3. Write an intStack interface that describes the abstract data type of a stack

Question

3. Write an intStack interface that describes the abstract data type of a stack of int values. Assume you have a class MyIntStack that implements this interface (you do not need to write it). Using this interface, write a program that evaluates a postfix arithmetic expression with single-digit integer operands and operators '-', '+','*', and '/'. For simplicity, assume that your input string representing the expression does not have any errors (e.g., is valid - no error detection is needed), and that it contains only digits and operator characters (e.g.. no spaces). You can use a string method charAt(int n) - look it up. (3 pts) public static int postfixExpr (String expr) { ???? }

Explanation / Answer

import java.util.*;

public class PostfixExpr
{
static Scanner console = new Scanner(System.in);

public static void main(String [] args)
   {
char ch = 0;
      int rand1 = 0;
      int rand2 = 0;
      int temp;
      int result = 0;
      String str;
   
      System.out.print("Enter the expression: ");
      str = console.nextLine();
   
   
      Stack aStack = new Stack(str.length());
   
      //enters the string into the stack and
      //converts the appropriate char to digits;
      //and then completes the appropriate
      //operations
      for(int i = 0; i < str.length(); i++)
      {
ch = str.charAt(i);
     
      if(Character.isDigit(ch))
      {
temp = Character.getNumericValue(ch);
      aStack.push(temp);
      }//end if
     
      if((ch == '+' || ch == '-' || ch == '*' || ch == '/'))
      {
      rand2 = aStack.pop();
      rand1 = aStack.pop();
         
        if(ch == '+')
result = rand1 + rand2;
         
         else if(ch == '-')
          result = rand1 - rand2;
         
          else if(ch == '*')
result = rand1 * rand2;
         
          else if(ch == '/')
{
if(rand2 == 0)
System.out.println("ERROR: divide by zero");

else
            result = rand1 / rand2;
}

aStack.push(result);
}//end if
    }//end for
if(aStack.count() == 1)
{
    result = aStack.pop();
    System.out.print("The result is: " + result);
}

else
System.out.println("ERROR: not enough operands");
}//end of main
}//end PostfixExpr

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