CMPS 390 Data Structures

Transcription

1 CMPS 390 Data Structures Programming Assignment #02 Infix to Postfix 1. Complete two methods in Java program (Postfix.java) to convert an infix expression into a postfix expression and evaluate the postfix expression. a. In the Postfix class, complete the method converttopostfix by adding code at the comment labeled ADD YOUR CODE HERE. This method converts an infix expression to a postfix expression with the following header: public static String converttopostfix(string infix) b. In the Postfix class, complete the method evaluatepostfix by adding code at the comment labeled ADD YOUR CODE HERE. This method evaluates a postfix expression with the following header: public static double evaluatepostfix(string postfix) c. In an expression string, a character is a token. A token can an operand or an operator. There are 4 possible operands and 6 operators. // 4 Operands: a = 2.0, b = 3.0, c = 4.0, d = 5.0 // 6 Operators: +, -, *, /, (, ) 2. Required Files Postfix.java // Programming Assignment #02: Complete 2 methods PostfixDriver.java // Test postfix main driver (no change) ArrayStack.java // Array implementation for stack (no change) 3. Here is a sample run: Testing postfix expressions with a = 2.0, b = 3.0, c = 4.0, d = 5.0 Infix: a+b Postfix: ab+ Value: 5.0 Infix: a-b+c*d Postfix: ab-cd*+ Value: 19.0 Infix: (a+b)*c-d Postfix: ab+c*d- Value: 15.0 Infix: a+b*(c-d) Postfix: abcd-*+ Value:

2 Infix: (a+b)/(c-d) Postfix: ab+cd-/ Value: -5.0 Infix: a*(b/(c-d)) Postfix: abcd-/* Value: Submission: Postfix.java 5. Your Programming Assignment #02 (Postfix.java) starts here: Postfix.java // CMPS390 // Postfix.java // Algorithms for two methods: converttopostfix and evaluatepostfix public class Postfix { /** Task: Creates a postfix expression that represents a given infix expression. infix a string that is a valid infix expression a string that is the postfix expression equivalent of infix /** Algorithm converttopostfix(infix) // Convert infix to postfix expression * operatorstack = a new empty stack * while (infix has characters left to parse) * Get an item * if (item is an operand) * else if (item == '(') * operatorstack.push(item) * else if (item == ')') * top = operatorstack.pop() * while (top is not '(') // not append '(' to postfix expression * top = operatorstack.pop() * else // item is an operator: '+', '-', '*', or '/' * while (!operatorstack.empty()) * top = operatorstack.peek() * if (item precedence <= top) * operatorstack.pop() * else * break * operatorstack.push(item) * while (!operatorstack.empty()) * top = operatorstack.pop(); * return the postfix expression public static String converttopostfix(string infix) { // ADD YOUR CODE HERE } // end converttopostfix /** Algorithm evaluatepostfix(postfix) // Evaluates a postfix expression. * valuestack = a new empty stack * while (postfix has characters left to parse) * Get an item * if item is an operand * valuestack.push(item) * else // item is an operator: '+', '-', '*', or'/' * operand2 = valuestack.pop() * operand1 = valuestack.pop() * result = operand1 item opernad2 * valuestack.push(result) * return valuestack.pop() 2

3 public static double evaluatepostfix(string postfix) { // ADD YOUR CODE HERE } // end evaluatepostfix /** Task: Determines the precedence of a given operator. operator a character that is (, ), +, -, *, or / an integer that indicates the precedence of operator: * 0 if ( or ), 1 if + or -, 2 if * or /, -1 if anything else private static int getprecedence(char operator) { switch (operator) { case '(': case ')': return 0; case '+': case '-': return 1; case '*': case '/': return 2; } // end switch return -1; } // end getprecedence private static boolean isvariable(char ch) { return (ch == 'a') (ch == 'b') (ch == 'c') (ch == 'd'); } // end isvariable private static double valueof(char variable) { switch (variable) { case 'a': return 2.0; case 'b': return 3.0; case 'c': return 4.0; case 'd': return 5.0; } // end switch return 0; } // end valueof } // end Postfix 3

4 ArrayStack.java (no change) public class ArrayStack { private Object [] stack; // array of stack entries private int topindex; // index of top entry private static final int DEFAULT_MAX_SIZE = 100; public ArrayStack() { stack = new Object[DEFAULT_MAX_SIZE]; topindex = -1; } // end default constructor public ArrayStack(int maxsize) { stack = new Object[maxSize]; topindex = -1; } // end constructor public void push(object newentry) { topindex++; stack[topindex] = newentry; } // end push public Object pop() { Object top = null; if (!isempty()) { top = stack[topindex]; stack[topindex] = null; topindex--; } // end if return top; } // end pop public Object peek() { Object top = null; if (!isempty()) top = stack[topindex]; return top; } // end peek public boolean isempty() { return topindex < 0; } // end isempty public void clear() { for (; topindex > -1; topindex--) stack[topindex] = null; } // end clear } // end ArrayStack 4

5 PostfixDriver.java (no change) // CMPS390 // PostfixDriver.java // Kuo-pao Yang public class PostfixDriver { public static void main(string[] args) { System.out.println("Testing postfix expressions with\n" + "a = 2.0, b = 3.0, c = 4.0, d = 5.0\n"); testpostfix("a+b"); testpostfix("a-b+c*d"); testpostfix("(a+b)*c-d"); testpostfix("a+b*(c-d)"); testpostfix("(a+b)/(c-d)"); testpostfix("a*(b/(c-d))"); } // end main public static void testpostfix(string infixexpression) { String postfixexpression = Postfix.convertToPostfix(infixExpression); System.out.println("Infix: " + infixexpression); System.out.println("Postfix: " + postfixexpression); System.out.println("Value: " + Postfix.evaluatePostfix(postfixExpression)); System.out.println(); } // end testpostfix } // end PostfixDriver /* OUTPUT Testing postfix expressions with a = 2.0, b = 3.0, c = 4.0, d = 5.0 Infix: a+b Postfix: ab+ Value: 5.0 Infix: a-b+c*d Postfix: ab-cd*+ Value: 19.0 Infix: (a+b)*c-d Postfix: ab+c*d- Value: 15.0 Infix: a+b*(c-d) Postfix: abcd-*+ Value: -1.0 Infix: (a+b)/(c-d) Postfix: ab+cd-/ Value: -5.0 Infix: a*(b/(c-d)) Postfix: abcd-/* Value: