Question

I’m trying to write a program that checks 15-16 digit inputs and see what bank they belong to. I’m not familiar with the language I’m coding in(c++), and would like some pointers. I know you can’t copy and paste without the rest of the code, but it would be to long to post all of it. I just need a little adivce on a couple things.

Right now I have the program checking the length of the input and what the first two values of the string are. I would like to know if there is an easier way then what I have right now.

if(cLen==15 && c[0]== 3 && c[1]==4)

and

if(cLen==15 && c[0]== 3 && c[1]==7)

cause all I need is to find Strings that have the first two nums to be 34 or 37

secondly I need to check if the string has first values of 51 through 55

and lastly I need to check if the string contains 6011 at the beginning.

string validatebankcc(string c, int cLen, bool& ccOK) {
    string bankcc;
    if(cLen==15 && c[0]== 3 && c[1]==4)
        bankcc = "AmericanExpress";
    if(cLen==15 && c[0]== 3 && c[1]==7)
        bankcc = "AmericanExpress";
    if(cLen==16 && "6011 in beginning")
        bankcc = "Discover";
    if(cLen==16 && c[0]==5 && c[1]==1)
        bankcc="MasterCard";
    if(cLen==16 && c[0]==5 && c[1]==5)
        bankcc="MasterCard";
    if(c[0]==4)
        bankcc="Visa";
    else
        bankcc = "Uknown Bank"
    return bankcc;

Answer 1

bool got_length_and_prefix(string s, int desired_length, string desired_prefix) {
  if (s.length() != desired_length) return false;
  if (s.find(desired_prefix) != 0) return false;
  return true;
}

string validatebankcc(string c, int /* cLen useless here*/, bool& ccOK) {
  ccOK = true;

  if (got_length_and_prefix(c, 15, "34")) return "AmericanExpress";
  if (got_length_and_prefix(c, 15, "37")) return "AmericanExpress";
  if (got_length_and_prefix(c, 16, "6011")) return "Discover";
  if (got_length_and_prefix(c, 16, "51")) return "MasterCard";
  if (got_length_and_prefix(c, 16, "55")) return "MasterCard";
  if (c[0] == '4') return "Visa";

  ccOK = false;
  return "Unkown Bank";
}

Though overall design is bad. Returning bank name as string is crying for trouble (which also leads to useless ccOK flag, which can be replaced by BANK_UNKNOWN or smth like that. Passing string length along with string which known about it length also smells like trouble.