Random numbers in C

1
Random numbers in C

• Nobody knows whats next...

2
Syntax

• Include the cstdlib library to get the functions
• Use srand(seed) to set the initial seed (seed
  must be an integer)
• Use rand() to get the next random number
• returns an integer between 0 and RAND_MAX
• Call srand ONCE per program, call rand many times

3
Seeds to start with

• srand(23) will always give you the same
  sequence of random numbers good when testing!
• Asking the user for a number and then using it as
  the seed - works but is a bit aggravating to the
  user
• Using the time function is most flexible (see
  example on web page)

4
rand() and RAND_MAX

• rand() returns a integer between 0 and RAND_MAX
  (a constant defined in cstdlib)
• What if you need an integer number between 1 and
  6? rand() 6 1
• What if you need a number between 0 and 1 (i.e, a
  fraction) ? rand() / RAND_MAX (but watch your
  TYPE here!)

5
Deterministic machines

• That means that computers do the same predictable
  thing every time you give them the same
  instructions
• we WANT this we don't want random happenings -
  most of the time
• when would you NOT want this?
• games
• simulations of reality traffic flows, nuclear
  explosions, star formation
• Cryptography!

6
What is "random"?

• A real random number is generated by a real
  world event like an atom decaying or not in a
  certain time
• Hard to do that in a computer
• Best we can do with a deterministic machine is
  make pseudorandom numbers
• They are good enough

7
good enough?

• A good random number is one that is distributed
  evenly in its range
• If you were rolling a die, you want numbers from
  1 to 6 to be equally likely to show up
• This is over the long run, not each individual
  trial

8
Lots of research

• Lots has been done on random numbers
• Trying to get faster algorithms
• With larger cycles all algorithms will
  eventually start repeating but the best ones not
  before a few million numbers at least
• Very heavy statistics and mathematics in the
  latest algorithms

9
A simple one mid square

• Take a number to start with (the seed)
• Square it
• Take the middle of it trim off some digits at
  front and end
• Thats the random number
• Repeat the process by feeding the number just
  generated back in as the starting number next time

10
An example

• 12345 squared 152399025
• chop it off and get 23990
• 23990 squared 575520100
• chop it off and get 55201
• 55201 squared 3047150401
• chop it off and get 47150
• And so on

11
Properties of an RNG

• Give the algorithm a DIFFERENT seed to start with
  and what comes out?
• Give the algorithm the SAME seed to start with
  and what comes out?