Mask a card number
Support staff can see the last four digits of a card and nothing else.
- Every character except the last four becomes an asterisk.
- Anything four characters or shorter comes back untouched — there is nothing safe to hide.
MaskCard(cardNo: string) → string
C# needs a compiler and Drill does not host one yet, so this page is the reference rather than an exercise: the problem, worked examples, and the solution in full. To type it out, the same problem runs in Python.
Where you start
public string MaskCard(string cardNo) {
}Worked examples
| Call | Result |
|---|---|
MaskCard("4506349012345678") | "************5678" |
MaskCard("12345") | "*2345" |
MaskCard("1234") | "1234" |
MaskCard("") | "" |
Hint
Build the asterisks from the length, then glue on the tail.
Reference solution in C#
public string MaskCard(string cardNo) {
if (cardNo.Length <= 4) return cardNo;
return new string('*', cardNo.Length - 4) + cardNo.Substring(cardNo.Length - 4);
}