1. Why they aren't really random
Random Numbers
Why they aren't really random

2. Why Random? Obvious uses Less obvious Games Simulations
Cryptography – shared secret

3. Pseudo Random Numbers Recipe: Start with number
Apply random looking math transformation eg : (num * 1009) % 100 12 8 72 48 32 88 92 28 52 68

4. C++ Random Need C standard library: #include <cstdlib>
rand(): returns value between 0 and RAND_MAX
RAND_MAX defined by library, usually ~32000

5. Seed Seed : starting value Same seed = same sequence 12 8 72 48 32 8892 28 52 68 12 8 72 48 32 88 92 28 52 68 12 8 72 48 32 88 92 28 52 68

6. Random Seed srand(num):
seeds random generator with integer value num
Reproducible random : seed with constant srand(12);

7. Random Seed Random sequence requires random seed Sources: System clock
Human input
Special device
Lavarand
Radio noise
Special hardware

8. Time From ctime library: #include <ctime>
time(0) returns seconds since Jan 1, 1970

9. Random Seed srand(num):
seeds random generator with integer value num
Reproducible random : seed with constant srand(12);
Less predictable random : seed with time srand(time(0));

10. C++ Random Range Use % to control range:
0 to 9: int num = rand() % 10;
0 to 29: int num = rand() % 30;

11. C++ Random Range Range Minimum Use + NUM for lower bound
1 to 6: int num = rand() % 6 + 1;
100 to 250: int num = rand() % ;
Range
Minimum

12. C++ Random Other Random double Divide by max size
Watch out for int division
double r = static_cast<double>(rand()) / RAND_MAX;

13. Theoretical Notes Cycles appear when we hit repeat values
Use prime numbers
Use larger range of values than you really need 12 8 72 48 32 88 92 28 52 68 …

14. Bad Tendencies Bad recipes may Cycle easily
Favor part of the number space
Show patterns
High always followed by low

15. C++ Random Numbers Aren't Good
Algorithm is weak
Poor distribution
Limited range
Serious about randomness?
Use an add on library