ga
can solve problems when certain variables
are integervalued. Give IntCon
, a vector of the x components
that are integers:
[x,fval,exitflag] = ga(fitnessfcn,nvars,A,b,[],[],... lb,ub,nonlcon,IntCon,options)
IntCon
is a vector of positive integers that
contains the x components that are integervalued.
For example, if you want to restrict x(2)
and x(10)
to
be integers, set IntCon
to [2,10]
.
Note:
Restrictions exist on the types of problems that 
Tip

This example shows how to find the minimum of Rastrigin's function restricted so the first component of x is an integer. The components of x are further restricted to be in the region .
Set up the bounds for your problem
lb = [5*pi,20*pi]; ub = [20*pi,4*pi];
Set a plot function so you can view the progress of ga
opts = gaoptimset('PlotFcns',@gaplotbestf);
Call the ga solver where x(1) has integer values
rng(1,'twister') % for reproducibility IntCon = 1; [x,fval,exitflag] = ga(@rastriginsfcn,2,[],[],[],[],... lb,ub,[],IntCon,opts)
Optimization terminated: average change in the penalty fitness value less than options.TolFun and constraint violation is less than options.TolCon. x = 16.0000 12.9325 fval = 424.1355 exitflag = 1
ga converges quickly to the solution.
There are some restrictions on the types of problems that ga
can
solve when you include integer constraints:
No linear equality constraints. You must have Aeq = []
and beq = []
. For a possible workaround, see No Equality Constraints.
No nonlinear equality constraints. Any nonlinear constraint
function must return []
for the nonlinear equality
constraint. For a possible workaround, see Example: Integer Programming with a Nonlinear Equality Constraint.
Only doubleVector
population type.
No custom creation function (CreationFcn
option),
crossover function (CrossoverFcn
option), mutation
function (MutationFcn
option), or initial scores
(InitialScores
option). If you supply any of these, ga
overrides
their settings.
ga
uses only the binary tournament
selection function (SelectionFcn
option), and overrides
any other setting.
No hybrid function. ga
overrides
any setting of the HybridFcn
option.
ga
ignores the ParetoFraction
, DistanceMeasureFcn
, InitialPenalty
,
and PenaltyFactor
options.
The listed restrictions are mainly natural, not arbitrary. For example:
There are no hybrid functions that support integer
constraints. So ga
does not use hybrid functions
when there are integer constraints.
To obtain integer variables, ga
uses
special creation, crossover, and mutation functions.
You cannot use equality constraints and integer constraints in the same problem. You can try to work around this restriction by including two inequality constraints for each linear equality constraint. For example, to try to include the constraint
3x_{1} – 2x_{2} = 5,
create two inequality constraints:
3x_{1} –
2x_{2} ≤ 5
3x_{1} –
2x_{2} ≥ 5.
To write these constraints in the form A x
≤ b
, multiply
the second inequality by 1
:
–3x_{1} + 2x_{2} ≤ –5.
You can try to include the equality constraint using A
= [3,2;3,2]
and b
= [5;5]
.
Be aware that this procedure can fail; ga
has
difficulty with simultaneous integer and equality constraints.
Example: Integer Programming with a Nonlinear Equality Constraint. This example attempts to locate the minimum of the Ackley function in five dimensions with these constraints:
x(1)
, x(3)
,
and x(5)
are integers.
norm(x) = 4
.
The Ackley function, described briefly in Resuming ga From the Final Population, is difficult to minimize. Adding integer and equality constraints increases the difficulty.
To include the nonlinear equality constraint, give a small tolerance tol
that
allows the norm of x
to be within tol
of 4
.
Without a tolerance, the nonlinear equality constraint is never satisfied,
and the solver does not realize when it has a feasible solution.
Write the expression norm(x) = 4
as two "less than zero" inequalities:
norm(x)  4
≤ 0
(norm(x)  4)
≤ 0
.
Allow a small tolerance in the inequalities:
norm(x)  4  tol
≤ 0
(norm(x)  4)  tol
≤ 0
.
Write a nonlinear inequality constraint function that implements these inequalities:
function [c, ceq] = eqCon(x) ceq = []; rad = 4; tol = 1e3; confcnval = norm(x)  rad; c = [confcnval  tol;confcnval  tol];
Set options:
StallGenLimit = 50
— Allow
the solver to try for a while.
TolFun = 1e10
— Specify
a stricter stopping criterion than usual.
Generations = 300
— Allow
more generations than default.
PlotFcns = @gaplotbestfun
—
Observe the optimization.
opts = gaoptimset('StallGenLimit',50,'TolFun',1e10,... 'Generations',300,'PlotFcns',@gaplotbestfun);
Set lower and upper bounds to help the solver:
nVar = 5; lb = 5*ones(1,nVar); ub = 5*ones(1,nVar);
Solve the problem:
rng(1,'twister') % for reproducibility [x,fval,exitflag] = ga(@ackleyfcn,nVar,[],[],[],[], ... lb,ub,@eqCon,[1 3 5],opts); Optimization terminated: stall generations limit exceeded and constraint violation is less than options.TolCon.
Examine the solution:
x,fval,exitflag,norm(x) x = 3.0000 0.8233 1.0000 1.1503 2.0000 fval = 5.1119 exitflag = 3 ans = 4.0001
The odd x
components are integers, as specified.
The norm of x
is 4
, to within
the given relative tolerance of 1e3
.
Despite the positive exit flag, the solution is not the global optimum. Run the problem again and examine the solution:
opts = gaoptimset(opts,'Display','off'); [x2,fval2,exitflag2] = ga(@ackleyfcn,nVar,[],[],[],[], ... lb,ub,@eqCon,[1 3 5],opts);
Examine the second solution:
x2,fval2,exitflag2,norm(x2) x2 = 1.0000 0.9959 2.0000 0.9960 3.0000 fval2 = 4.2359 exitflag2 = 1 ans = 3.9980
The second run gives a better solution (lower fitness function
value). Again, the odd x
components are integers,
and the norm of x2
is 4
, to
within the given relative tolerance of 1e3
.
Be aware that this procedure can fail; ga
has
difficulty with simultaneous integer and equality constraints.
Integer programming with ga
involves several
modifications of the basic algorithm (see How the Genetic Algorithm Works). For integer programming:
Special creation, crossover, and mutation functions enforce variables to be integers. For details, see Deep et al. [2].
The genetic algorithm attempts to minimize a penalty function, not the fitness function. The penalty function includes a term for infeasibility. This penalty function is combined with binary tournament selection to select individuals for subsequent generations. The penalty function value of a member of a population is:
If the member is feasible, the penalty function is the fitness function.
If the member is infeasible, the penalty function is the maximum fitness function among feasible members of the population, plus a sum of the constraint violations of the (infeasible) point.
For details of the penalty function, see Deb [1].
[1] Deb, Kalyanmoy. An efficient constraint handling method for genetic algorithms. Computer Methods in Applied Mechanics and Engineering, 186(2–4), pp. 311–338, 2000.
[2] Deep, Kusum, Krishna Pratap Singh, M.L. Kansal, and C. Mohan. A real coded genetic algorithm for solving integer and mixed integer optimization problems. Applied Mathematics and Computation, 212(2), pp. 505–518, 2009.