[Help-glpk] order of basis variables

[Olivier Liess]

Hi everyone,

Still playing with the simplex tableau (ST). I'll start with a small 
example.
Here consider the following ST, where x1-3 are structural variables 
whereas x4-5 are slack variables, and . are some coefficient.

  | x1 x2 x3 x4 x5 |
---------------------
x4 |  .  .  .  1  0 |
x5 |  .  .  .  0  1 |
---------------------
Z |  .  .  .  0  0 |


Now suppose that after some iterations of the simplex method we have the 
optimal solution and the following ST:

  | x1 x2 x3 x4 x5 |
---------------------
x2 |  .  1  0  .  . |
x3 |  .  0  1  .  . |
---------------------
Z |  .  0  0  .  . |

Here what we want is to know that x2 is associated with x4 (1st line in 
the ST), and x3 with x5 (2nd line).

GLPK says nothing about the order of variables returned by functions 
like lpx_eval_tab_row. While parsing the sourcecode, it seems that the 
"basis" array in the LPX structure gives me the right order (I checked 
on small examples), but I wanted someone to confirm my guess. So ?

Anyway, thanks again for the good job done while providing this solver 
to the community.

--
Olivier Liess

Doctorant en Recherche Operationnelle
Universite d'Avignon et des Pays de Vaucluse
Laboratoire d'Informatique d'Avignon CNRS FRE 2487

BP 1228
84911 Avignon cedex 9
France

http://www.lia.univ-avignon.fr/equipes/RO/index.html

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