Pr10

 Implement C++ program for expression conversion as infix to postfix and its evaluation using

stack based on given conditions:

1. Operands and operator, both must be single character.

2. Input Postfix expression must be in a desired format.

3. Only '+', '-', '*' and '/ ' operators are expected.

OBJECTIVE:

1. Understand Stack ADT.

2. Understand stack application for Expression conversion and evaluation.




#include <iostream>

#include <stack>

#include <cctype>

#include <string>

using namespace std;


// Function to check if the character is an operator

bool isOperator(char c) {

    return (c == '+' || c == '-' || c == '*' || c == '/');

}


// Function to get the precedence of the operator

int precedence(char op) {

    if (op == '+' || op == '-') {

        return 1; // lower precedence

    }

    if (op == '*' || op == '/') {

        return 2; // higher precedence

    }

    return 0;

}


// Function to perform arithmetic operations

int applyOperator(int a, int b, char op) {

    switch (op) {

        case '+': return a + b;

        case '-': return a - b;

        case '*': return a * b;

        case '/': return a / b;

    }

    return 0;

}


// Function to convert infix expression to postfix

string infixToPostfix(const string& infix) {

    stack<char> operators;

    string postfix = "";


    for (char c : infix) {

        // Skip spaces

        if (c == ' ') continue;


        // If character is an operand (letter or digit)

        if (isalnum(c)) {

            postfix += c;

        }

        // If character is an operator

        else if (isOperator(c)) {

            while (!operators.empty() && precedence(operators.top()) >= precedence(c)) {

                postfix += operators.top();

                operators.pop();

            }

            operators.push(c);

        }

        // If character is an opening parenthesis

        else if (c == '(') {

            operators.push(c);

        }

        // If character is a closing parenthesis

        else if (c == ')') {

            while (!operators.empty() && operators.top() != '(') {

                postfix += operators.top();

                operators.pop();

            }

            operators.pop(); // Remove '(' from the stack

        }

    }


    // Pop all the operators remaining in the stack

    while (!operators.empty()) {

        postfix += operators.top();

        operators.pop();

    }


    return postfix;

}


// Function to evaluate a postfix expression

int evaluatePostfix(const string& postfix) {

    stack<int> values;


    for (char c : postfix) {

        // Skip spaces

        if (c == ' ') continue;


        // If the character is a digit (operand)

        if (isalnum(c)) {

            values.push(c - '0'); // Convert char to integer

        }

        // If the character is an operator

        else if (isOperator(c)) {

            int val2 = values.top();

            values.pop();

            int val1 = values.top();

            values.pop();

            int result = applyOperator(val1, val2, c);

            values.push(result);

        }

    }


    return values.top(); // The result will be the last value in the stack

}


int main() {

    string infixExpression;

    

    // Input infix expression

    cout << "Enter infix expression (single character operands, operators (+, -, *, /), and no spaces): ";

    getline(cin, infixExpression);


    // Convert infix to postfix

    string postfixExpression = infixToPostfix(infixExpression);

    cout << "Postfix Expression: " << postfixExpression << endl;


    // Evaluate the postfix expression

    int result = evaluatePostfix(postfixExpression);

    cout << "Evaluation Result: " << result << endl;


    return 0;

}



// Explanation:

// Infix to Postfix Conversion:


// We iterate through each character in the infix expression.

// Operands (A-Z, a-z, or digits) are directly added to the postfix expression.

// *Operators (+, -, , /) are handled using a stack. If the operator on the stack has higher or equal precedence than the current operator, we pop it to the postfix expression.

// Parentheses are used to group operations. We push a ( to the stack when we encounter it and pop operators to the postfix expression until we encounter a ).

// Postfix Expression Evaluation:


// We evaluate the postfix expression by iterating through it.

// Operands (digits) are pushed to a stack.

// Operators pop two values from the stack, apply the operation, and push the result back onto the stack.

// The final value in the stack will be the result of the evaluation.

// Operator Precedence:


// The precedence of * and / is higher than that of + and -. The algorithm ensures that higher-precedence operators are applied first.


// sample input:A+B*c

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