Random

Random Number Generation

• Produce a number within some bounds following some statistical rules.
• A true random number is a number that is nondeterministically selected from a set of numbers wherein each possible selection has an equal probability of occurrence.
• Usually in computer science we contend with pseudo-random numbers. These are not truly nondeterministic, but an approximation of random selection based on some algorithm.
• Since pseudo-random selections are “determined” by an algorithm, or set of rules, they are technically deterministic.

Random Class in C#

• Instantiate a random number generator and use to select numbers:

  Random rand = new Random();
  Random randB = new Random(seed_int);

• Notice that we can create a generator with or without an argument. The argument is called a seed for the generator.

• A seed tells the generator where to start its sequence. Using the same seed will always reproduce the same sequence of numbers.

• The default constructor still has a seed value, but it is a hidden value pulled from the clock time during instantiation.

• Time-based seeds only reset approximately every 15 milliseconds.

• The random class is not “random enough” for cryptography.

• For cryptographic randomness, use the RNGCryptoServiceProvider class or System.Security.Cryptography.RandomNumberGenerator.

Using Random

• Next() method returns a pseudo-random number between 0 and 2,147,483,647 (max signed int), inclusive.

• By default, the number is always non-negative and within that range.

  int randomInt = rand.Next();

• What if we wanted to create a random number between 0 and 100?

• We could use rand.Next() and then use modulo to cut down the answer range!

• Alternatively, we could give the Next() method an int argument to set a ceiling.

  int randomUpto100 = rand.Next(101);

• The ceiling value is exclusive, so remember to use one number higher than what you want to be your max number.

• We can also pass two arguments in order to set a range for the values.

  int random50to100 = rand.Next(50,101);

• The ceiling value is still exclusive, but the floor is inclusive.

• NextDouble() returns a normalized value (value between 0.0 and 1.0 inclusive).

• What if we want a different range? Adjust with math!

  double randNeg2to3 - (rand.NextDouble()*5)-2;

• NextBytes() method takes a byte array as an argument and generates a random byte value for each index.

• Remember, a byte has an unsigned value between 0 and 255 inclusive.

  byte[] byteArray = new byte[10];
  rand.NextBytes(byteArray);

Creating Random Strings

• What if we want to construct random strings made of a, b, c, and d?

• Other techniques are available, but we can use a loop and switch!

  Random rand = new Random();
  string answer = "";
  int selection = 0;
   
  for(int i = 0; i < 10; i++)
  {
      selection = rand.Next(4);
      switch(selection){
      case(0):
          answer+="a";
          break;
      case(1):
          answer+="b";
          break;
      case(2):
          answer+="c";
          break;
      default:
          answer+="d";
          break;
      }
  }