RenderWindow object breaks srand

[dunaevai135]

When

srand((unsigned)time(NULL));

called before RenderWindow class object creation, rand() return same value, like srand() initialize by const var.
If i will put srand after RenderWindow creation, everything OK.
see attached screenshot

I'm confused that the example "examples/pong/Pong.cpp" uses a similar code:
https://github.com/SFML/SFML/blob/fae3b65f0567f87fa9925cd42d28df15eb69e79c/examples/pong/Pong.cpp
srand before RenderWindow

P.S. sorry for my English, it's my second language.

code and compile parameters:

(click to show/hide)

#include <SFML/Graphics.hpp>
#include <iostream>
#include <time.h>
using namespace sf;

int main()
{
        VideoMode vdm = VideoMode(400, 533);

        srand((unsigned)time(NULL));

        RenderWindow app(vdm, "Game!");

        for (int i=0;i<10;i++)
        {
                std::cout << rand()%100 << " " << rand()%100 << "\n";
        }
        return 0;
}

// g++ main2.cpp -lsfml-graphics -lsfml-window -lsfml-system -std=c++11

[Turbine]

I don't think you can use srand in a loop. Non-SFML related.

http://stackoverflow.com/questions/4926622/how-to-generate-different-random-numbers-in-a-loop-in-c

[dunaevai135]

see screenshot
srand out of loop before RenderWindow initialise
it only once, at the start of main()
If i will put srand after RenderWindow creation, everything OK.

[Mario]

I don't think you can use srand in a loop. Non-SFML related.

http://stackoverflow.com/questions/4926622/how-to-generate-different-random-numbers-in-a-loop-in-c

You can. The only problem is, time() has limited resolution, so if you loop fast enough, you'll start with the same seed for several iterations, therefore returning the samepseudo-random number every time for a while.

As for initial problem: Not sure how SFML would influence srand()/rand().

Edit:

Just tried the code and I get random/different numbers every time I run the example snippet. Everything fine. Could you also print your seed, i.e. the return value of time()? I have the suspicion there's something off with your system for whatever reason.

[dunaevai135]

As for me, it looks like RenderWindow object initializes srand with one const number

[Mario]

Which version of SFML are you running? There's no call to "srand()" in SFML's source.

Edit:
Also, what happens if you create a second sf::RenderWindow and run the loop again (without calling srand() a second time)?

[dunaevai135]

I tried the latest stable version
code:

(click to show/hide)

#include "SFML-2.4.2/include/SFML/Graphics.hpp"
#include <iostream>
#include <time.h>
using namespace sf;

int main()
{
        VideoMode vdm = VideoMode(400, 533);

        unsigned int init_time = time(NULL);
        srand(init_time);
        std::cout << init_time << "\n";

        RenderWindow app(vdm, "Game!");

        for (int i=0;i<5;i++)
        {
                std::cout << rand()%100 << " " << rand()%100 << "\n";
        }
        return 0;
}

// g++ main2.cpp -lsfml-graphics -lsfml-window -lsfml-system -std=c++11

out:

(click to show/hide)

1491746044
87 83
25 80
71 54
75 87
54 20
$ ./a.out
1491746045
87 83
25 80
71 54
75 87
54 20
$ ./a.out
1491746048
87 83
25 80
71 54
75 87
54 20

the same problem
putting srand() after RenderWindow app(vdm, "Game!"); repairs for me.

It can be bag?

$ uname -a
Linux dunaev-desktop 4.4.0-71-generic #92-Ubuntu SMP Fri Mar 24 12:59:01 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
 

What else about the system to provide?

[dunaevai135]

It really strange
see :

code:

(click to show/hide)

#include "SFML-2.4.2/include/SFML/Graphics.hpp"
#include <iostream>
#include <time.h>
using namespace sf;

int main()
{
        VideoMode vdm = VideoMode(400, 533);

        unsigned int init_time = time(NULL);
        std::cout << init_time << "\n";
        srand(init_time);
       
        for (int i=0;i<5;i++)
        {
                std::cout << rand()%100 << " " << rand()%100 << "\n";
        }
        std::cout << "\n";
       
        RenderWindow app(vdm, "Game!");

        for (int i=0;i<5;i++)
        {
                std::cout << rand()%100 << " " << rand()%100 << "\n";
        }
        return 0;
}

// g++ main2.cpp -lsfml-graphics -lsfml-window -lsfml-system -std=c++11
 

out:

(click to show/hide)

./a.out
1491746597
64 56
79 33
16 81
8 17
53 6

87 83
25 80
71 54
75 87
54 20
$ ./a.out
1491746599
62 63
94 82
31 56
12 90
35 64

87 83
25 80
71 54
75 87
54 20
$ ./a.out
1491746647
73 15
67 39
69 95
3 75
63 76

87 83
25 80
71 54
75 87
54 20
$
 

like

RenderWindow app(vdm, "Game!");

reinitializes srand()

[dunaevai135]

Which version of SFML are you running? There's no call to "srand()" in SFML's source.

Edit:
Also, what happens if you create a second sf::RenderWindow and run the loop again (without calling srand() a second time)?

(click to show/hide)

#include "SFML-2.4.2/include/SFML/Graphics.hpp"
#include <iostream>
#include <time.h>
using namespace sf;

int main()
{
        VideoMode vdm = VideoMode(400, 533);

        unsigned int init_time = time(NULL);
        std::cout << init_time << "\n";
        srand(init_time);
       
        RenderWindow app(vdm, "Game!");

        for (int i=0;i<5;i++)
        {
                std::cout << rand()%100 << " " << rand()%100 << "\n";
        }

        RenderWindow app2(vdm, "Game!");

        std::cout << "\n";
        for (int i=0;i<5;i++)
        {
                std::cout << rand()%100 << " " << rand()%100 << "\n";
        }
        return 0;
}
 

out:

(click to show/hide)

$ ./a.out
1491747014
87 83
25 80
71 54
75 87
54 20

73 57
8 64
90 79
61 86
93 34
$ ./a.out
1491747015
87 83
25 80
71 54
75 87
54 20

73 57
8 64
90 79
61 86
93 34
$ ./a.out
1491747016
87 83
25 80
71 54
75 87
54 20

73 57
8 64
90 79
61 86
93 34

 

[eXpl0it3r]

How about you forget srand and rand and use some proper RNG in the <random> header introduced with C++11.

https://channel9.msdn.com/Events/GoingNative/2013/rand-Considered-Harmful

[Laurent]

As for me, it looks like RenderWindow object initializes srand with one const number

A quick search in source code of master shows that the only call to srand is in the Pong example. So if SFML does something, it's hidden in one of its dependencies.

I really suspect something in your environment. Definitely not SFML itself.