Generating multiple random numbers

Question

I want to generate 25 unique random numbers and list them in a console. The numbers should be atleast 10 characters long. Any easy way to do that?

Answer 1

Try building the numbers up as strings, and use a HashSet to ensure they are unique:

Random random = new Random();
HashSet<string> ids = new HashSet<string>();

while (ids.Count < 25)
{
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < 10; ++i)
    {
        sb.Append(random.Next(10));
    }
    ids.Add(sb.ToString());
}

Example output:

7895499338
2643703497
0126762624
8623017810
...etc...

The class HashSet is present in .NET 3.5 and newer.

Answer 2

The problem lies a little in "25 unique random". Displaying 25 random numbers is as easy as

Random r = new Random();
for(int i=0; i<25; i++)
    Console.WriteLine(r.Next(1,100).ToString());

These are not necessarily unique, though. If you do not want to allow duplicates, you need to store previously generated numbers somehow, and roll again if you hit an old one.

Be aware that you change the probability distribution of your generated numbers this way.

Edit: I've just noticed that these numbers should be ten characters long. Since 9,999,999,999 exceeds Int32.MaxValue, I'd suggest using Math.Floor(r.NextDouble() * 10000000000 + 1000000000) instead of r.Next(1,100).

Since your numbers are that long, you should not need to worry about duplicates. They are very very unlikely.