Roman Numeral Calculator

/********************************************************
*                                                       *
*                   Roman Numeral Calc                  *
*                   Author: wmays                       *
*                                                       *
*           This was my first attempt to make a program *
*           that would use Roman Numerals.              *
*                                                       *
*********************************************************/

#include <string>
#include <iostream>
//Definitions for messages (We don't want instructions for Latin math in English, do we?)
#define ENTER_MSG "Ingressus operator et numero: "
#define WRONG_FORMAT "Omnes numeros oportet esse Numeris Romanis."
#define RESULT "Exitu: "

using namespace std;
// All the values for Roman Numerals
char numerals[] = {'I','V','X','L','C','D','M'};
short int values[] = {1,5,10,50,100,500,1000};
//Just a useful function to find a letter's location in the array
int indIn(char ch){
    int ret = -1;
    for(int j = 0; j < sizeof(numerals); j++){
        if(ch == numerals[j]){
            ret = j;
            break;
        }
    }
}
// Detects if a string is a valid Roman Numeral
bool isRNum(string str){
    for(int i = 0; i < str.length(); i++){
        bool worked = false;
        for(int j = 0; j < sizeof(numerals); j++){
            if(numerals[j] == str[i]){
                worked = true;
            }
        }
        if(!worked)
            return(false);
    }
    return(true);
}

// Roman Numeral Parser
int pNum(string stri){
    int ret = 0;
    for(int i = 0; i < stri.length(); i++){
        char cur = stri[i];
        int cval = values[indIn(cur)];
        for(int j = i+1; j < stri.length(); j++){
            char cchar = stri[j];
            if(indIn(cchar) > indIn(cur)){
                cval *= -1;
                break;
            }
        }
        ret += cval;
    }
    return(ret);
}
// Convert from integers to Roman Numerals
string tNum(int n){
    int t = n;
    string ret = "";
    for(int i = sizeof(numerals)-1; i >= 0; i--){
        while(true){
            if(t >= values[i]){
                t -= values[i];
                ret += numerals[i];
            }else{
                if(i > 0){
                    if(i > 1){
                        if(t >= values[i] - values[i-2]){
                            t -= (values[i] - values[i-2]);
                            ret += numerals[i-2];
                            ret += numerals[i];
                            break;
                        }
                    }else if(t >= values[i] - values[i-1]){
                        t -= (values[i] - values[i-1]);
                        ret += numerals[i-1];
                        ret += numerals[i];
                    }
                }
                break;
            }
        }
    }
    return(ret);
}

char oper = ' ';
string numer = "";
int temp = 0;
int main(){
    while(true){
        cout <<ENTER_MSG;
        cin >> oper >> numer;
        if(isRNum(numer)){
            switch(oper){
                case '+':
                    temp += pNum(numer);
                    break;
                case '-':
                    temp -= pNum(numer);
                    break;
                case '*':
                    temp *= pNum(numer);
                    break;
                case '/':
                    temp /= pNum(numer);
                default:
                    break;
            }
            cout << RESULT << tNum(temp) << '\n';
        }else{
            cout << WRONG_FORMAT << '\n';
        }
    }
    return(0);
}