Converts infix arithmetic expressions to postfix
import java.io.IOException; public class InToPost { private Stack theStack; private String input; private String output = ""; public InToPost(String in) { input = in; int stackSize = input.length(); theStack = new Stack(stackSize); } public String doTrans() { for (int j = 0; j < input.length(); j++) { char ch = input.charAt(j); switch (ch) { case '+': case '-': gotOper(ch, 1); break; // (precedence 1) case '*': // it's * or / case '/': gotOper(ch, 2); // go pop operators break; // (precedence 2) case '(': // it's a left paren theStack.push(ch); // push it break; case ')': // it's a right paren gotParen(ch); // go pop operators break; default: // must be an operand output = output + ch; // write it to output break; } } while (!theStack.isEmpty()) { output = output + theStack.pop(); } System.out.println(output); return output; // return postfix } public void gotOper(char opThis, int prec1) { while (!theStack.isEmpty()) { char opTop = theStack.pop(); if (opTop == '(') { theStack.push(opTop); break; }// it's an operator else {// precedence of new op int prec2; if (opTop == '+' || opTop == '-') prec2 = 1; else prec2 = 2; if (prec2 < prec1) // if prec of new op less { // than prec of old theStack.push(opTop); // save newly-popped op break; } else // prec of new not less output = output + opTop; // than prec of old } } theStack.push(opThis); } public void gotParen(char ch){ while (!theStack.isEmpty()) { char chx = theStack.pop(); if (chx == '(') break; else output = output + chx; } } public static void main(String[] args) throws IOException { String input = "1+2*4/5-7+3/6"; String output; InToPost theTrans = new InToPost(input); output = theTrans.doTrans(); System.out.println("Postfix is " + output + '\n'); } class Stack { private int maxSize; private char[] stackArray; private int top; public Stack(int max) { maxSize = max; stackArray = new char[maxSize]; top = -1; } public void push(char j) { stackArray[++top] = j; } public char pop() { return stackArray[top--]; } public char peek() { return stackArray[top]; } public boolean isEmpty() { return (top == -1); } } }
1. | Parse postfix arithmetic expressions |