Example .

. Parse array [ num dotdot num ] of integer

. Cannot proceed as non terminal "simple" never generates a string beginning with token "array". Therefore, requires back-tracking.

. Back-tracking is not desirable, therefore, take help of a "look-ahead" token. The current token is treated as look- ahead token. ( restricts the class of grammars )

To construct a parse tree corresponding to the string array [ num dotdot num ] of integer , we start with the start symbol type . Then, we use the production type à simple to expand the tree further and construct the first child node. Now, finally, the non-terminal simple should lead to the original string. But, as we can see from the grammar, the expansion of the non-terminal simple never generates a string beginning with the token "array". So, at this stage, we come to know that we had used the wrong production to expand the tree in the first step and we should have used some other production. So, we need to backtrack now. This backtracking tends to cause a lot of overhead during the parsing of a string and is therefore not desirable. To overcome this problem, a " look-ahead " token can be used. In this method, the current token is treated as look-ahead token and the parse tree is expanded by using the production which is determined with the help of the look-ahead token.