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Random numbers in C++

Random numbers in C++. Nobody knows what’s next. 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

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Random numbers in C++

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  1. Random numbers in C++ Nobody knows what’s next...

  2. 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!

  3. 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”

  4. “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

  5. 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

  6. 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 • That’s the random number • Repeat the process by feeding the number just generated back in as the starting number next time

  7. 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

  8. 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?

  9. 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

  10. 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)

  11. 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!)

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