"Srand ((unsigned) time (NULL));" What the hell does that mean?

1. Random numbers are calculated from random seeds according to a certain calculation method. So, as long as the calculation method is certain, random seed is certain, then the random number that produces will not change.

#include 
using namespace std; 

int main () 
{ 
unsigned int seed=5; 
Srand (seed); 
unsigned int r=rand (); 
cout<<r<<endl;

return 0;

2, as long as the user or third party does not set random seed, then by default, random seeds from the system clock (that is, the Timer/counter value)

#include 
using namespace std; 

int main () 
{ 
srand (unsigned) time (NULL)); 
unsigned int r=rand (); 
cout<<r<<endl;

return 0; 

Here the user and other programs do not set random seeds, the value of the system Timer/counter is used as a random seed, so, in the same platform environment, after compiling the build exe, each time it is run, the random number displayed will be pseudo random number, that is, the results of each run display will be different.

3 . Suggestion: If you want to generate a sequence of random numbers in a program, you need to set up a random seed at most before generating a random number. that is, simply call Srand ((unsigned) time (NULL) at the beginning of the main program; You can use Rand directly behind it. Do not place Srand ((unsigned) time (NULL)) in a for-wait loop;

From: Srand ((unsigned) time (NULL)); -lizhi200404520 's Column-Blog channel-csdn.net
http://blog.csdn.net/lizhi200404520/article/details/6707885