Lecture 246:- Program for Evaluation of Postfix

Here's a C++ implementation to evaluate a postfix expression using a stack:

cppCopy code

#include <iostream> #include <stack> #include <string> #include <cctype> int evaluatePostfix(const std::string& postfix) {    std::stack<int> operandStack;    for (char c : postfix) {        if (std::isspace(c)) {            continue; // Skip whitespaces        } else if (std::isdigit(c)) {            operandStack.push(c - '0'); // Convert character to integer and push to stack        } else { // Operator            int operand2 = operandStack.top();            operandStack.pop();            int operand1 = operandStack.top();            operandStack.pop();            switch (c) {                case '+':                    operandStack.push(operand1 + operand2);                    break;                case '-':                    operandStack.push(operand1 - operand2);                    break;                case '*':                    operandStack.push(operand1 * operand2);                    break;                case '/':                    operandStack.push(operand1 / operand2);                    break;            }        }    }    return operandStack.top(); } int main() {    std::string postfix;    std::cout << "Enter a postfix expression: ";    std::getline(std::cin, postfix);    int result = evaluatePostfix(postfix);    std::cout << "Result: " << result << std::endl;    return 0; }

The evaluatePostfix function takes a string containing the postfix expression as input. It uses a stack (operandStack) to store the operands while iterating through the postfix expression. When it encounters an operand (a digit), it converts it to an integer and pushes it onto the stack. When it encounters an operator, it pops the top two operands from the stack, performs the corresponding operation, and pushes the result back to the stack.

The evaluation continues until the end of the postfix expression. Finally, the top element of the stack contains the result of the entire expression, which is returned as the final evaluation result.

In the main function, the user is prompted to enter a postfix expression. The program then calls the evaluatePostfix function to evaluate the expression and prints the result.