SECTION:Frequently asked questions about SoPlex

    QUESTION: I have installation problems. What can I do?
    LABEL:installationproblems
    ANSWER:
    <p>
        Please have a look at the <code>INSTALL</code> file.
    </p>

    QUESTION: How can I contribute?
    LABEL:howtocontribute
    ANSWER:
    <p>
        Please report
        <a href="http://soplex.zib.de/#bugs">bugs and problems</a>.<br/>

        In fact, a couple of improvements of SoPlex are
        possible. If you are interested in helping please contact
        us.
    </p>

    QUESTION:So, how do I start working with SoPlex?
    LABEL:howtostart
    ANSWER:
    <p>
        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 <a href="http://scipopt.org">SCIP</a>.
   </p>

    QUESTION:Can I use SoPlex with Branch-and-cut or MIP solvers?
    LABEL:usesoplexinsidemipsolvers
    ANSWER:
    <p>
        Yes, but the corresponding software needs an interface.

        Currently, the following frameworks have an
        interface to SoPlex.
    </p>
    <ul>
        <li>
            <a  href="http://scipopt.org">SCIP</a> - Solving
            Constraint Integer Programs.
        </li>
        <li>
            <a href="http://www.informatik.uni-koeln.de/abacus/">ABACUS</a>
            has an older interface to 1.2.1 only.
        </li>
        <li>
            There is also an interface to <a
            href="https://projects.coin-or.org/Osi">COIN
            OSI</a>.
        </li>
    </ul>

    QUESTION:Why is &lt;iostream&gt; used but &lt;assert.h&gt; and not &lt;cassert&gt;?
    LABEL:assertvscassert

    <p>
        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 &lt;cassert&gt;. 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.
    </p>

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

    QUESTION:Can SoPlex solve Integer Programs (IPs)?
    LABEL:solvingips
    ANSWER:
    <p>
        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
        <a href="http://scipopt.org/">here</a>.
    </p>


    QUESTION:Is there a Windows version?
    LABEL:windoof
    ANSWER:
    <p>
        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 <a href="http://www.cygwin.com/">Cygwin</a> or minGW
    </p>

    QUESTION:I want a primal and a dual simplex, where are they?
    LABEL:primaldualsimplex
    ANSWER:
    <p>
        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)".
    </p>
    <p>
        When using the low-level class SPxSolver, you can specify to use the
        ENTERing and LEAVEing algorithm and COLUMN and ROW representation.
    </p>
    <center>
        <table border="1" rules="all">
            <tr><td>&nbsp;</td><td>ENTER </td><td>LEAVE </td></tr>
            <tr><td>COLUMN</td><td>Primal</td><td>Dual  </td></tr>
            <tr><td>ROW   </td><td>Dual  </td><td>Primal</td></tr>
        </table>
    </center>
    <p>
        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.
    </p>

    QUESTION:I got a segment violation or a signal 11.
    LABEL:segmentationfault
    ANSWER:
    <p>
        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.
    </p>

    QUESTION:I got messages or exceptions stating &quot;This shall not be.&quot; or similar.
    LABEL:numericaltroubles
    ANSWER:
    <p>
        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.
    </p>


    QUESTION:Can SoPlex handle basis files?
    LABEL:basisfiles
    ANSWER:
    <p>
        Yes, the binary version allows to read and write basis files using the
        options <code>--readbas=</code> and <code>writebas=</code>, 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).
    </p>

    QUESTION:SoPlex means &quot;Sequential Simplex&quot;. Is there a parallel version
        available?
    LABEL:parallelversion
    ANSWER:
    <p>
        No. Some research was done in this direction. You can find most of
        the results in
        <a href="http://www.zib.de/PaperWeb/abstracts/TR-96-09">
        http://www.zib.de/PaperWeb/abstracts/TR-96-09</a> and
        <a href="http://www.zib.de/PaperWeb/abstracts/SC-95-45">
        http://www.zib.de/PaperWeb/abstracts/SC-95-45</a>.
    </p>

    QUESTION:Is there a wrapper class/library to use SoPlex instead of CPLEX ?
    LABEL:soplexwrapperclass
    ANSWER:
    <p>
        No.
    </p>

    QUESTION:How can I make LP generation easier?
    LABEL:lpgeneration
    ANSWER:
    <p>
        You can use <a href="http://zimpl.zib.de/">ZIMPL</a>, 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.
    </p>

    QUESTION:What kind of preprocessing is done?
    LABEL:preprocessing
    ANSWER:
    <p>
        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.
    </p>
    <p>
        Handled are:
    </p>
    <ul>
        <li> empty rows / columns                                  </li>
        <li> unconstrained rows                                    </li>
        <li> row singletons                                        </li>
        <li> forcing rows                                          </li>
        <li> zero objective column singletons                      </li>
        <li> (implied) free column singletons                      </li>
        <li> doubleton equations combined with a column singleton  </li>
        <li> (implicitly) fixed columns                            </li>
        <li> redundant lhs / rhs                                   </li>
        <li> redundant variable bounds                             </li>
        <li> variables that are free in one direction              </li>
        <li> (weakly) dominated columns                            </li>
        <li> duplicate rows / columns                              </li>
    </ul>

    <p>
        The preprocessing can be switched off by using the option  <code>-s0</code>.
    </p>
