You can combine the algorithms for converting between infix to postfix and for e

Question

  You can combine the algorithms for converting between infix to postfix and for evaluating postfix to evaluate an infix expression directly. To do so you need two stacks:

1. stack containing OPERATORS

2. stack containing OPERANDS.

  When an operand is encountered, it is processed as described in the infix to postfix algorithm. When an operator is popped off the operator stack, it is processed as described in the postfix evaluation algorithm:

   The top two operands are popped off the operand stack, the operation is performed, and the result is pushed back onto the operand stack.

* Write a program to evaluate infix expressions directly using this combined algorithm. *

* Your calculator must be able to compute any expression of floating point numbers using the operands +, -, *, or / (you need not implement parentheses, nor do you need to handle negative input numbers such as "-5").

* The essence of your calculator is a class Calculator with at least the method double calculate(String expression). Your program should be able to accept arbitrary, valid infix expressions that use these four operators.

* You must also write test code to accept input from the user and print the result

calc 2 + 2
4
calc 2.1 + 2.4
4.5
calc 3 * 3 - 2 - 1
6
calc 1 + 2 + 1.5 * 8 / 3
7

Explanation / Answer

import java.util.Stack;/** * Class to evaluate infix and postfix expressions. * public class InfixPostfixEvaluator {/** * Operators in reverse order of precedence. */private static final String operators = "-+/*";private static final String operands = "0123456789";public int evalInfix(String infix) {return evaluatePostfix(convert2Postfix(infix));}public String convert2Postfix(String infixExpr) {char[] chars = infixExpr.toCharArray();Stack<Character> stack = new Stack<Character>();StringBuilder out = new StringBuilder(infixExpr.length());for (char c : chars) {if (isOperator(c)) {while (!stack.isEmpty() && stack.peek() != '(') {if (operatorGreaterOrEqual(stack.peek(), c)) {out.append(stack.pop());} else {break;}} stack.push(c);} else if (c == '(') { stack.push(c);} else if (c == ')') {while (!stack.isEmpty() && stack.peek() != '(') {out.append(stack.pop());}if (!stack.isEmpty()) { stack.pop();}} else if (isOperand(c)) {out.append(c);}}while (!stack.empty()) {out.append(stack.pop());}return out.toString();}public int evaluatePostfix(String postfixExpr) {char[] chars = postfixExpr.toCharArray();Stack<Integer> stack = new Stack<Integer>();for (char c : chars) {if (isOperand(c)) { stack.push(c - '0'); // convert char to int val} else if (isOperator(c)) {int op1 = stack.pop();int op2 = stack.pop();int result;switch (c) {case '*': result = op1 * op2; stack.push(result);break;case '/': result = op2 / op1; stack.push(result);break;case '+': result = op1 + op2; stack.push(result);break;case '-': result = op2 - op1; stack.push(result);break;}}}return stack.pop();}private int getPrecedence(char operator) {int ret = 0;if (operator == '-' || operator == '+') { ret = 1;} else if (operator == '*' || operator == '/') { ret = 2;}return ret;}private boolean operatorGreaterOrEqual(char op1, char op2) {return getPrecedence(op1) >= getPrecedence(op2);}private boolean isOperator(char val) {return operators.indexOf(val) >= 0;}private boolean isOperand(char val) {return operands.indexOf(val) >= 0;}}

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