looping, and inputing this info

#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <iostream>

using namespace std;

// CPC = Cost Per Credit
const int CPC_NONRES = 400;
const int CPC_RES    = 200;

// max tuition
const int MAX_NONRES = 5500;
const int MAX_RES    = 3300;

template<class T> inline T MIN(T a, T b) { return (a < b) ? a : b; }
template<class T> inline T MAX(T a, T b) { return (a > b) ? a : b; }

// -----------------------------------------------------------------

string prompt(const string& s)
{
    string ans;

    while (true) {
        cout << s;
        cin >> ans;

        if (cin.bad() || cin.fail()) {
            cout << "** Input error. Try again." << endl << endl;
            continue;
        }
        if (!ans.length())
            break;

        cin.ignore(256, '\n');
        break;
    }

    return ans;
}

// -----------------------------------------------------------------

int main()
{
    string fname, lname, is_res, ncredits;
    int credits = 0;
    int cpc = CPC_NONRES;
    int max = MAX_NONRES;

    fname  = prompt("\nEnter your first name: ");
    lname  = prompt("Enter your last name: ");
    is_res = prompt("Are you a resident? (y/n): ");

    while (true) {
        ncredits = prompt("Number of credits: ");
        credits  = atoi(ncredits.c_str());

        if (credits > 0 && credits <= 20)
            break;

        cout << "** Invalid answer. Must be 1-20." << endl << endl;
    }

    if (is_res.at(0) == 'Y' || is_res.at(0) == 'y') {
        cpc = CPC_RES;
        max = MAX_RES;
    }

    char* total;
    char* cost;
    sprintf(cost,  "$%.2f", (float)cpc);
    sprintf(total, "$%.2f", (float)MIN<int>((credits * cpc), max));

    printf("\nTuition for %s %s:\n\n", fname.c_str(), lname.c_str());
    printf("   CREDITS   |   PER CREDIT   |      TOTAL\n");
    printf("-------------+----------------+-----------\n");
    printf("   %7d   |   %10s   |   %8s\n\n", credits, cost, total);

    return EXIT_SUCCESS;
}