Conversion of infix string to postfix and evaluation of postfix string to make a simple calculator application in C++.

(A) Algorithm for converting an infix expression into postfix operation

(B) Algorithm for evaluation of postfix string

Source code for both infix to postfix and postfix evaluation

The code is also available on GitHub.

Program: Conversion of Infix to Postfix String and Evaluation
Language: C/C++
by Bibek Subedi
June 13, 2011
Operators Used
1. '+' For addition
2. '-' For Subtraction
3. '*' For Multiplication
4. '/' For Division
5. '^' For Exponentiation
Program Limitations
* This program Only process single digit operations
* Can't handle unary operation
* Only process left to right associativity
***************************************************/
#include <iostream>
#include <cmath>
#include <cstdlib>
#include <string>
#define MAX_SIZE 20
using namespace std;
template <class T>
class Stack {
private:
    T item[MAX_SIZE];
    int top;
 
public:
    Stack()
    {
        top = -1;
    }
    void push(T data)
    {
        if (!this->is_full())
            item[++top] = data;
        else {
            cout << "Stack Error" << endl;
            exit(10);
        }
    }
    T pop()
    {
        if (!this->is_empty())
            return item[top--];
        else {
            cout << "Stack is Empty" << endl;
            exit(11);
        }
    }
    int size()
    {
        return top + 1;
    }
    bool is_empty()
    {
        if (top == -1)
            return true;
        else
            return false;
    }
    bool is_full()
    {
        if (top == MAX_SIZE - 1)
            return true;
        else
            return false;
    }
    void display()
    {
        for (int i = 0; i < this->size(); i++) {
            cout << item[i] << " ";
        }
        cout << endl;
    }
    T return_top()
    {
        return item[top];
    }
};
class Convert {
private:
    bool num_flag;
    bool two_digit_flag;
 
