Random number generator - why seed every time

Question

I am relative new to c and c++. In java, the language I am used to program in, its very easy to implement random number generation. Just call the the static random-method from a class called Math.

int face = ((int)(Math.random() * 6) + 1);

simulates a dice-throw ...

In c and c++ you have to "seed the random number generator" , by calling the srand-function

srand ( time(NULL) );

What is the point of doing this - I mean is there any advantage of having to seed the random number generator every time the code is run?

Answer 1

Given the same seed, a pseudo random number generator will produce the same sequence every time. So it comes down to whether you want a different sequence of pseudo random numbers each time you run, or not.

It really depends on your needs. There are times when you want to repeat a sequence. And times when you do not. You need to understand the needs of each specific application.

One thing you must never do is seed repeatedly during generation of a single sequence. Doing so very likely will destroy the distribution of your sequence.

Answer 2

What's usually called a random number generator is actually a pseudo-random number generator. This typically means that you can generate the same random sequence if you provide the "key" to that sequence, referred to as the "seed". This is very useful when you wish to test your algorithm that is based on randomization, and you need to ensure repeatable results.

If you do not "seed" your Random number generator, it is seeded with some (usually based on system time) random number by default, and therefore produces the different sequence every time that you run your program.

Answer 3

If you don't seed the generator, it will have the same seed every time you run your program, and the random number sequence will be the same each time.

Also note that you only should to seed the generator once, at the beginning of the program.

Answer 4

The advantage is that you can repeat a random number sequence by supplying the same seed.

The game Elite used this to store an entire world, consisting of thousands of stars, as a single number. To generate the exact same world a second time, the just supplied the same seed.