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.