Conditionally add a term in sum or other set operation

The condition

z=sum(i$(y(i) gt 0),x(i));;

says include the term for set element i only if y(i) > 0,  while

z=sum((i,j)$(sameas(i,j)),x(i,j));

says add the term corresponding to a pair of set elements i and j only if the set elements have the same name (thus if the name of element i was Chicago then the j term would be included in the sum only if the name of element j was Chicago).

For more on this class of conditionals see the discussion in the Conditionals chapter.  For more on Sameas also see the Conditionals chapter.