assignment6a - Massachusetts Institute of Technology...

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Massachusetts Institute of Technology Department of Electrical Engineering and Computer Science 6.087: Practical Programming in C IAP 2010 Problem Set 6 Solutions Part 1: Pointers to pointers. Multidimensional arrays. Stacks and queues. Out: Wednesday, January 20, 2010. Due: Friday, January 22, 2010. This is Part 1 of a two-part assignment. Part 2 will be released Thursday. Problem 6.1 In this problem, we will implement a simple “four-function” calculator using stacks and queues. This calculator takes as input a space-delimited in±x expression (e.g. 3 + 4 * 7 ), which you will convert to post±x notation and evaluate. There are four (binary) operators your calculator must handle: addition ( + ), subtraction ( - ), multiplication ( * ), and division ( / ). In addition, your calculator must handle the unary negation operator (also - ). The usual order of operations is in effect: the unary negation operator - has higher precedence than the binary operators, and is eval- uated right-to-left ( right-associative ) * and / have higher precedence than + and - all binary operators are evaluated left-to-right ( left-associative ) To start, we will not consider parentheses in our expressions. The code is started for you in prob1.c , which is available for download from Stellar. Read over the ±le, paying special attention to the data structures used for tokens, the stack, and queue, as well as the functions you will complete. (a) We have provided code to translate the string to a queue of tokens, arranged in in±x (natural) order. You must: ±ll in the infix to postfix() function to construct a queue of tokens arranged in post±x order (the in±x queue should be empty when you’re done) complete the evaluate postfix() function to evaluate the expression stored in the post±x queue and return the answer may assume the input is a valid in±x expression, but it is good practice to make your code robust by handling possible errors (e.g. not enough tokens) . Turn in a printout of your code, along with a printout showing the output from your program for a few test cases (in±x expressions of your choosing) to demonstrate it works properly. 1
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Answer: Here’s one possible implementation: (only functions infix to postfix() and evaluate postfix() shown) / creates a queue of tokens in postfix order from a i n f i x / / postcondition : returned contains a l l the tokens , and pqueue infix should be empty / struct token i n f i x t o p o s t f i x ( ) { / TODO: construct ordered ; a l l i n f i x added to or freed / p expr stack top = NULL, ptoken ; ; postfix . front = postfix . back = NULL; for ( ptoken = dequeue ( pqueue infix ) ; ptoken ; = infix )) { switch > type ) { case OPERAND: / operands d i r e c t l y / enqueue(&queue , ) ; break ; OPERATOR: / operator , after operators higher precedence are moved / while ( && ( op precedences [ > value .
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assignment6a - Massachusetts Institute of Technology...

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