InfixToPostfix.cpp

// Infix to Postfix Expression 

#include <iostream>
#include <string>
#include <stack>

using namespace std;
//*********************************************
//function to convert Infix to PPostfix
string InfixToPostfix(string );

//check whether a particular character is operand or not
bool IsOperand(char );

//check whether a particular character is operator or not
bool IsOperator(char );

//check for operator precedence
bool hasHigherPrecedence(char ,char );

//finds weight to calculate operator precedence
int getWeight(char );

//**************************************************

int main(){
    string expression;
    cout << "Enter a infix expression :" << endl;
    getline(cin,expression);
    string postfix = InfixToPostfix(expression);
    cout << "Postfix Expression : " << postfix << endl ;
    return 0;
}

string InfixToPostfix(string expression){
    stack<char> S;
    string postfix = "";
    for (int i=0;i<expression.length();i++){
        if (expression[i] == ' ' || expression[i] == ',')continue;
        
        if(IsOperand(expression[i])){
            postfix += expression[i];
            continue;
        }
        if(expression[i] == '('){
            S.push(expression[i]);
            continue;
        }
        if(expression[i] == ')'){
            while(!S.empty() && S.top() != '('){
                postfix += S.top();
                S.pop();
            }
        S.pop();
        continue;
        }
        if(IsOperator(expression[i])){
            while(!S.empty() && S.top() != '(' && hasHigherPrecedence(S.top(), expression[i])){
                postfix += S.top();
                S.pop();
            }
            S.push(expression[i]);
            continue;
        }
        
    }
    while (!S.empty()){
        postfix += S.top();
        S.pop();
    }
    return postfix;
}

bool IsOperand(char exp){
    if(exp >= '0' && exp <= '9')return true;
    if(exp >= 'a' && exp <= 'z')return true;
    if(exp >= 'A' && exp <= 'Z')return true;
    return false;
}

bool IsOperator(char exp){
    if(exp == '+' || exp == '-' || exp == '*' || exp == '/' || exp == '^' )return true;
    return false;
}

bool hasHigherPrecedence(char op1, char op2){
    int op1Weight = getWeight(op1);
    int op2Weight = getWeight(op2);
    
    if(op1Weight == op2Weight){
        if(op1 == '^')return false; //bcoz exponent(^) is right associative.
        return true;
    }
    return (op1Weight > op2Weight) ? true :false ;
    
}

int getWeight(char exp){
    int weight = -1;
    switch(exp){
        case '+':
        case '-':   
            weight = 1;
            break;
        case '*':
        case '/':   
            weight = 2;
            break;
        case '^':
            weight = 3;
            break;
    }
    return weight;
}