Consider the following grammar for IF-THEN-ELSE statements:


	S --> IF b THEN S ELSE S
	     |IF b THEN S 
 	
	     |a

where b represents a boolean condition and a represents some other statements. Then IF b THEN IF b THEN a ELSE a


has two parse trees. 

The second parse, with ELSE  associated with the closest IF 
is considered the way this should be parsed. 


We can rewrite this grammar to be unambiguous: 





S1 --> IF b THEN S1 | IF b THEN S2 ELSE S1 | a
S2 --> IF b THEN S2 ELSE S2 | a



Then,  IF b THEN IF b THEN a ELSE a has only 1 parse 





 Send questions and comments to:
Karen Lemone