C++ problem random

[Kingrap]

Hi community,
i have a small problem with the random function in the thread!

int rand_func()
{
    srand((unsigned int)time(NULL));
    int output = rand()%3;
    return output;
}

void test(void *id)
{
   int n = rand_func();
}

i opened two threads with _beginthread() but n of first thread is equal to n of second thread ..

help me please, i need a random number from 0 to 3..
the second n should not be equal to the first n!

if possible I would like to know the reason

sorry for my english *-*

[Schlüsselbein]

You should call srand only once (for instance in your main routine).

[Raz9r]

Originally Posted by Schlüsselbein

You should call srand only once (for instance in your main-routine).

Also, this is rather C than C++. Note that there exists no language named "C/C++" for a reason.
What about the headers <random> and, not to forget, <thread>?

[MrSm!th]

Btw. did you try the code more than once? That range is rather small, it could be just an accident that both threads got the same result.

[Kingrap]

i have solved the problem thank

[Schlüsselbein]

Btw. did you try the code more than once? That range is rather small, it could be just an accident that both threads got the same result.

I dont think so.
For example:

#include <iostream>
#include <ctime>
#include <vector>

int main()
{
	const unsigned int length = 500000;

	std::vector<unsigned int> v;
	v.reserve(length);

	for(int i = 0; i < length; ++i)
		v.push_back(static_cast<unsigned int>(::time(NULL)));

	std::cout << "Element 0: \t" << v.front() << std::endl;
	std::cout << "Element " << length << ": \t" << v.back() << std::endl;
}

For me with my rather slow pc (1,9ghz Dual Core processor) i need up to 500k iterations in debug mode before i get t0 + 1 second.
Therefore he seeds the random number generator with the same seed -> same result.

[jomergallano]

you can have some other codes
here :