Free Variables
By default, a LINGO variable has a lower bound of zero and an upper bound of infinity. @FREE removes the lower bound of zero and lets a variable take negative values, rendering it unconstrained in sign, or free. The syntax is:
@FREE( variable_name);
where variable_name is the name of the variable you wish to make free.
The @FREE function may be used in a model anywhere you would normally enter a constraint. The @FREE function can be embedded in an @FOR statement to allow you to easily make all, or selected, variables of an attribute to be free. Some examples of @FREE are:
Example 1: @FREE( X);
makes the scalar variable X free,
Example 2: @FREE( QUANTITY( 4));
makes the variable QUANTITY( 4) free,
Example 3: @FOR( ITEMS: @FREE( QUANTITY));
makes all variables in the QUANTITY attribute free.