How do you generate a random number in C#?

Question

I would like to generate a random floating point number between 2 values. What is the best way to do this in C#?

Answer 1

The only thing I'd add to Eric's response is an explanation; I feel that knowledge of why code works is better than knowing what code works.

The explanation is this: let's say you want a number between 2.5 and 4.5. The range is 2.0 (4.5 - 2.5). NextDouble only returns a number between 0 and 1.0, but if you multiply this by the range you will get a number between 0 and range.

So, this would give us random doubles between 0.0 and 2.0:

rng.NextDouble() * 2.0

But, we want them between 2.5 and 4.5! How do we do this? Add the smallest number, 2.5:

2.5 + rng.NextDouble() * 2.0

Now, we get a number between 0.0 and 2.0; if you add 2.5 to each of these values we see that the range is now between 2.5 and 4.5.

At first I thought that it mattered if b > a or a > b, but if you work it out both ways you'll find it works out identically so long as you don't mess up the order of the variables used. I like to express it with longer variable names so I don't get mixed up:

double NextDouble(Random rng, double min, double max)
{
    return min + (rng.NextDouble() * (max - min));
}

Answer 2

System.Random r = new System.Random();

double rnd( double a, double b )
{
   return a + r.NextDouble()*(b-a);
}

Answer 3

// generate a random number starting with 5 and less than 15
Random r = new Random();
int num = r.Next(5, 15);  

For doubles you can replace Next with NextDouble

Answer 4

How random? If you can deal with pseudo-random then simply:

Random randNum = new Random();
randNum. NextDouble(Min, Max);

If you want a "better" random number, then you probably should look at the Mersenne Twister algorithm. Plenty of people hav already implemented it for you though

Answer 5

Here is a snippet of how to get Cryographically safe random numbers: This will fill in the 8 bytes with a crytographically strong sequence of random values.

byte[] salt = new byte[8];
RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider();
rng.GetBytes(salt);

For more details see How Random is your Random??" (inspired by a CodingHorror article on deck shuffling)

Answer 6

For an explaination of why Longhorn has been downmodded so much: http://msdn.microsoft.com/en-us/magazine/cc163367.aspx Look for the implementation of NextDouble and the explanation of what is a random double.

That link is also a goo example of how to use cryptographic random numbers (like Sameer mentioned) only with actual useful outputs instead of a bit stream.