Rand function generates pseudo-random numbers

call rand() Before the function, you need to call srand function

srand The function sets the starting point for generating a series of pseudo-random integers .

To reinitialize the generator , Need to be for srand Provide a random seed parameter .

The value of the seed will set the generator to a random starting point .

We use time stamps as seeds -- srand((unsigned int)time(NULL))

rand Retrieves the generated pseudo-random number .

Show the phenomenon through the results ( To compare a series of codes ):

（1）

Once you pass the random seed ( Time stamp ) Set it up srand The starting point , Will pass through rand Generate a series of real pseudo-random numbers

Note that random seed setting is used here srand The starting point is not called ！！ If the concept here is vague , Skip first and look back , You can understand .

// True pseudorandom 
int main()
{
    srand(time(NULL));
    while (1)
    {
        int num1 = rand();
        int num2 = rand();
        printf("%d %d\n", num1, num2);
    }
    return 0;
}

 （2）

If you set it by calling multiple times srand The starting point , And each setting time is less than 1s, So here 1s Inside , rand The function generates the same value , Until the next second . Summed up in ：srand Starting point every 1s Reset it again

// Error model 1
int main()
{
    while (1)
    {
        srand(time(NULL));
        int num1 = rand();
        int num2 = rand();
        printf("%d %d\n", num1, num2);
    }
    return 0;
}

（3）

Set with a fixed value as the seed srand The starting point ,rand The generated value will never change .

 Error model 2：
int main()
{
    while (1)
    {
        srand(1);
        int num1 = rand();
        int num2 = rand();
        printf("%d %d\n", num1, num2);
    }
    return 0;
}

（4）

Here are the results of two runs , It's not hard to see. , A single time seems random , Each actual run rand The generated values correspond to equal The reason is that it is not used srand Set the starting point , The default starting point is seed 1 Of srand The starting point .

 Error model 3：
int main()
{
    while (1)
    {
        int num1 = rand();
        int num2 = rand();
        printf("%d %d\n", num1, num2);
    }
    return 0;
}

  summary

Level co., LTD. , Let's explain it in a more popular way .

And temporarily understood as ：

1.

Provide to srand If the seed is fixed ,rand Never change .

2.

Using timestamp as seed , stay 1s Many times in Set by calling srand The starting point , actually srand every other 1s can By calling Change the starting point ,

Equivalent to the 1s The seed value is not changed in , Lead to 1s Set the starting point of the same seed multiple times , So the result is similar to 1 identical ,rand stay 1s Never change

3.

Using timestamp as seed , Set it up srand After the starting point , stay 1s Do not change the starting point or in 1s Then change the starting point ,rand The generated value is always a random value

4.

If not provided srand The starting point ,rand Although each generated value seems random , But in fact, when the program is run many times , It is found that after each run , The generated values correspond to each other and are equal .

5.

Using timestamp as seed ,srand Every time Recall settings The starting interval of is 1s！ Attention is to recall ！ But once the starting point is set , Each generation srand The starting interval is very small , I don't know the exact value, but it must be less than 1s.

Be careful ： The most confusing point is ！

By calling srand Function to set srand The starting point ,

Or through srand Set the starting point by changing the seed parameters of the function itself .