR=3)
!
INTEGER MAXFCN, NOUT
REAL A(NCON,NVAR), ACC, B(NCON), OBJ, &
SOL(NVAR), XGUESS(NVAR), XLB(NVAR), XUB(NVAR)
EXTERNAL FCN
!
! Set values for the following problem.
!
! Min -X(1)*X(2)*X(3)
!
! -X(1) - 2*X(2) - 2*X(3) .LE. 0
! X(1) + 2*X(2) + 2*X(3) .LE. 72
!
! 0 .LE. X(1) .LE. 20
! 0 .LE. X(2) .LE. 11
! 0 .LE. X(3) .LE. 42
!
DATA A/-1.0, 1.0, -2.0, 2.0, -2.0, 2.0/, B/0.0, 72.0/
DATA XLB/3*0.0/, XUB/20.0, 11.0, 42.0/, XGUESS/3*10.0/
