I need help

[Day Vide]

Why is this giving me 25 times the same number and not all random ?

#include <iostream>
#include <windows.h>
#include <cstdlib>
#include <ctime>

using namespace std;

int randomNumber(int a);

int main(){


    int num = randomNumber(4);

    for(int x = 0; x < 25; x++){

        num = randomNumber(4);
        cout << num << endl;

    }


}

int randomNumber(int a){

    srand(time(0));

    int num = rand()%a;

    return num;


}

[AbstractionAnon]

Line 28: Do not call srand() within a loop or a random number function. srand() sets the RNG to a particular starting point. Calling srand() repeatedly can cause the RNG to return the same random numbers. srand() should be called ONCE at the beginning of main().
http://www.cplusplus.com/reference/cstdlib/srand/

Move line 28 to line 13.

[Day Vide]

Oh thanks man lol

[ne555]

man wrote:

The srand() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned by rand(). These sequences are repeatable by calling srand() with the same seed value.

[E0p9LyTq]

You seed the pseudo random number generator each time you call your function, you reset the generator within a very short period of time, ending up with the same sequence of un-random numbers.

#include <iostream>
// #include <windows.h> // not needed for this!
#include <cstdlib>
#include <ctime>

int randomNumber(int a);

int main()
{
   // seed the random generator ONCE
   srand(time(0));

   int num = randomNumber(4);

   for(int x = 0; x < 25; x++)
   {
      num = randomNumber(4);

      std::cout << num << "  ";
   }
   std::cout << "\n";
}


int randomNumber(int a)
{
   int num = rand() % a;

   return num;
}

The C library random functions are inefficient, have horrible distribution and grossly outdated. C++ has much better random number generation utilities in <random>:

#include <iostream>
#include <chrono>
#include <random>

int main()
{
   // obtain a seed from the system clock:
   unsigned seed = static_cast<unsigned> (std::chrono::system_clock::now().time_since_epoch().count());

   // seeds the random number engine, the standard default random engine
   std::default_random_engine generator(seed);

   // discard some values for better randomization
   generator.discard(generator.state_size);

   // set a distribution range (0 - 3)
   std::uniform_int_distribution<int> distribution(0, 3);

   for (int loop = 0; loop < 25; loop++)
   {
      // pick and store the random number
      int numPicked = distribution(generator);

      std::cout << numPicked << "  ";
   }
   std::cout << "\n";
}

Even the C standard (footnote 295) recommends NOT using rand():

There are no guarantees as to the quality of the random sequence produced and some implementations are known to produce sequences with distressingly non-random low-order bits. Applications with particular requirements should use a generator that is known to be sufficient [read: not this one] for their needs. (emphasis added)

http://cpp.indi.frih.net/blog/2014/12/the-bell-has-tolled-for-rand/
http://www.gztscf.com/0716/c-srand-does-not-give-same-sequences-of-random-numbers/