Recommended way to initialize srand?

Question

I need a \'good\' way to initialize the pseudo-random number generator in C++. I\'ve found an article that states:

In order to generate random-like

Answer 1

This is what I've used for small command line programs that can be run frequently (multiple times a second):

unsigned long seed = mix(clock(), time(NULL), getpid());

Where mix is:

// http://www.concentric.net/~Ttwang/tech/inthash.htm
unsigned long mix(unsigned long a, unsigned long b, unsigned long c)
{
    a=a-b;  a=a-c;  a=a^(c >> 13);
    b=b-c;  b=b-a;  b=b^(a << 8);
    c=c-a;  c=c-b;  c=c^(b >> 13);
    a=a-b;  a=a-c;  a=a^(c >> 12);
    b=b-c;  b=b-a;  b=b^(a << 16);
    c=c-a;  c=c-b;  c=c^(b >> 5);
    a=a-b;  a=a-c;  a=a^(c >> 3);
    b=b-c;  b=b-a;  b=b^(a << 10);
    c=c-a;  c=c-b;  c=c^(b >> 15);
    return c;
}

Answer 2

On windows:

srand(GetTickCount());

provides a better seed than time() since its in milliseconds.

Answer 3

#include <stdio.h>
#include <sys/time.h>
main()
{
     struct timeval tv;
     gettimeofday(&tv,NULL);
     printf("%d\n",  tv.tv_usec);
     return 0;
}

tv.tv_usec is in microseconds. This should be acceptable seed.

Answer 4

i suggest you see unix_random.c file in mozilla code. ( guess it is mozilla/security/freebl/ ...) it should be in freebl library.

there it uses system call info ( like pwd, netstat ....) to generate noise for the random number;it is written to support most of the platforms (which can gain me bonus point :D ).

Answer 5

Include the header at the top of your program, and write:

srand(time(NULL));

In your program before you declare your random number. Here is an example of a program that prints a random number between one and ten:

#include <iostream>
#include <iomanip>

using namespace std;

int main()
{
   //Initialize srand
   srand(time(NULL));

   //Create random number
   int n = rand() % 10 + 1;

   //Print the number
   cout << n << endl; //End the line

   //The main function is an int, so it must return a value
   return 0;
}

Answer 6

The best answer is to use the Boost random number stuff. Or if you have access to C++11 use the <random> header.

But if we are talking about rand() and srand()
The best way is just to use time():

int main()
{
    srand(time(NULL));

    ...
}

Be sure to do this at the beginning of your program, and not every time you call rand()!

Every time you start up, time() will return a unique value (unless you start the application multiple times a second). In 32 bit systems, it will only repeat every 60 years or so.

I know you don't think time is unique enough but I find that hard to believe. But I have been known to be wrong.

If you are starting a lot of copies of your application simultaneously you could use a timer with a finer resolution. But then you run the risk of a shorter time period before the value repeats.

OK, so if you really think you are starting multiple applications a second.
Then use a finer grain on the timer.

 int main()
 {
     struct timeval time; 
     gettimeofday(&time,NULL);

     // microsecond has 1 000 000
     // Assuming you did not need quite that accuracy
     // Also do not assume the system clock has that accuracy.
     srand((time.tv_sec * 1000) + (time.tv_usec / 1000));

     // The trouble here is that the seed will repeat every
     // 24 days or so.

     // If you use 100 (rather than 1000) the seed repeats every 248 days.

     // Do not make the MISTAKE of using just the tv_usec
     // This will mean your seed repeats every second.
 }