Using Solver in VBA

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Using Solver in VBA

Contents

Solver and Conventional VBA

Avoiding Solver Reference Problems

Preparing Solver for First Use

Solver Links

Solver and Conventional VBA

Solver is a powerful analysis tool, bundled with Excel and used for optimization and simulation of business

and engineering models. It can be even more powerful if used in conjunction with VBA, to automate solving of

multiple models which use different input parameters and constraints.

In a simple example, there are two factors in B5 and B6. The product (=B5*B6) is calculated in B8. Solver will

be used to find the maximum value of the target cell (the product in B8), subject to the constraint that both

factors (B5:B6) shall not exceed a value of 4. Select Solver from the Tools menu, and enter the appropriate

conditions and constraints in the Solver Parameters dialog.

Click the Solve button, and another dialog indicates whether a solution is found and offers some options.

Using Solver in VBA http://peltiertech.com/Excel/SolverVBA.html

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If you record a macro while you use Solver, you will get something like the following:

Sub SolverMacro1()'' SolverMacro1 Macro' Macro recorded by Jon Peltier' SolverOk SetCell:="$B$8", MaxMinVal:=1, ValueOf:="0", ByChange:="$B$5:$B$6" SolverAdd CellRef:="$B$5:$B$6", Relation:=1, FormulaText:="4" SolverOk SetCell:="$B$8", MaxMinVal:=1, ValueOf:="0", ByChange:="$B$5:$B$6" SolverSolveEnd Sub

SolverAdd adds constraints to the Solver model. SolverOK defines the cell to optimize, how to optimize it, and

what cells to change during the Solver optimization. The macro recorder wrote this line twice, so the first

occurrence can be removed. To prevent parameters from a different Solver optimization interfering with the

macro's optimization, Solver should be reset prior to running, using SolverReset. SolverSolve has an optional

UserFinish argument; if UserFinish is False or omitted, the second dialog shown above will ask the user

whether to save the optimization, but if UserFinish is True, Solver will end without the dialog. A modified

Solver macro is shown below:

Sub SolverMacro2()'' SolverMacro2 Macro' Macro fixed up by Jon Peltier' SolverReset SolverAdd CellRef:="$B$5:$B$6", Relation:=1, FormulaText:="4" SolverOk SetCell:="$B$8", MaxMinVal:=1, ValueOf:="0", ByChange:="$B$5:$B$6" SolverSolve TrueEnd Sub

When you try to run this macro, you get a compile error. The command SolverReset is highlighted, and the

following error message appears.

In order to use a macro based on an installed add-in, you must first make sure that the add-in is installed,

then you must set a reference to the add-in in the workbook containing the code that calls the add-in's

procedures.

To install an add-in, on Excel's Tools menu, choose Add-Ins. If the add-in is shown on the list, check the box

in front of its name. If the add-in is not found, click Browse, navigate to the add-in file*, then when it appears


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on the add-in list, check its checkbox. Solver was already installed, or we would not have been able to record

a macro using it.

*Depending on your Office and Windows versions, the default Excel add-ins library is "C:\Program

Files\Microsoft Office\OFFICE11\Library" or "C:\Documents and Settings\{username}\Application

Data\Microsoft\AddIns". By default in Excel 2003, Solver is located in "C:\Program Files\Microsoft

Office\OFFICE11\Library\SOLVER".

To set a reference to an add-in, it must first be installed. Then on the VB Editor's Tools menu, select

References. This lists all open workbooks and installed add-ins, as well as a huge list of resources installed on

the host computer. Find the add-in in the list, and check the box in front of its name.


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With a reference set to Solver, SolverMacro2

will run as expected. In addition, the Solver

library will be accessible through the VB

Editor's Object Browser (right), and you will

have the benefit of Intellisense (below) while

editing code that uses members of the Solver

library.


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Avoiding Solver Reference Problems

The code you write to run Solver will work on your computer, and on any computer with the same versions of

Excel and Solver. In fact, it should work on any computer that has later versions of Excel and Solver. If you

want to distribute your workbook with VBA code written for Solver, you should write the code using the

earliest expected version of Excel (e.g., Excel 2000), so it will work on all versions that users may have

installed (e.g., Excel 2000, 2002, and 2003). When the workbook is first opened on a given computer, it finds

the references resources, or more recent versions if available.

