Question

I’m working on a .NET application in C# that will generate a variable-length string of random digits. I’d like to add a checklist that will allow the user to select any combination of digits between zero and nine and prevent them from appearing in the string. Currently, I simply do this to get the string:

do
{
   int num = rnd.Next(10);
   output += num.ToString();
   i++;
}
while (i < stringLength);

I can think of a way to exclude selected digits by throwing them out once they are generated and not incrementing the counter, but it seems like there would be a less wasteful algorithm. The program will support generating a number of strings, so if a user is creating millions of strings, I’d like to keep overhead at a minimum.

Bonus: I forgot to mention that I’d also like if someone could point me to a resource for patterns such as this. I’ll be working a lot with random numbers based on parameters in the near future and I’d like to learn some principles instead of asking questions about individual problems such as this.

Answer 1

If you generate an array of allowed characters then pick from that:

int[] allowedDigits = {1, 2, 3, 6, 7, 0};
var output = new StringBuilder();

do
{
   int num = rnd.Next(allowedDigits.Length);
   output.Append(allowedDigits[num]);
   i++;
}
while (i < stringLength);

obviously allowedDigits is generated rather than hard coded 😉

Using a StringBuilder is more efficient and you don’t have to explicitly do the ToString.