Question

Okay this is probably more a maths question but since its related to programming and my web application i’ll ask here first:

I’m trying to create short id’s that are 8 characters long . The “pool” to draw the id from is a combination of numbers, upper and lower case letters.

string charPool = "ABCDEFGOPQRSTUVWXY1234567890ZabcdefghijklmHIJKLMNnopqrstuvwxyz"

And if you’re interested here’s the method:

private string GenerateRandomCode(int length)
{
    string charPool = "ABCDEFGOPQRSTUVWXY1234567890ZabcdefghijklmHIJKLMNnopqrstuvwxyz";
    StringBuilder rs = new StringBuilder();

    for (int i = 0; i < length; i++)
    {
        rs.Append(charPool[(int)(_random.NextDouble() * charPool.Length)]);
    }

    return rs.ToString();
}

How many possible combinations are there for 8 character id’s? Grateful if you can post the equation as well 🙂

Thanks

Answer 1

options per slot ^ number of slots = number of combinations

a-z is 26, times 2 (for uppers as well) is 52, plus 10 (0-9) is 62. Each ID is 8 chars long, so the result is 62^8, which is pretty big:

218,340,105,584,896 possible unique ID's

I would suggest doing:

_random.Next(charPool.Length - 1)

(and saving charPool.Length - 1 in a variable outside of the loop), instead of:

_random.NextDouble() * charPool.Length

Because you might get an exact 1.0 with .nextDouble(), which means you will be accessing the array at an index that equals the length, and you will get IndexOutOfRangeException.