Question

I need to create a non sequential list of numbers that fit within a range. For instance i need to a generate a list of numbers from 1 to 1million and make sure that non of the numbers are in a sequential order, that they are completly shuffled. I guess my first question is, are there any good algorithms out there that could help and how best to implement this.

I currently am not sure the best way to implement, either via a c# console app that will spit out the numbers in an XML file or in a database that will spit out the numbers into a table or a set of tables, but that is really secondary to actually working out the best way of “shuffling” the set of numbers.

Any advice guys?

Rob

Answer 1

How “non-sequential” do you want it?

You could easily generate a list of random numbers from a range with the Random class:

Random rnd1 = new Random();
List<int> largeList = new List<int>();

for (int i = 0, i < largeNumber, i++)
{
  largeList.Add(rnd1.Next(1, 1000001);
}

---

Edit to add

Admittedly the Durstenfeld algorithm (modern version of the Fisher–Yates shuffle apparently) is much faster:

var fisherYates = new List<int>(upperBound);
for (int i = 0; i < upperBound; i++)
{
  fisherYates.Add(i);
}

int n = upperBound;

while (n > 1)
{
   n--;
   int k = rnd.Next(n + 1);
   int temp = fisherYates[k];
   fisherYates[k] = fisherYates[n];
   fisherYates[n] = temp;
}

For the range 1 to 10000 doing a brute force “find a random number I’ve not yet used” takes around 4-5 seconds, while this takes around 0.001.

Props to Greg Hewgill for the links.