Postfix Evaluation (greater value).cpp

#include <iostream>
#include <stack>
#include <cmath>
using namespace std;

bool isOperator(char data){
    return (data == '+' || data == '-' || data == '*' || data == '/' || data == '^');
}

float calculate(char op, float A, float B){
    float sum = 0.0;
    switch(op){
        case '^':
            sum = pow(B, A);
            break;
        case '/':
            sum = B / A;
            break;
        case '*':
            sum = B * A;
            break;
        case '+':
            sum = B + A;
            break;
        case '-':
            sum = B - A;
            break;
    }
    return sum;
}

bool isInt(float data){
    return ((int)data == data);
}

struct Expression {
    string type;
    bool valid;
};
Expression *expression(string expression){
    Expression *exp = new Expression;
    bool strExp = expression.find_first_not_of("(+-*/^) abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ") == string::npos;
    bool numExp = expression.find_first_not_of("(+-*/^) 0123456789") == string::npos;
    
    if(strExp && !numExp){
        exp->type = "letter";
    } else if(numExp && !strExp){
        exp->type = "number";
    } else {
        exp->type = "!";
    }
    exp->valid = (strExp || numExp) && (strExp != numExp);
    return exp;
}

string evaluate(string postfix){
    stack<float> stack;
    float A, B, sum;
    string number;

    Expression *exp = new Expression;
    exp = expression(postfix);
    bool error = (!exp->valid || exp->type != "number") ? true : false;

    if(!error){
        for(int i = 0; i < postfix.size(); i++){
            char data = postfix[i];
            if(data == ' '){
                if(number.length() > 0) stack.push(stoi(number)); number = ""; 
            } else if(isdigit(data)){
                number += data;
            } else if(isOperator(data)){
                if(stack.size() >= 2){
                    float A, B, sum;
                    A = stack.top(); stack.pop();
                    B = stack.top(); stack.pop();
                    sum = calculate(data, A, B);
                    stack.push(sum);
                } else {
                    error = true;
                }
            } else {
                error = true;
            }
            if(error) break;
        }
    }

    string finalResult;
    if(!stack.empty() && stack.size() == 1){
        float result = stack.top();
        finalResult = isInt(result) ? to_string((int)result) : to_string(result); // Check if the result has no fractional
    } else {
        error = true;
    }
    return (error) ? "!" : finalResult;
}

int main(){
    string postfix = "2 3 4 * + 8 23 2 ^ / -";
    cout << "Postfix: " << postfix << endl;

    string result = evaluate(postfix);
    result = (result == "!") ? "Invalid Expression!" : result;
    cout << "Postfix evaluated result = " << result << endl;
}