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DAWEN LEE
Greenhorn
Posts: 1
how to uses java to write a program like this:
The input string, such as "7 * ( 23 - 105 / 15 ) - 19", is converted by the tokenizer into a sequence of tokens that are enqueued into the first buffer (i.e. queue) q1.

The validate method then dequeues the tokens, one at a time, and checks that the expression be valid. Each token is copied (i.e. enqueued) into the second buffer q2. If the expression is not valid an InvalidExpressionException is thrown and the whole process is terminated.

More detail on this method is given below.

The infix2postfix method converts the sequence of tokens (from q2) by re-ordering into postfix sequence and enqueues them into q3. Postfix notation is known commonly as reverse Polish notation.

The expression

7 * ( 23 - 105 / 15 ) - 19
with infix operators would appear as
7 23 105 15 / - * 19 -
in reverse Polish. The operator appears after the two values to which it applies. Brackets are not needed.

A means of converting infix expressions into reverse Polish is given below.

The evaluate method uses a stack to support evaluation of the reverse Polish expression and returns the result.

Mike Gershman
Ranch Hand
Posts: 1272
Parsing is a bigger topic than will fit in one post on a bulletin board, and it's not a beginner's task unless you were given an exact algorithm. I therefore assume that your homework assignment gave you an algorithm to follow which you did not post here. If not, you can start with the link below, choose a parsing algorithm, study it, and code it. For further help, you can try the dragon book (see this link).

http://en.wikipedia.org/wiki/Parser#Top-down_parsers

Ernest Friedman-Hill
author and iconoclast
Marshal
Posts: 24212
35
please don't post the same question to multiple forums -- it just wastes people's time and makes them less likely to want to help you. I'm closing this thread and leaving this copy open.