An LP basis (BAS) file is used to specify an initial basis for a continuous model. The file provides basis status information for each variable and constraint in the model. If a basis has been imported into a continuous model before optimization begins (using GRBread, for example), and if a simplex optimizer has been selected (through the Method parameter), the Gurobi simplex optimizer begins from the specified basis.
A BAS file begins with a
NAME line, and ends with an
ENDDATA statement. No information is retrieved from these lines,
but they are required by the format. Between these two lines are
basis status lines, each consisting of two or three fields. If the
first field is
BS, the variable
named in the second field is non-basic at its lower bound, non-basic
at its upper bound, or basic, respectively. Any additional fields are
ignored. If the first field is
XU, the variable
named in the second field is basic, while the variable named in the third
field is non-basic at its lower or upper bound, respectively.
The following is a simple example:
NAME example.bas XL x1 c1 XU x2 c2 BS c3 UL x3 LL x4
A near-optimal basis can speed the solution of a difficult LP model. However, specifying a start basis that is not extremely close to an optimal solution will often slow down the solution process. Exercise caution when providing start bases.