GAMS: debugging tools

Earlier this week I was sitting next to an experienced GAMS modeler working together on a model, but I was nevertheless able to show him some tricks:

• Use $stop to stop GAMS at a certain point and use GDX=X command line parameter to inspect the symbols as they are when stopped.
• The IDE gdx viewer is more powerful than many users know.
• If inside a loop, use
  abort$1 "stopped for debugging";
• If inside a loop and you want to stop at a certain iteration use something like:
  abort$(sameas(i,"iter10")) "stopped for debugging";
• Always simplify equations as much as possible by create intermediate sets and parameters. Equations are much more difficult to debug than sets or parameters.
• Loops are difficult to debug, not in the least because of an old issue in GAMS:
  loop(i,
  display i;
  );
  will display something else than expected: it will not show the current i but rather the complete set i. In addition when we use the abort$1 trick we don't see at what point we stopped (I would like to see a loop-stack). Sometimes we need to introduce another singleton set -- a set with one element -- that keeps track of the current state of the loop set in order to create a simulated loop-stack.

Note: a bug in GAMS may introduce a symbol SameAs in the gdx file. This is largely a harmless bug.