first rand() generates the same value after srand(time(0))

Question

srand(time(0)) does not seem to be seeding rand() correctly, the 1st number generated is always the same.

This is running on OS X, is there something wrong with my settings or my code? I do wait a couple of seconds to rerun the program. The 1st number increments only a day later, the rest of the numbers are always random.

#include<iostream>
using namespace std;

int main ()
{
    int Num1,Num2;

    // seed the random number generator

    srand((unsigned int )time(NULL));

    Num1 = rand() %49 +1 ;

    do {Num2 = rand() % 49 + 1;}
    while(Num2 == Num1);

    ///Display Random Numbers

    cout<<"Numbers are: "<<endl;
    cout<<Num1<<" "<<Num2<<endl<<endl;

    return 0;
}

Can't spot anything Xcode specific in your question. Removed the tag. — πάντα ῥεῖ, Dec 04 '14 at 18:25
How long are you waiting between runs of the program? I get different numbers on Windows as long as I wait at least a second between runs. — nobody, Dec 04 '14 at 18:27
You should include stdlib.h or cstdlib and time.h or ctime to make sure the prototypes for the functions you're using are visible. — Retired Ninja, Dec 04 '14 at 18:28
No repro on Linux, after including cstdlib to pick up the missing declarations for rand() and srand(). — Jim Lewis, Dec 04 '14 at 18:29
I can confirm that on my Mac the first number is always 29 no matter what the seed value is. A good reason to use `` instead. — Retired Ninja, Dec 04 '14 at 18:39
I've tried to include cstlib, time.h, ctime and cstdlib. No change. — Visions23, Dec 04 '14 at 18:45
I do wait a couple of seconds to rerun the program. The 1st number increments only a day later, the rest of the numbers are random as always. — Visions23, Dec 04 '14 at 18:47
related: [Q: Rand() % 14 only generates the values 6 or 13](http://stackoverflow.com/q/20263187/457406) — Matthias Bauch, Dec 04 '14 at 18:56

score 3 · Accepted Answer · answered Dec 04 '14 at 19:47

3

You are getting the same numbers because the seeds are so close in value and the random generator hasn't had time to diverge yet. On OS X use sranddev() instead of srand(), it will give you a better seed and avoid this problem.

#include<iostream>
using namespace std;

int main()
{
  int a, b;

  // seed the random number generator
  sranddev();

  a = rand();
  b = rand();

  cout << a << " " << (a % 49) + 1 << endl
       << b << " " << (b % 49) + 1 << endl << endl;

  return 0;
}

answered Dec 04 '14 at 19:47

Variable Length Coder

7,958
2
25
29

sranddev() did the trick. Many thanks. I'l read up on the differences between srand and sranddev on OX. – Visions23 Dec 04 '14 at 19:52
2

On most modern POSIX systems `arc4random_uniform()` should be go to random generator, as it avoids both seeding and modulus bias. Only reason not to use it is if you need the higher performance of the lower quality generators. – Jon Shier Dec 04 '14 at 20:12

score 0 · Answer 2 · answered Dec 04 '14 at 19:00

Assuming you have access to C++11, you would have better luck with something like this:

#include <iostream>
#include <random>

int main()
{
    std::random_device rd;
    std::mt19937 gen(rd());
    std::uniform_int_distribution<> dis(1, 50);
    std::cout << "Numbers are: " << dis(gen) << " " << dis(gen) << "\n";
    return 0;
}

rand is typically a pretty poor random generator with limited range of values and using % N to limit the values makes it even worse. There's a presentation about why rand isn't a good choice here: http://channel9.msdn.com/Events/GoingNative/2013/rand-Considered-Harmful

Thank you for the responses so far, I'm still learning C++, (Information Systems Undergrad), i ran the code on an online compiler and it works, i'l try on codeblocks but the problem seems to be with xcode. — Visions23, Dec 04 '14 at 19:21

score 0 · Answer 3 · answered Dec 04 '14 at 20:08

0

Besides using sranddev() instead of rand(). Increasing the range also form %49 to 50 or higher also seemed to work.

answered Dec 04 '14 at 20:08

Visions23

85
7

first rand() generates the same value after srand(time(0))

3 Answers3

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