Learning C++ - Credit Card Verification

#include <iostream> #include <string> using namespace std; int isNumber(string temp) { // check whether the input is a // 16-digit number by comparing the items // in the string with the ascii values // for numbers starting at 0:48 to 9:57 int tc; for (int x =0; x<temp.length(); x++) { tc = temp[x]; if (tc>=48 && tc <= 57) { if (x==temp.length()-1) return 1; } else { return 0; } } } int val(char ch) { // change char of a number to its int value. // 0 in ascii starts at 48 hence the value in the return function. int tc; tc = ch; return (tc-48); } int luhn (string temp) { // luhn algorithm or modulo 10. // more info on wikipedia. int sum,total,valtemp; sum=0; for (int x=0; x< temp.length();x++) { if (x % 2 == 0) { valtemp = val(temp[x]) *2; if (valtemp > 9) valtemp -= 9; } else { valtemp = val(temp[x]); } sum += valtemp; } if (sum%10 == 0) { //number is valid. return 1; } else { //number is not valid return 0; } } int process(string temp) { // process user input if (temp.length() < 16 || temp.length()> 16) { cout << "Error: Please insert 16 digits." << endl; } else { if (isNumber(temp) == 1) { cout<<"Calculating..." << endl; if (luhn(temp)==1) { cout << "Validated: Credit card number is valid." << endl; } else { cout << "Credit card number is not valid" << endl; } } else { cout << "Only numbers are permitted." << endl; } } return 0; } int main() { string temp; //User Input. cout << "Check credit card(16 digits):"<< endl; cin >> temp; process(temp); return 0; }