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Using Solver in Excel VBA
ContentsSolver and Conventional VBAAvoiding Solver Reference ProblemsPreparing Solver for First UseSolver LinksSolver and Conventional VBASolver 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. If you record a macro while you use Solver, you will get something like the following:
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:
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 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\OFFICE15\Library" or "C:\Documents and Settings\{username}\Application Data\Microsoft\AddIns". By default in Excel 2013, Solver is located in "C:\Program Files\Microsoft Office\OFFICE15\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.
Avoiding Solver Reference ProblemsThe 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, so it will work on all versions that users may have installed. 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 2013 for your department to use, but you have to send it to a supplier, and the supplier hasn't upgraded past Excel 2007. 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, 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:
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. Note that before Excel 2007, the name of the solver add-in was "Solver.xla", not "Solver.xlam".
The results of the SolverSolve function include: 0 Solver found a solution. All constraints and optimality conditions are satisfied. 1 Solver has converged to the current solution. All constraints are satisfied. 2 Solver cannot improve the current solution. All constraints are satisfied. 3 Stop chosen when the maximum iteration limit was reached. 4 The Set Cell values do not converge. 5 Solver could not find a feasible solution. 6 Solver stopped at user's request. 7 The conditions for Assume Linear Model are not satisfied. 8 The problem is too large for Solver to handle. 9 Solver encountered an error value in a target or constraint cell. 10 Stop chosen when maximum time limit was reached. 11 There is not enough memory available to solve the problem. 12 Another Excel instance is using SOLVER.DLL. Try again later. 13 Error in model. Please verify that all cells and constraints are valid. Preparing Solver for First UseOne frequent complaint about automating Solver is that it doesn't work using VBA until it has been used at least once manually. This is because Solver installs itself in a kind of "on demand" mode. Unlike a regularly-installed add-in, it is not opened until it is first used. And until it is first used, it hasn't run its Auto_Open procedure, which is what actually prepares it to run. Using VBA you can bypass the initial manual Solver operation with this command: Application.Run "Solver.xlam!Solver.Solver2.Auto_open" This command should be run before the first Solver optimization procedure is executed. I have developed a Solver initialization routine that first makes sure the computer even has Solver, then it installs it and runs its Auto_Open procedure. The procedure is written as a function, which returns True if Solver is available and ready to use. I usually call this procedure from the parent workbook's Workbook_Open event procedure. If CheckSolver is False, I usually have the workbook close itself after a brief warning to the user.
The function above works fine for English versions of Excel, but in other languages, the name of the add-in may not be "Solver Add-In". We have to be a bit more clever, and introduce a loop to check the filenames of all add-ins. The CheckSolverIntl function below calls two additional functions which perform the loops. This function still relies on Solver being named "solver.xlam". If this is not the case, for example, in different language versions of Excel, change the value of the constant sAddIn in this procedure, and please email me about it.
Solver LinksFrontline Systems
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