SECTION:Frequently asked questions about SoPlex QUESTION: I have installation problems. What can I do? LABEL:installationproblems ANSWER:

Please have a look at the INSTALL file.

QUESTION: How can I contribute? LABEL:howtocontribute ANSWER:

Please report bugs and problems.
In fact, a couple of improvements of SoPlex are possible. If you are interested in helping please contact us.

QUESTION:So, how do I start working with SoPlex? LABEL:howtostart ANSWER:

To just read LPs from a file and solve them without modifying the LPs, compile SoPlex and use it on the command line. Calling the binary without parameters gives you some usage information. To use SoPlex as a callable library, see src/example.cpp or src/soplexmain.cpp (from which the SoPlex binary is compiled) on how to construct an LP, solve it, and extract the solution. For further details, you have to consult the doxygen documentation of the code (or even the code itself). Another good starting point might also be the SoPlex interface of SCIP.

QUESTION:Can I use SoPlex with Branch-and-cut or MIP solvers? LABEL:usesoplexinsidemipsolvers ANSWER:

Yes, but the corresponding software needs an interface. Currently, the following frameworks have an interface to SoPlex.

QUESTION:Why is <iostream> used but <assert.h> and not <cassert>? LABEL:assertvscassert

The reason is twofold. From the theoretical point we were not able to exactly find out in TC++PL in what namespace cassert should load its declarations. Surely in std. But since these are normally functions with C linkage, this won't work. Then some of them, like assert, are macros which have no namespace. The practical point is that the compiler vendors seem to be unsure as well. Most of them put everything in both namespaces std and global. So there is no advantage in using <cassert>. Compaq even left them off because it seemed unclear to them. So our reasoning was: If it is a C++ header we use the correct form without the .h and in std. If it is a C header, according to the standard the .h header has to be there and uses the global namespace. That seems acceptable to us, especially for C functions.

QUESTION:Why is malloc/free sometimes used and not new/delete? LABEL:mallocvsnew ANSWER:

Because there is no realloc with new/delete. Because malloc is faster. And we only use it for builtin types or so called "Data Objects" . If you do not like this decision, it is quite easy to change spxalloc.h such as to use new/delete.

QUESTION:Can SoPlex solve Integer Programs (IPs)? LABEL:solvingips ANSWER:

No. You need an IP-Solver for this. Most IP-Solver use LP-Solvers as a subroutine and do some kind of Branch-and-Bound. For instance, you can use SCIP (Solving Constraint Integer Programs) together with SoPlex to solve IPs. SCIP can be obtained at here.

QUESTION:Is there a Windows version? LABEL:windoof ANSWER:

The code is tested to compile under Microsoft Visual Studio 2008 and 2010. It should also be possible to create a Windows version by compiling it with the GCC under Cygwin or minGW

QUESTION:I want a primal and a dual simplex, where are they? LABEL:primaldualsimplex ANSWER:

SoPlex implements a composite simplex, i.e., it switches between primal and dual simplex and the user can only specify the starting algorithm via the integer parameter "int:algorithm = 0 (primal) / 1 (dual)".

When using the low-level class SPxSolver, you can specify to use the ENTERing and LEAVEing algorithm and COLUMN and ROW representation.

 ENTER LEAVE
COLUMNPrimalDual
ROW Dual Primal

COLUMN oriented is the "usual" representation. Then Entering is the Primal and Leaving is the Dual algorithm. In ROW oriented representation, we have in principle the explicit dual and then the algorithms are reversed.

QUESTION:I got a segment violation or a signal 11. LABEL:segmentationfault ANSWER:

If all of the test instances from Netlib work, but your LP gives this problem, mail your LP in as an gzip'ed MPS of LP file and we will check. If you have this problem also with the test instances, check your stack space: ulimit -s will report the current size in kilobytes. Try a higher value. If this doesn't help, maybe your compiler is broken. Try compiling without optimization.

QUESTION:I got messages or exceptions stating "This shall not be." or similar. LABEL:numericaltroubles ANSWER:

Such messages are an indication of numerical trouble, which may happen from time to time especially on LPs with weird numbers. If the problem occurs with the binary, try using another scaler (call soplex without parameters to see how to specify this). If the problem occurs while using the callable library of SoPlex, you can try to add a scaler and a preprocessor yourself (see the SoPlex class) unless you solve the same LP repeatedly with small changes (as in a Branch-and-cut process, for instance). If you send us an LP or MPS file with the offending linear program together with a description of your settings, we may have a closer look at it to see if we can do something about it.

QUESTION:Can SoPlex handle basis files? LABEL:basisfiles ANSWER:

Yes, the binary version allows to read and write basis files using the options --readbas= and writebas=, respectively. The library version allows to read or write a basis file from within the code as well. This can be useful for debugging. If you cannot reconstruct an error occurring in the library code with the binary version, try to save a basis and try again. Note that the LP needs to be in MPS format in order to ensure compatability of the basis and the LP (the LP format cannot store ranged rows).

QUESTION:SoPlex means "Sequential Simplex". Is there a parallel version available? LABEL:parallelversion ANSWER:

No. Some research was done in this direction. You can find most of the results in the PhD thesis of Roland Wunderling Paralleler und Objektorientierter Simplex-Algorithmus (in German) and ZIB-Report SC-45-95 Parallelizing the Dual Simplex Method by Robert E. Bixby and Alexander Martin.

QUESTION:Is there a wrapper class/library to use SoPlex instead of CPLEX ? LABEL:soplexwrapperclass ANSWER:

No.

QUESTION:How can I make LP generation easier? LABEL:lpgeneration ANSWER:

You can use ZIMPL, for example. It takes a (human readable) file describing the linear program together with a data file as input and generates LPs or MIPs in LP- or MPS-format.

QUESTION:What kind of preprocessing is done? LABEL:preprocessing ANSWER:

In the default setting SoPlex presolves the given LP in order to simplify the problem as far as possible by removing rows, columns, and bounds. Moreover, infeasibility or unboundedness may be detected. The simplification cannot be undone, but given a primal/dual solution for the simplified LP, the simplifier can reconstruct the primal/dual solution and basis of the unsimplified LP.

Handled are:

The preprocessing can be switched off by using the option -s0.