Forum Discussion
Different path taken by Xpress Optimizer for solving
We are using Xpress Optimizer version 9.4.0 and have observed differences in how the same problem is solved when: The model is constructed programmatically by creating constraints, variables, etc. ...
The simple answer is that the order of columns (variables) will be different, and that leads to different results. This is because when reading an lp file the variables are discovered on the fly. This is not specific to Xpress, most solvers behave the same way.
One way to try to mitigate that is to put all the variables in the correct order in the objective, with a coefficient of 0 if needed. For example:
Minimize
__OBJ___: 0 x1 + 0 x2 + 0 x3 + x4
This way the order of columns is x1, x2, x3, x4, while using
Minimize
__OBJ___: x4
x4 will be the very first variable.
There can be more subtle differences, for example the accuracy to which the numbers are written to the lp file.
