string typeVisa (string str); int main(){ string credit_card; string type; getline(cin, credit_card); type = typeVisa(credit_card); } string typeVisa (string str){ int digit1; int digit2; digit1 = atoi(str[0]); digit2 = atoi(str[1]); // code continues and returns a string } argument of type "char" is incompatible with parameter of type "const char *" What does this mean ^^?
atoi() takes a null-terminated string of characters, but you are trying to pass it a single character instead. You could do this instead:
char arr[] = {str[0], '\0'}; digit1 = atoi(arr); arr[0] = str[1]; digit2 = atoi(arr); ... But an easier way to convert a char in the range '0'..'9' to an integer of equivalent value is to subtract '0' from the character, thus:
'0' - '0' = 48 - 48 = 0 '1' - '0' = 49 - 48 = 1 '2' - '0' = 50 - 48 = 2 And so on... digit1 = str[0] - '0'; digit2 = str[1] - '0'; 
atoi(str.c_str())
If you want to convert each digit separately, you write something like below
//Instead of this: digit1 = atoi(str[0]); digit2 = atoi(str[1]); //use this: digit1 = (int)str[0] - '0'; digit2 = (int)str[1] - '0'; PS: you might need add validation for passed string is numeric.
0 is 48.