title: gurobi - Error code = 10004 Unable to retrieve attribute 'X'
body: <p>I am getting an error in my c++/gurobi file:</p>
  
  <p>Error code = 10004 Unable to retrieve attribute 'X'</p>
  
  <p>I read that this might have something to do with labels? But I don't see how there is a problem.
  It works for some input files, but not for others. So I have created a toy file, t5.txt in attachment. This file does not work, but removing the last column and setting 8 to 7 fixes it. I am puzzled...</p>
  
  <p>Below is the output of model.write. Everything seems to make sense, any Ideas what I am doing wrong?
  Whenever I do a model.write(test.sol), the program stops, so there seems to be something wrong with the solution> </p>
  
  <p>Attachments: 
  main.cpp ->  <a href="https://dl.dropboxusercontent.com/u/13564139/main.cpp" rel="nofollow">https://dl.dropboxusercontent.com/u/13564139/main.cpp</a></p>
  
  <p>input.txt -> <a href="https://dl.dropboxusercontent.com/u/13564139/t5.txt" rel="nofollow">https://dl.dropboxusercontent.com/u/13564139/t5.txt</a></p>
  
  <pre><code>Maximize
    15 student_has_projects4.1
  Subject To
   R0: student_has_projects0.0 + student_has_projects1.0
     + student_has_projects2.0 + student_has_projects3.0
     + student_has_projects4.0 + student_has_projects5.0
     + student_has_projects6.0 + student_has_projects7.0 &lt;= 4
   R1: student_has_projects1.0 + student_has_projects2.0 &gt;= 1
   R2: student_has_projects2.0 + 2 student_has_projects5.0 &lt;= 2
   R3: student_has_projects2.0 + 2 student_has_projects5.0 &gt;= 1
   R4: student_has_projects0.0 + student_has_projects3.0
     + student_has_projects4.0 + student_has_projects6.0
     + student_has_projects7.0 &gt;= 1
   R5: student_has_projects2.0 + student_has_projects5.0 &lt;= 1
   R6: student_has_projects0.1 + student_has_projects1.1
     + student_has_projects2.1 + student_has_projects3.1
     + student_has_projects4.1 + student_has_projects5.1
     + student_has_projects6.1 + student_has_projects7.1 &lt;= 4
   R7: student_has_projects1.1 + student_has_projects2.1 &gt;= 1
   R8: student_has_projects2.1 + 2 student_has_projects5.1 &lt;= 2
   R9: student_has_projects2.1 + 2 student_has_projects5.1 &gt;= 1
   R10: student_has_projects0.1 + student_has_projects3.1
     + student_has_projects4.1 + student_has_projects6.1
     + student_has_projects7.1 &gt;= 1
   R11: student_has_projects2.1 + student_has_projects5.1 &lt;= 1
   R12: student_has_projects0.2 + student_has_projects1.2
     + student_has_projects2.2 + student_has_projects3.2
     + student_has_projects4.2 + student_has_projects5.2
     + student_has_projects6.2 + student_has_projects7.2 &lt;= 4
   R13: student_has_projects1.2 + student_has_projects2.2 &gt;= 1
   R14: student_has_projects2.2 + 2 student_has_projects5.2 &lt;= 2
   R15: student_has_projects2.2 + 2 student_has_projects5.2 &gt;= 1
   R16: student_has_projects0.2 + student_has_projects3.2
     + student_has_projects4.2 + student_has_projects6.2
     + student_has_projects7.2 &gt;= 1
   R17: student_has_projects2.2 + student_has_projects5.2 &lt;= 1
   R18: student_has_projects0.0 + student_has_projects0.1
     + student_has_projects0.2 = 1
   R19: student_has_projects1.0 + student_has_projects1.1
     + student_has_projects1.2 = 1
   R20: student_has_projects2.0 + student_has_projects2.1
     + student_has_projects2.2 = 1
   R21: student_has_projects3.0 + student_has_projects3.1
     + student_has_projects3.2 = 1
   R22: student_has_projects4.0 + student_has_projects4.1
     + student_has_projects4.2 = 1
   R23: student_has_projects5.0 + student_has_projects5.1
     + student_has_projects5.2 = 1
   R24: student_has_projects6.0 + student_has_projects6.1
     + student_has_projects6.2 = 1
   R25: student_has_projects7.0 + student_has_projects7.1
     + student_has_projects7.2 = 1
  Bounds
  Binaries
   student_has_projects0.0 student_has_projects0.1 student_has_projects0.2
   student_has_projects1.0 student_has_projects1.1 student_has_projects1.2
   student_has_projects2.0 student_has_projects2.1 student_has_projects2.2
   student_has_projects3.0 student_has_projects3.1 student_has_projects3.2
   student_has_projects4.0 student_has_projects4.1 student_has_projects4.2
   student_has_projects5.0 student_has_projects5.1 student_has_projects5.2
   student_has_projects6.0 student_has_projects6.1 student_has_projects6.2
   student_has_projects7.0 student_has_projects7.1 student_has_projects7.2
  End
  </code></pre>