The LISTFIX command displays a report on variables that the model generator determined as being fixed in value.

Consider the following small model:

[ROW_OBJ] MAX= 20*X + 30*Y + 12*Z;

[ROW_1] X = 2*Y;

[ROW_2] X + Y + Z <= 110;

[ROW_3] Y = 30;

Examining the model, we can easily determine that Y is fixed in value at 30, while X is fixed at 60. Determining the fixed variables in a small model like this is an easy task, however, this would not be the case for larger models. LINGO's Fixed Variable Reduction feature substitutes fixed variables out of the generated model that gets passed to the solver. In which case, you might find it useful when viewing a generated model (Solver|Generate|Display command) to know which variables have been substituted out of the model. For our particular model here, running the LISTFIX command yields the following report:

    Fixed Variable Report

    ---------------------

Variable     Key Row        Value

      Y       ROW_3     30.00000

      X       ROW_1     60.00000

As mentioned, the fixed variables X and Y are listed along with their values. The Key Row column indicates the row in which the variable was substituted out in.

Running the Solver|Generate|Display Model command on this same model, we get:

MODEL:

[ROW_OBJ] MAX= 12 * Z + 2100;

[ROW_2] Z <= 20;

END

And we see that both the fixed variables and their key rows have been substituted out of the generated formulation that get passed to the solver.