public:
    Convert();
    string return_with_bracket(string infix);
    void to_Postfix(string infix, char postfix[]);
    bool prcd(char op1, char op2);
    int isOperand(char op);
    int isOperator(char op);
    bool return_flag()
    {
        return num_flag;
    }
};
Convert::Convert()
{
    this->num_flag = false;
    this->two_digit_flag = false;
}
string Convert::return_with_bracket(string infix)
{
    return ("(" + infix + ")");
}
bool Convert::prcd(char op1, char op2)
{
    if ((op1 == '+' || op1 == '-' || op1 == '*' || op1 == '/') && op2 == '(')
        return true;
    if (op1 == '+' && op2 == '+')
        return true;
    if (op1 == '-' && op2 == '-')
        return false;
    if (op1 == '-' && op2 == '+')
        return false;
    if (op1 == '+' && op2 == '-')
        return false;
    if (op1 == '/' && op2 == '/')
        return false;
    if (op1 == '/' && (op2 == '-' || op2 == '+'))
        return true;
    if (op1 == '*' && (op2 == '+' || op2 == '-'))
        return true;
    if ((op1 == '-' || op1 == '+') && (op2 == '*' || op2 == '/'))
        return false;
    if ((op1 == '$' || op1 == '+') && (op2 == '*' || op2 == '/' || op2 == '-'))
        return true;
    if ((op1 == '-' || op1 == '+' || op1 == '*' || op1 == '/') && op2 == '^')
        return false;
    if (op1 == '^' && (op2 == '+' || op2 == '*' || op2 == '/' || op2 == '-'))
        return false;
}
int Convert::isOperand(char op)
{
    return (op >= '0' && op <= '9');
}
int Convert::isOperator(char op)
{
    return (op == '+' || op == '-' || op == '/' || op == '*' || op == '^');
}
void Convert::to_Postfix(string infix, char postfix[])
{
    int position, outpos = 0;
    char c;
    int count = 0;
    char temp;
    char stacktop;
    Stack<char> stack;
    for (position = 0; (c = infix[position]) != '\0'; position++) {
        if (this->isOperand(c)) {
            postfix[outpos++] = c;
            this->num_flag = true;
            count++;
            if (count >= 2) {
                this->two_digit_flag = true;
            }
        }
        else if (this->isOperator(c)) {
            count = 0;
            if (isOperator(infix[position]) && isOperator(infix[position + 1])) {
                cout << "\aMissing argument in between " << infix[position] << " and " << infix[position + 1]
                     << " in column " << position + 1 << endl;
                exit(9);
            }
            if (this->prcd(c, stacktop)) {
                stacktop = stack.return_top();
                stack.push(c);
                stacktop = c;
            }
            else {
                while (true) {
                    temp = stack.pop();
                    postfix[outpos++] = temp;
                    stacktop = stack.return_top();
                    if (prcd(c, stacktop) || stacktop == '(')
                        break;
                }
                stack.push(c);
                stacktop = stack.return_top();
            }
        }
        else if (c == '(') {
            count = 0;
            stack.push(c);
            stacktop = stack.return_top();
        }
        else if (c == ')') {
            count = 0;
            while (1) {
                if (stack.size() == 0) {
                    cout << "Warning!! Number of ')' is greater than '('" << endl;
                    exit(2);
                }
                temp = stack.pop();
                if (temp != '(') {
                    postfix[outpos++] = temp;
                }
                else {
                    break;
                }
            }
            stacktop = stack.return_top();
        }
        else {
            cout << "Invalid input";
            exit(3);
        }
        if (infix[position] == ')' && infix[position + 1] == '(') {
            stack.push('*');
            stacktop = stack.return_top();
        }
    }
    if (stack.size() != 0) {
        cout << "Warning!!Number of '(' is greater than ')'" << endl;
        // exit(6);
    }
    if (!this->return_flag()) {
        cout << "You must Enter Numeric value for calculation" << endl;
        cout << "This program cannot perform operations on variables";
        exit(5);
    }
    if (this->two_digit_flag) {
        cout << "Sory! Althoug u may  have entered right string" << endl;
        cout << "this program is only for single digit operation" << endl;
        exit(8);
    }
    postfix[outpos] = '\0';
}
class Evaluate {
public:
    double eval(char expr[], Convert&);
    double oper(int symb, double op1, double op2);
};
double Evaluate::oper(int symb, double op1, double op2)
{
    switch (symb) {
    case '+':
        return (op1 + op2);
    case '-':
        return (op1 - op2);
    case '*':
        return (op1 * op2);
    case '/':
        return (op1 / op2);
    case '^':
        return (pow(op1, op2));
    }
}
double Evaluate::eval(char expr[], Convert& convert)
{
    int c, position;
    char temp1;
    int count = 0;
    double opnd1, opnd2, value;
    Stack<double> stack;
    for (position = 0; (c = expr[position]) != '\0'; position++) {
        if (convert.isOperand(c)) {
            temp1 = double(c - '0');
            stack.push(temp1);
        }
        else {
            opnd2 = stack.pop();
            if (stack.size() == 0) {
                cout << "This program cannot process unary operation";
                exit(1);
            }
            opnd1 = stack.pop();
            value = oper(c, opnd1, opnd2);
            stack.push(value);
        }
    }
    if (stack.size() >= 2) {
        cout << "Sory! this program cannot calculate this" << endl;
        cout << "Enter +, *, /, - or ^ between bracket" << endl;
        exit(4);
    }
    return (stack.pop());
}
int main()
{
    Convert convert;
    Evaluate evaluate;
    string bracketted_infix;
    char infix[50], postfix[50];
    char choice;
    while (1) {
        cout << "Enter string: ";
        cin >> infix;
        cout << endl;
        cout << "Entered String: " << infix << endl;
        bracketted_infix = convert.return_with_bracket(infix);
        convert.to_Postfix(bracketted_infix, postfix);
        cout << "Equivalent Postfix string: " << postfix << endl;
        cout << "RESULT: ";
        cout << evaluate.eval(postfix, convert);
        cout << "\nCalculate another string?(y/n) ";
        cin >> choice;
        cout << endl;
        if (choice == 'n')
            break;
    }
    return 0;
}