for looping

Does FOR LOOP work on SELECT?  The following simple function looks good to
me but does not compile because of "missing .. at end of SQL expression".


CREATE FUNCTION get_all_diffs()
RETURNS FLOAT AS '
DECLARE
 i INTEGER := 0;
sym_diff FLOAT := 0.0;

BEGIN
 FOR row IN SELECT * FROM "tblTrades" LOOP
  i := i+1;
 END LOOP;

 RETURN sym_diff;
END;'

LANGUAGE 'plpgsql'
;

SELECT get_all_diffs();