C++ Notes: Random Numbers

Random positive integers - rand()

The rand function returns a "random" positive integer from 0 to a large value (at least 32,767) every time it is called. To scale the value into the range you want, use the mod (%) operator and addition. For example to generate a random number in the range 1 to 10 and assign it to r: 
#include <ctime>    // For time()
#include <cstdlib>  // For srand() and rand()
    . . .
    srand(time(0));  // Initialize random number generator.
    . . .
    r = (rand() % 10) + 1;

Pseudo-random numbers and seeds - srand(...)

The sequence of numbers returned by rand() are called random because they satisfy statistical tests for randomness, eg, uniform distribution, coorelation between sequential numbers is zero, no apparent patterns). But of course they really can't be truly random (whatever that means) because computers are deterministic. Therefore they are more properly called pseudorandom numbers.

For a given seed (starting value), the sequence of numbers that rand() returns will always be the same. Because the starting point for the pseudorandom sequence can easily be varied (see below) and because the sequence is very long (perhaps billions before the sequence repeats), these pseudorandom numbers are as good as random.

Having the same sequence generated each time can be useful for debugging, but it isn't very useful when you're actually using the program. To generate a different random sequence, it's necessary to set the seed that starts the sequence. The srand() function takes a positive integer parameter which tells where to start the sequence. 

srand(2345);
The above call sets the initial seed to 2345. However, this still isn't very useful. If you want to have what appears to be a truly random sequence each time you run the program, you need to start with a different seed each time.

Using the time as a seed - srand(time(0))

The standard way to start with a different initial value, the seed, is to use the current time as a seed. Use the time() function as follows: 
srand(time(0));  // Initialize random number generator.
at the beginning of the program to initialize the random seed. time(0) returns the integer number of seconds from the system clock. This will almost always be a different value.

Include files

Use the following include files. 
#include <ctime>    // For time()
#include <cstdlib>  // For srand() and rand()