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1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Matrix Classes</title><meta name="generator" content="DocBook XSL Stylesheets V1.66.1"><link rel="start" href="index.html" title="CLP User Guide"><link rel="up" href="ch03.html" title="Chapter 3. 
2  Not-Quite-So-Basic Model Classes
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6  Not-Quite-So-Basic Model Classes
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8  The next abstract class of interest is <tt class="function">ClpMatrixBase</tt>.
9  CLP encapsulates its knowledge of how a matrix is stored in this class.  The
10  default instance of this is the <tt class="function">ClpPackedMatrix</tt> class.
11  This is identical in format to <tt class="function">CoinPackedMatrix</tt>.  Below
12  is a diagram summarizing the hierarchy of the most important matrix classes:
13  </p><div class="mediaobject"><img src="figures/clpbasicmatrixhier.gif"></div><p>
14  The important new methods implemented are for filling a basis, checking validity
15  of elements and faster "times" and "transposeTimes" when
16  the input array is sparse and/or we have a row copy of the matrix.  Advanced
17  users should note that not all methods have to be implemented.  In particular,
18  <tt class="function">scaling</tt> need not be implemented and
19  <tt class="function">reverseOrderedCopy</tt> can return <i class="parameter"><tt>NULL</tt></i>
20  if a row copy does not make sense.
21  </p><p>
22  In addition to the default class, there are two others at present:
23  <tt class="function">ClpPlusMinusOneMatrix</tt> and
24  <tt class="function">ClpNetworkMatrix</tt>.  As the name implies, the first one is
25  useful when all elements are ±1.  In this case multiplies are not needed
26  and more importantly less memory is used and there are fewer cache misses.  A
27  class for a matrix where all elements are +1 would be trivial to create. If
28  there were fewer than 64000 rows one could even store row indices as shorts
29  etc.
30  </p><p>
31  The use of <tt class="function">ClpPlusMinusOneMatrix</tt> involves some work as one
32  cannot simply read-in an MPS file.  The key is to use
33  <tt class="function">loadProblem</tt> to pass in a matrix.  So if
34  <tt class="varname">matrix</tt> was a <tt class="function">CoinPackedMatrix</tt> one
35  could do the following:
36  </p><pre class="programlisting">
37  ClpPlusMinusOneMatrix plusMinus(matrix);
38  assert (plusMinus.getIndices()); // would be zero if not +- one
39  model.loadProblem(plusMinus,
40    lowerColumn,upperColumn,objective,
41    lower,upper);
42  </pre><p>
43  <tt class="function">ClpNetworkMatrix</tt> is similar, but represents a network,
44  thus may only have one element per column.  Fortunately, using is is very
45  easy.  Given <tt class="varname">head</tt> and <tt class="varname">tail</tt>, one could
46  do the following:
47  </p><pre class="programlisting">
48  ClpNetworkMatrix network(numberColumns,head,tail);
49  model.loadProblem(network,
50    lowerColumn,upperColumn,objective,
51    lower,upper);
52  </pre><p>
53  Actual code is in <tt class="filename">COIN/Clp/Test/unitTest.cpp</tt>.  A quick
54  glance at the output of this program shows that use of
55  <tt class="function">ClpNetworkMatrix</tt> gives much faster run times.  This is
56  not because of storage issues, but because CLP recognizes the network and uses
57  a network basis factorization which is much faster.  However, in this mode CLP
58  is not a genuine network code as it does not take full advantage of the
59  structure by combining operations but it does have the advantage of
60  flexibility.
61  </p><p>
62  Other instances are possible.  In particular, it should be possible to use the
63  abstract class for column generation or for dynamic matrices which change over
64  time.  Minor modifications may be needed but it should work quite smoothly
65  (there is already a dummy "refresh" method which would be used).
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