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Should I use bounds or 'Singleton Equations'?

Q: What is the difference with respect to model size between

x.lo(i) = 1000;


equation xmin(i);
xmin(i) .. x(i) =g= 1000;

and between

x.fx('i0') = 777;


equation xinit;
xinit .. x('i0') =e= 777;

In general bounds (i.e. x.lo, and x.up) are more efficient than equations: a bound does not make the model bigger while an equation does. For solvers with good presolvers this advantage is less pronounced, as the presolver will convert “singleton equations” like x(i) =g= 1000; into bounds automatically.

Similarly preferably you should not generate many .fx fixed variables. In many cases you can use dollar conditions in the model so that GAMS will not generate them. You can also use the .holdfixed model suffix; this will cause GAMS to consider fixed variables as constants. Again, for a solver with a good presolver many fixed variables are not an issue; they will be removed from the model automatically.

gams/should_i_use_bounds_or_singleton_equations.txt · Last modified: 2007/10/20 05:44 by franz