This sounds easy, but sometimes it isn't. Perhaps you developed a workbook in Excel 2003 for your

department to use, but you have to send it to a supplier, and the supplier hasn't upgraded past Excel 2000. Or

perhaps the workbook must be shared amongst a group of users who have different versions of Excel and

Solver installed. In these cases, a computer with an earlier version of Solver installed will choke on the

reference to a later version of Solver.

It is possible, of course, to install add-ins and set references using VBA. This can be tricky, and in Microsoft

Office 2002 and later, the user has to grant permission for VBA code to access any VB projects. Without this

permission, references to installed components cannot be set.

To avoid issues with installing add-ins and setting references to various resources, your code can be modified

so that it is called using Application.Run. Without a reference to the add-in, you lose IntelliSense and the

Object Browser, and your code suffers from a small (probably imperceptible) performance penalty. However,

you gain simpler, more reliable execution. The syntax is straightforward: Application.Run is followed by the

procedure name in double quotes, followed by a comma separated list of arguments being passed to the

procedure:

Application.Run "SubName", Argument1, Argument2,...

If Application.Run is used to return the calculated result of a function, the syntax is slightly different, with a

variable set equal to Application.Run, with the procedure and arguments enclosed within parentheses:

MyVariable = Application.Run("Function", Argument1, Argument2,...)

The SolverMacro2 procedure above is easily modified to use Application.Run:

Sub SolverMacro3()'' SolverMacro3 Macro' Macro fixed up by Jon Peltier' Edited to use Application.Run to avoid reference problems' Application.Run "SolverReset" Application.Run "SolverAdd", "$B$5:$B$6", 1, "4" Application.Run "SolverOk", "$B$8", 1, "0", "$B$5:$B$6" Application.Run "SolverSolve", TrueEnd Sub

A more general version of a Solver procedure is shown below. This includes more informative comments, and

it provides a notice to the user about the success of the Solver optimization.

Sub RunSolver() '' Adjusted for Application.Run() to avoid Reference problems with Solver '' Peltier Technical Services, Inc., Copyright 2007. All rights reserved.


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' reset Application.Run "Solver.xla!SolverReset"

' set up new analysis Application.Run "Solver.xla!SolverOk", "Blah1", 1, , "BlahBlah1"

' add constraints Application.Run "Solver.xla!SolverAdd", "Blah2", 3, 0 Application.Run "Solver.xla!SolverAdd", "Blah3", 2, "BlahBlah3"

' run the analysis Result = Application.Run("Solver.xla!SolverSolve", True)

' finish the analysis Application.Run "Solver.xla!SolverFinish"

' report on success of analysis If Result

If Not bSolverInstalled Then ' (re)install Solver Application.AddIns("Solver Add-In").Installed = True ' check whether Solver is installed (should be true) bSolverInstalled = Application.AddIns("Solver Add-In").Installed End If

If Not bSolverInstalled Then MsgBox "Solver not found. This workbook will not work.", vbCritical CheckSolver = False End If

If CheckSolver Then ' initialize Solver Application.Run "Solver.xla!Solver.Solver2.Auto_open" End If

On Error GoTo 0

End Function

Solver Links

Frontline Systems

Frontline Systems has developed Solver add-ins for Excel and other applications. The standard Excel Solver

add-in can be upgraded to a premium Solver version or to other specialized Solvers, and there are versions

for use with other programming platforms. While the capabilities of Solver are very extensive, the online

documentation is somewhat sparse.

Solver Tutorial

Standard Excel Solver

Optimization Solutions with the Microsoft Excel Solver (examples)

Controlling the Solver with VBA

Microsoft

Solver (list of articles about Solver and Excel 2003)

Introduction to optimization with the Excel (2003) Solver tool

How to create Visual Basic macros by using Excel Solver in Excel 97

Perform What-If Analysis with the Excel 2007 Solver Tool

Around the Web

Microsoft Excel Solver add-in Examples (Vertex42)

Teaching Linear Programming using Microsoft Excel Solver

Using Solver for LP Problems

Solving a Linear Program Using the Excel Solver

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Peltier Technical Services, Inc.We can customize or automate any example from this site, or build a new one for you. Todiscuss your project or obtain a quote, please contact me at [email protected]

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