Is uninitialized local variable the fastest random number generator?

Question

I know the uninitialized local variable is undefined behaviour(*UB*), and also the value may have trap representations which may affect further operation, but sometimes I want to use the random number only for visual representation and will not further use them in other part of program, for example, set something with random color in a visual effect, for example: void updateEffect(){ for(int i=0;i<1000;i++){ int r; int g; int b; star[i].setColor(r%255,g%255,b%255); bool isVisible; star[i].setVisible(isVisible); } } is it that faster than void updateEffect(){ for(int i=0;i<1000;i++){ star[i].setColor(rand()%255,rand()%255,rand()%255); star[i].setVisible(rand()%2==0?true:false); } } and also faster than other random number generator?

Answer 1

You actually don't need to call rand() more than once, if you can generate a good starting seed. Lot's of good science in computer science has been put into creating random numbers, as computers are not very good at it. (Google Donald Knuth and "The Art of Computer Programming") Most random number generators use a large table, and when you call srand() to initialize your random numbers, it is picking a location in that table. If you provide srand() with the same number, you get the same sequence of random numbers each call.

Most programs, to get around this, seed srand() with the current time, usually the time when the program start. I know a guy who broke the Keno games at a Casino in Atlantic City, because someone told him what time the machine was turned on. My company's software used the time, but we reset srand() every time a human touched a button that requested the ball draw -- human interaction is something truly random, as you could not know in advance the exact time they would press the button, and by the time you found out, the draw was already done.

That said, try creating your own Random() class, and have the constructor seed srand() at instantiation. Add additional calls to srand() at infrequent (ie, random) times that your program requests a random number.

Random::Random()

{

const short StartupRands = 16;

const unsigned int MaxSeed = 0xccd569eL;

for( short starts = 0; starts < StartupRands; starts++ )

{

time_t now;

srand((unsigned int)time(&now));

do

{

seed_ = rand()*rand() + rand()*rand();

}

while( seed_ > MaxSeed );

}

srand((unsigned int)seed_);

}