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- import java.io.*;
- import java.util.*;
- public class Postfix
- {
- private static Stack operators = new Stack();
- private static Stack operands = new Stack();
- public static void main(String argv[]) throws IOException
- {
- String infix;
- //create an input stream object
- BufferedReader keyboard = new BufferedReader (new
- InputStreamReader(System.in));
- //get input from user
- System.out.print("\nEnter the algebraic expression in infix: ");
- infix = keyboard.readLine();
- //output as postfix
- System.out.println("The expression in postfix is:" + toPostfix(infix));
- //get answer
- System.out.println("The answer to the equation is: " + evaluate
- (toPostfix(infix)) + "\n");
- }
- private static String toPostfix(String infix)
- //converts an infix expression to postfix
- {
- StringTokenizer s = new StringTokenizer(infix);
- //divides the input into tokens
- String symbol, postfix = "";
- while (s.hasMoreTokens())
- //while there is input to be read
- {
- symbol = s.nextToken();
- //if it's a number, add it to the string
- if (Character.isDigit(symbol.charAt(0)))
- postfix = postfix + " " + (Integer.parseInt
- (symbol));
- else if (symbol.equals("("))
- //push (
- {
- Character operator = new Character('(');
- operators.push(operator);
- }
- else if (symbol.equals(")"))
- //push everything back to (
- {
- while (((Character)operators.peek()).charValue() != '(')
- {
- postfix = postfix + " " + operators.pop();
- }
- operators.pop();
- }
- else
- //print operators occurring before it that have greater precedence
- {
- while (!operators.empty() && !(operators.peek()).equals("(") && prec(symbol.charAt(0)) <= prec(((Character)operators.peek()).charValue()))
- postfix = postfix + " " + operators.pop();
- Character operator = new Character(symbol.charAt(0));
- operators.push(operator);
- }
- }
- while (!operators.empty())
- postfix = postfix + " " + operators.pop();
- return postfix;
- }
- private static int evaluate(String postfix)
- {
- StringTokenizer s = new StringTokenizer(postfix);
- //divides the input into tokens
- int value;
- String symbol;
- while (s.hasMoreTokens())
- {
- symbol = s.nextToken();
- if (Character.isDigit(symbol.charAt(0)))
- //if it's a number, push it
- {
- Integer operand = new Integer(Integer.parseInt(symbol));
- operands.push(operand);
- }
- else //if it's an operator, operate on the previous two operands
- {
- int op2 = ((Integer)operands.pop()).intValue();
- int op1 = ((Integer)operands.pop()).intValue();
- int result = 0;
- switch(symbol.charAt(0))
- {
- case '*': {result = op1 * op2; break;}
- case '+': {result = op1 + op2; break;}
- case '-': {result = op1 - op2; break;}
- case '/': {result = op1 / op2; break;}
- case '%': {result = op1 % op2; break;}
- }
- Integer operand = new Integer(result);
- operands.push(operand);
- }
- }
- value = ((Integer)operands.pop()).intValue();
- return value;
- }
- private static int prec(char x)
- {
- if (x == '+' || x == '-')
- return 1;
- if (x == '*' || x == '/' || x == '%')
- return 2;
- return 0;
- }
- }