| 1 | <?xml version="1.0" encoding="ISO-8859-1" standalone="no"?> |
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| 2 | <html xmlns="http://www.w3.org/1999/xhtml"><head><title>Follow-On Branching</title><meta name="generator" content="DocBook XSL Stylesheets V1.61.2"/><link rel="home" href="index.html" title="CBC User Guide"/><link rel="up" href="ch05.html" title="Chapter 5. Branching "/><link rel="previous" href="ch05s02.html" title="Pseudo Cost Branching"/><link rel="next" href="ch06.html" title="Chapter 6. Cutting planes"/></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Follow-On Branching</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="ch05s02.html">Prev</a> </td><th width="60%" align="center">Chapter 5. |
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| 3 | Branching |
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| 4 | </th><td width="20%" align="right"> <a accesskey="n" href="ch06.html">Next</a></td></tr></table><hr/></div><div class="section" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="followOn"/>Follow-On Branching</h2></div></div><div/></div><p> |
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| 5 | In crew scheduling, the problems are long and thin. A problem may have a few rows but many thousands of variables. Branching a variable to 1 is very powerful as it fixes many other variables to zero, but branching to zero is very weak as thousands of variables can increase from zero. In crew scheduling problems, each constraint is a flight leg, e.g., JFK airport to DFW airport. |
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| 6 | From DFW there may be several flights the crew could take next - suppose one flight is |
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| 7 | the 9:30 flight from DFW to LAX airport. A binary branch is that the crew arriving |
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| 8 | at DFW either take the 9:30 flight to LAX or they don't. This "follow-on" branching does not |
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| 9 | fix individual variables. Instead this branching divides all the variables with entries in the JFK-DFW |
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| 10 | constraint into two groups - those with entries in the DFW-LAX constraint and those without |
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| 11 | entries. |
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| 12 | </p><p> |
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| 13 | The full sample code for follow-on brancing is in <tt class="filename">crew.cpp</tt> |
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| 14 | located in the CBC Samples directory, see |
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| 15 | <a href="ch08.html" title="Chapter 8. More Samples ">Chapter 8, <i> |
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| 16 | More Samples |
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| 17 | </i></a>). In this case, the simple integer |
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| 18 | variables are left which may be necessary if other sorts of constraints exist. Follow-on branching rules are to be considered first, so the priorities are set to indicate the follow-on rules take precedence. Priority 1 is the highest priority. |
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| 19 | |
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| 20 | </p><div class="example"><a id="id3000566"/><p class="title"><b>Example 5.2. <tt class="classname">CbcFollowOn</tt></b></p><pre class="programlisting"> |
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| 21 | |
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| 22 | int iColumn; |
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| 23 | int numberColumns = solver3->getNumCols(); |
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| 24 | /* We are going to add a single follow-on object but we |
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| 25 | want to give low priority to existing integers |
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| 26 | As the default priority is 1000 we don't actually need to give |
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| 27 | integer priorities but it is here to show how. |
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| 28 | */ |
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| 29 | // Normal integer priorities |
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| 30 | int * priority = new int [numberColumns]; |
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| 31 | int numberIntegers=0; |
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| 32 | for (iColumn=0;iColumn<numberColumns;iColumn++) { |
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| 33 | if (solver3->isInteger(iColumn)) { |
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| 34 | priority[numberIntegers++]= 100; // low priority |
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| 35 | } |
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| 36 | } |
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| 37 | /* Second parameter is set to true for objects, |
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| 38 | and false for integers. This indicates integers */ |
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| 39 | model.passInPriorities(priority,false); |
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| 40 | delete [] priority; |
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| 41 | /* Add in objects before we can give them a priority. |
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| 42 | In this case just one object |
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| 43 | - but it shows the general method |
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| 44 | */ |
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| 45 | CbcObject ** objects = new CbcObject * [1]; |
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| 46 | objects[0]=new CbcFollowOn(&model); |
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| 47 | model.addObjects(1,objects); |
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| 48 | delete objects[0]; |
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| 49 | delete [] objects; |
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| 50 | // High priority |
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| 51 | int followPriority=1; |
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| 52 | model.passInPriorities(&followPriority,true); |
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| 53 | |
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| 54 | </pre></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ch05s02.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch05.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ch06.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Pseudo Cost Branching </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 6. Cutting planes</td></tr></table></div></body></html> |
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