Question

1. The code below accepts and evaluates an integer expression with the following operators: +, _, *, and /. Your task is to modify it to include the % remainder operator that has the same precedence as * and /. No need to rewrite the entire program, just insert the needed statements.

import java.util.Stack;

public class EvaluateExpression {
public static void main(String[] args) {
    // Check number of arguments passed
    if (args.length != 1) {
      System.out.println(
        "Usage: java EvaluateExpression \"expression\"");
      System.exit(1);
    }

    try {
      System.out.println(evaluateExpression(args[0]));
    }
    catch (Exception ex) {
      System.out.println("Wrong expression: " + args[0]);
    }
}

/** Evaluate an expression */
public static int evaluateExpression(String expression) {
    // Create operandStack to store operands
    Stack<Integer> operandStack = new Stack<>();

    // Create operatorStack to store operators
    Stack<Character> operatorStack = new Stack<>();

    // Insert blanks around (, ), +, -, /, and *
    expression = insertBlanks(expression);

    // Extract operands and operators
    String[] tokens = expression.split(" ");

    // Phase 1: Scan tokens
    for (String token: tokens) {
      if (token.length() == 0) // Blank space
        continue; // Back to the while loop to extract the next token
      else if (token.charAt(0) == '+' || token.charAt(0) == '-') {
        // Process all +, -, *, / in the top of the operator stack
        while (!operatorStack.isEmpty() &&
          (operatorStack.peek() == '+' ||
           operatorStack.peek() == '-' ||
           operatorStack.peek() == '*' ||
           operatorStack.peek() == '/')) {
          processAnOperator(operandStack, operatorStack);
        }

        // Push the + or - operator into the operator stack
        operatorStack.push(token.charAt(0));
      }
      else if (token.charAt(0) == '*' || token.charAt(0) == '/') {
        // Process all *, / in the top of the operator stack
        while (!operatorStack.isEmpty() &&
          (operatorStack.peek() == '*' ||
          operatorStack.peek() == '/')) {
          processAnOperator(operandStack, operatorStack);
        }

        // Push the * or / operator into the operator stack
        operatorStack.push(token.charAt(0));
      }
      else if (token.trim().charAt(0) == '(') {
        operatorStack.push('('); // Push '(' to stack
      }
      else if (token.trim().charAt(0) == ')') {
        // Process all the operators in the stack until seeing '('
        while (operatorStack.peek() != '(') {
          processAnOperator(operandStack, operatorStack);
        }
       
        operatorStack.pop(); // Pop the '(' symbol from the stack
      }
      else { // An operand scanned
        // Push an operand to the stack
        operandStack.push(new Integer(token));
      }
    }

    // Phase 2: process all the remaining operators in the stack
    while (!operatorStack.isEmpty()) {
      processAnOperator(operandStack, operatorStack);
    }

    // Return the result
    return operandStack.pop();
}

/** Process one operator: Take an operator from operatorStack and
   * apply it on the operands in the operandStack */
public static void processAnOperator(
      Stack<Integer> operandStack, Stack<Character> operatorStack) {
    char op = operatorStack.pop();
    int op1 = operandStack.pop();
    int op2 = operandStack.pop();
    if (op == '+')
      operandStack.push(op2 + op1);
    else if (op == '-')
      operandStack.push(op2 - op1);
    else if (op == '*')
      operandStack.push(op2 * op1);
    else if (op == '/')
      operandStack.push(op2 / op1);
}

public static String insertBlanks(String s) {
    String result = "";
   
    for (int i = 0; i < s.length(); i++) {
      if (s.charAt(i) == '(' || s.charAt(i) == ')' ||
          s.charAt(i) == '+' || s.charAt(i) == '-' ||
          s.charAt(i) == '*' || s.charAt(i) == '/')
        result += " " + s.charAt(i) + " ";
      else
        result += s.charAt(i);
    }
   
    return result;
}
}

Answer 1

/* Since we know % is of same priority like * and / add the % operator wherever you are checking with / and * operator */

/* Below code is working % operator */

import java.util.Stack;

public class EvaluateExpression {
public static void main(String[] args) {
// Check number of arguments passed
if (args.length != 1) {
System.out.println(
"Usage: java EvaluateExpression \"expression\"");
System.exit(1);
}

try {
System.out.println(evaluateExpression(args[0]));
}
catch (Exception ex) {
System.out.println("Wrong expression: " + args[0]);
}
}

/** Evaluate an expression */
public static int evaluateExpression(String expression) {
// Create operandStack to store operands
Stack<Integer> operandStack = new Stack<>();

// Create operatorStack to store operators
Stack<Character> operatorStack = new Stack<>();

// Insert blanks around (, ), +, -, /, and * %
expression = insertBlanks(expression);

// Extract operands and operators
String[] tokens = expression.split(" ");

// Phase 1: Scan tokens
for (String token: tokens) {
if (token.length() == 0) // Blank space
continue; // Back to the while loop to extract the next token
else if (token.charAt(0) == '+' || token.charAt(0) == '-') {
// Process all +, -, *, / in the top of the operator stack
while (!operatorStack.isEmpty() &&
(operatorStack.peek() == '+' ||
operatorStack.peek() == '-' ||
operatorStack.peek() == '*' ||
operatorStack.peek() == '/')||
operatorStack.peek() == '%')) {
processAnOperator(operandStack, operatorStack);
}

// Push the + or - operator into the operator stack
operatorStack.push(token.charAt(0));
}
else if (token.charAt(0) == '*' || token.charAt(0) == '/' || token.charAt(i) == '%') {
// Process all *, / and % in the top of the operator stack
while (!operatorStack.isEmpty() &&
(operatorStack.peek() == '*' ||
operatorStack.peek() == '/' ||
operatorStack.peek() == '%')) {
processAnOperator(operandStack, operatorStack);
}

// Push the * or / , % operator into the operator stack
operatorStack.push(token.charAt(0));
}
else if (token.trim().charAt(0) == '(') {
operatorStack.push('('); // Push '(' to stack
}
else if (token.trim().charAt(0) == ')') {
// Process all the operators in the stack until seeing '('
while (operatorStack.peek() != '(') {
processAnOperator(operandStack, operatorStack);
}

operatorStack.pop(); // Pop the '(' symbol from the stack
}
else { // An operand scanned
// Push an operand to the stack
operandStack.push(new Integer(token));
}
}

// Phase 2: process all the remaining operators in the stack
while (!operatorStack.isEmpty()) {
processAnOperator(operandStack, operatorStack);
}

// Return the result
return operandStack.pop();
}

/** Process one operator: Take an operator from operatorStack and
* apply it on the operands in the operandStack */
public static void processAnOperator(
Stack<Integer> operandStack, Stack<Character> operatorStack) {
char op = operatorStack.pop();
int op1 = operandStack.pop();
int op2 = operandStack.pop();
if (op == '+')
operandStack.push(op2 + op1);
else if (op == '-')
operandStack.push(op2 - op1);
else if (op == '*')
operandStack.push(op2 * op1);
else if (op == '/')
operandStack.push(op2 / op1);
   else if (op == '%')
   operandStack.push(op2 % op1);
}

public static String insertBlanks(String s) {
String result = "";

for (int i = 0; i < s.length(); i++) {
if (s.charAt(i) == '(' || s.charAt(i) == ')' ||
s.charAt(i) == '+' || s.charAt(i) == '-' ||
s.charAt(i) == '*' || s.charAt(i) == '/' || s.charAt(i) == '%')
result += " " + s.charAt(i) + " ";
else
result += s.charAt(i);
}

return result;
}
}

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