9 - CMPSCI 187 Programming With Data Structures Lecture#9...

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CMPSCI 187: Programming With Data Structures Lecture #9: The Evaluator Program 26 September 2011
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The Evaluator Program • The Problem: Postfix Evaluation • The Method: Pending Arguments on a Stack • The Structure of L&C’s Solution • The Driver Class Postfix • The Class PostfixEvaluator • Complications: I/O, StringTokenizer , Wrapper classes, Exceptions
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The Problem: Postfix Evaluation • We have arithmetic expressions where the constants are int values and the operators are + , - , * , and / . • We are more used to infix expressions like (b*b)-(4*a*c) . • It turns out that postfix expressions like b b * 4 a * c * - are easier to evaluate with a stack -- they have no ambiguities to be resolved by operator hierarchy or parentheses. • To translate infix to postfix, find the highest-priority operator, so that the infix expression can be read as R op S, then write “R S op”, then translate both R and S to postfix. • We want to input the postfix expression as a string and output an int.
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The Method: Pending Arguments on a Stack • We create a Stack<Integer> object to store arguments still to be used. • If our first input is “4”, we cannot process it yet so we push it.
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