Lecture 245:- Evaluation of Postfix Expression

To evaluate a postfix expression, we can use a stack to keep track of the operands and perform the appropriate operations when encountering operators.

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

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; }

In this implementation, we use a stack to store the operands while iterating through the postfix expression. When we encounter an operand (a digit), we convert it to an integer and push it onto the stack. When we encounter an operator, we pop the top two operands from the stack, perform the corresponding operation, and push 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 we return 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.