Creating Random Numbers

# Creating pseudo-random numbers with rand()

In Numerical Recipes in C: The Art of Scientific Computing (William H. Press, Brian P. Flannery, Saul A. Teukolsky, William T. Vetterling; New York: Cambridge University Press, 1992 (2nd ed., p. 277)), the following comments are made:

"If you want to generate a random integer between 1 and 10, you should always do it by using high-order bits, as in
j = 1 + (int) (10.0 * (rand() / (RAND_MAX + 1.0)));

and never by anything resembling
j = 1 + (rand() % 10);

(which uses lower-order bits)."
Random-number generation is a complex topic. The Numerical Recipes in C book (see reference above) provides an excellent discussion of practical random-number generation issues in Chapter 7 (Random Numbers).
Source: http://linux.die.net/man/3/rand

## Example: randomly sleep between 1 to 3 seconds

```int sleepTime = 0;
srand(time(NULL)); // set the seed
sleepTime = 1 + (int) (2000000 * (rand() / (RAND_MAX + 1.0)));
sleepTime += 1000000;

#ifdef DBG
fprintf(stderr, "\nDBG: sleepTime: %d\n", sleepTime);
#endif
```

### Sample output (20 iterations):

DBG: sleepTime: 2456105
DBG: sleepTime: 2592463
DBG: sleepTime: 1038192
DBG: sleepTime: 2758857
DBG: sleepTime: 1907930
DBG: sleepTime: 2341565
DBG: sleepTime: 1779641
DBG: sleepTime: 1212501
DBG: sleepTime: 2919997
DBG: sleepTime: 1078757
DBG: sleepTime: 1787229
DBG: sleepTime: 2222557
DBG: sleepTime: 1662914
DBG: sleepTime: 1094766
DBG: sleepTime: 2805637
DBG: sleepTime: 2956669
DBG: sleepTime: 1098023
DBG: sleepTime: 1825648
DBG: sleepTime: 1256375
DBG: sleepTime: 1976356

page revision: 3, last edited: 06 Jun 2008 13:58