6.8 Integer Optimization

An optimization problem where one or more of the variables are constrained to integer values is called a (mixed) integer optimization problem. MOSEK supports integer variables in combination with linear, quadratic and quadratically constrtained and conic problems (except semidefinite). See the previous tutorials for an introduction to how to model these types of problems.

6.8.1 Example MILO1

We use the example

(6.24)\[\begin{split}\begin{array}{lccl} \mbox{maximize} & x_0 + 0.64 x_1 & & \\ \mbox{subject to} & 50 x_0 + 31 x_1 & \leq & 250, \\ & 3 x_0 - 2 x_1 & \geq & -4, \\ & x_0, x_1 \geq 0 & & \mbox{and integer} \end{array}\end{split}\]

to demonstrate how to set up and solve a problem with integer variables. It has the structure of a linear optimization problem (see Sec. 6.1 (Linear Optimization)) except for integrality constraints on the variables. Therefore, only the specification of the integer constraints requires something new compared to the linear optimization problem discussed previously.

This is easily programmed in R using the piece code shown in Listing 6.11,

Listing 6.11 R implementation of problem (6.24). Click here to download.
#  Copyright : Copyright (c) MOSEK ApS, Denmark. All rights reserved.
#  File :      milo1.R
#  Purpose :   To demonstrate how to solve a small mixed integer linear
#              optimization problem using the Rmosek package.

milo1 <- function()
    # Specify the continuous part of the problem.
    prob <- list(sense="max")
    prob$c  <- c(1, 0.64)
    prob$A  <- Matrix(rbind(c(50, 31),
                            c( 3, -2)), sparse=TRUE)
    prob$bc <- rbind(blc=c(-Inf,  -4),
                     buc=c( 250, Inf))
    prob$bx <- rbind(blx=c(  0,   0),
                     bux=c(Inf, Inf))

    # Specify the integer constraints
    prob$intsub <- c(1 ,2)

    # Solve the problem
    r <- mosek(prob)

    # Return the solution
    stopifnot(identical(r$response$code, 0))


where \(x_1\) and \(x_2\) are pointed out as integer variables.

The input arguments follow those of a linear or conic program with the additional identification of the integer variables. The column vector intsub should simply contain indexes to the subset of variables for which integrality is required. For instance if \(x\) should be a binary \(\{0,1\}\)-variable, its index in the problem formulation should be added to intsub, and its bounds \(0 \leq x \leq 1\) should be specified explicitly.

If executed correctly you should be able to see the log of the interface and optimization process printed to the screen. The output structure will only include an integer solution int, since we are no longer in the continuous domain for which the interior-point algorithm operates. The structure also contains the problem status as well as the solution status based on certificates found by the MOSEK optimization library.

6.8.2 Specifying an initial solution

It is a common strategy to provide a starting feasible point (if one is known in advance) to the mixed-integer solver. This can in many cases reduce solution time.

There are two modes for MOSEK to utilize an initial solution.

  • A complete solution. MOSEK will first try to check if the current value of the primal variable solution is a feasible point. The solution can either come from a previous solver call or can be entered by the user, however the full solution with values for all variables (both integer and continuous) must be provided. This check is always performed and does not require any extra action from the user. The outcome of this process can be inspected via information items "MSK_IINF_MIO_INITIAL_FEASIBLE_SOLUTION" and "MSK_DINF_MIO_INITIAL_FEASIBLE_SOLUTION_OBJ", and via the Initial feasible solution objective entry in the log.

  • A partial integer solution. MOSEK can also try to construct a feasible solution by fixing integer variables to the values provided by the user (rounding if necessary) and optimizing over the remaining continuous variables. In this setup the user must provide initial values for all integer variables. This action is only performed if the parameter MSK_IPAR_MIO_CONSTRUCT_SOL is switched on. The outcome of this process can be inspected via information items "MSK_IINF_MIO_CONSTRUCT_SOLUTION" and "MSK_DINF_MIO_CONSTRUCT_SOLUTION_OBJ", and via the Construct solution objective entry in the log.

In the following example we focus on inputting a partial integer solution.

(6.25)\[\begin{split}\begin{array} {ll} \mbox{maximize} & 7 x_0 + 10 x_1 + x_2 + 5 x_3 \\ \mbox{subject to} & x_0 + x_1 + x_2 + x_3 \leq 2.5\\ & x_0,x_1,x_2 \in \integral \\ & x_0,x_1,x_2,x_3 \geq 0 \end{array}\end{split}\]

Solution values can be set using the appropriate fields in the problem structure.

Listing 6.12 Implementation of problem (6.25) specifying an initial solution. Click here to download.
    # Specify start guess for the integer variables.
    prob$sol$int$xx <- c(1, 1, 0, NaN)

    # Request constructing the solution from integer variable values
    prob$iparam <- list(MIO_CONSTRUCT_SOL=1)

The log output from the optimizer will in this case indicate that the inputted values were used to construct an initial feasible solution:

Construct solution objective       : 1.950000000000e+01

The same information can be obtained from the API:

Listing 6.13 Retrieving information about usage of initial solution Click here to download.

6.8.3 Example MICO1

Integer variables can also be used arbitrarily in conic problems (except semidefinite). We refer to the previous tutorials for how to set up a conic optimization problem. Here we present sample code that sets up a simple optimization problem:

(6.26)\[\begin{split}\begin{array}{ll} \mbox{minimize} & x^2+y^2 \\ \mbox{subject to} & x \geq e^y+3.8, \\ & x, y \ \mbox{integer}. \end{array}\end{split}\]

The canonical conic formulation of (6.26) suitable for Rmosek package is

(6.27)\[\begin{split}\begin{array}{llr} \mbox{minimize} & t & \\ \mbox{subject to} & (t,x,y)\in\Q^3 & (t\geq\sqrt{x^2+y^2}) \\ & (x-3.8, 1, y) \in\EXP & (x-3.8\geq e^y) \\ & x, y \ \mbox{integer}, & \\ & t\in\real. \end{array}\end{split}\]
Listing 6.14 Implementation of problem (6.27). Click here to download.

mico1 <- function()
    # Specify the continuous part of the problem.
    # Variables are [t; x; y]
    prob <- list(sense="min")
    prob$c <- c(1, 0, 0)
    prob$A <- Matrix(nrow=0, ncol=3)   # 0 constraints, 3 variables
    prob$bx <- rbind(blx=rep(-Inf,3), bux=rep(Inf,3))

    # Conic part of the problem
    prob$F <- rbind(diag(3), c(0,1,0), c(0,0,0), c(0,0,1))
    prob$g <- c(0, 0, 0, -3.8, 1, 0)
    prob$cones <- cbind(matrix(list("QUAD", 3, NULL), nrow=3, ncol=1),
                        matrix(list("PEXP", 3, NULL), nrow=3, ncol=1))
    rownames(prob$cones) <- c("type","dim","conepar")

    # Specify the integer constraints
    prob$intsub <- c(2 ,3)

    # It is as always possible (but not required) to input an initial solution
    # to start the mixed-integer solver. 
    prob$sol$int$xx <- c(100, 9, -1)

    # Solve the problem
    r <- mosek(prob, list(getinfo=TRUE))

    # Return the solution
    stopifnot(identical(r$response$code, 0))

Error and solution status handling were omitted for readability.