Trading memory for time

Sometimes an extensive calculation that is repeated in the model many times can be restructured so it is calculated once then saved.  For example, a component of a .. equation specification for a model solved a lot of times can be restructured to be done once and stored then only accessed later as follows.

obj.. Z=SUM(( CROP, TILLAGE, LANDTREAT, ROTATION ),

             ACREPLANT( CROP, TILLAGE, LANDTREAT, ROTATION)*

             SUM(INPUT, USAGE(INPUT,  CROP)));

Here the code is revised by defining a parameter for the input usage sum and substituting i.e.;

INPUTUSE( CROP) = SUM(INPUT, USAGE (INPUT, CROP));

obj.. Z=SUM(( CROP, TILLAGE, LANDTREAT, ROTATION ),

  ACREPLANT( CROP, TILLAGE, LANDTREAT, ROTATION)*

          INPUTUSE (CROP));

This can be a huge time saver but one needs to watch out for problems with static calculations as discussed in the calculating items chapter.