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C++ Program for expression conversion as Infix to Postfix and Postfix Evaluation using Stack Data Structure

 Problem Statement: 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.

Note: Scroll Horizontally to see full line of code.

#include <iostream>
#include <string>
using namespace std;

class stack
{
    char *arr;
    int top = -1;

public:
    stack()
    {
        arr = new char[100];
        top = -1;
    }

    void push(char x)
    {
        top++;
        arr[top] = x;
    }

    void pop()
    {
        top--;
    }

    char Top()
    {
        return arr[top];
    }

    bool empty()
    {
        return top == -1;
    }
};

class intstack
{
    int *arr;
    int top = -1;

public:
    intstack()
    {
        arr = new int[100];
        top = -1;
    }

    void push(int x)
    {
        top++;
        arr[top] = x;
    }

    void pop()
    {
        top--;
    }

    int Top()
    {
        return arr[top];
    }

    bool empty()
    {
        return top == -1;
    }
};

int prec(char c)
{
    if (c == '^')
    {
        return 3;
    }
    else if (c == '*' || c == '/')
    {
        return 2;
    }
    else if (c == '+' || c == '-')
    {
        return 1;
    }
    else
    {
        return -1;
    }
}

int postfixEvaluation(string s)
{
    intstack st1;
    for (int i = 0; i < s.length(); i++)
    {
        if ((s[i] >= 'a' && s[i] <= 'z') || (s[i] <= 'Z' && s[i] >= 'A'))
        {
            int x;
            cout << "Enter the value of " << s[i] << " : ";
            cin >> x;
            st1.push(x);
        }
        else
        {
            int op2 = st1.Top();
            st1.pop();
            int op1 = st1.Top();
            st1.pop();
            switch (s[i])
            {
            case '+':
                st1.push(op1 + op2);
                break;
            case '-':
                st1.push(op1 - op2);
                break;
            case '/':
                st1.push(op1 / op2);
                break;
            case '*':
                st1.push(op1 * op2);
                break;
            }
        }
    }
    return st1.Top();
}

string infixToPostfix(string s)
{
    string ans;
    stack st;
    for (int i = 0; i < s.length(); i++)
    {

        if ((s[i] >= 'a' && s[i] <= 'z') || (s[i] <= 'Z' && s[i] >= 'A'))
        {
            ans += s[i];
        }
        else if (s[i] == '(')
        {
            st.push(s[i]);
        }
        else if (s[i] == ')')
        {
            while (!st.empty() && st.Top() != '(')
            {
                ans += st.Top();
                st.pop();
            }
            if (!st.empty())
            {
                st.pop();
            }
        }
        else
        {
            while (!st.empty() && (s[i] == '^' ? prec(st.Top()) > prec(s[i]) : prec(st.Top()) >= prec(s[i])))
            {
                ans += st.Top();
                st.pop();
            }
            st.push(s[i]);
        }
    }

    while (!st.empty())
    {
        ans += st.Top();
        st.pop();
    }

    return ans;
}

int main()
{
    string s;
    char cont = 'y';
    while (cont == 'y')
    {
        cout << "Enter the infix Expression: ";
        cin >> s;
        cout << "The postfix expression is: " << infixToPostfix(s) << endl;
        int ans = postfixEvaluation(infixToPostfix(s));
        cout << "The ans after postfix evaluation is: " << ans << endl;
        cout << "Do you want to continue? (y/n): ";
        cin >> cont;
    }
    if (cont == 'n')
    {
        cout << "Program Ended Successfully!!!" << endl;
    }
    return 0;
}


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