Re: strange behavior of random numbers with gtkmm

[Alexey Savartsov <asavartsov gmail com>]

The problem is that you always calling srand with same seed parameter. So you always get same random sequence.

Usually, srand called with the current time value like:

srand(time(0)); 

May be you need type casting of time() return value - follow compiler warnings there.

On 08.06.2010, at 13:27, jody wrote:

> Hi
> I've found a strange effect in gtkmm programs involving random numbers:
> for a given seed not always produce the same sequence is produced.
> 
> I wrote a test program which shows this. There are two buttons in the window,
> 'reset' to set the random seed to a constant value, and 'step' to get a random
> number and print it.
> The reset function is called in the constructor.
> 
> This is the output i get when i run the program and do the following:
> - press the step-button 3 times
> - press the reset-button
> - press the step-button 3 times
> - press the reset-button
> - press the step-button 3 times
> 
> [jody localhost rtest]$ ./gtkrand
> Reset
> One step: 1851975387
> One step: 1789185830
> One step: 545709798
> Reset
> One step: 1889402444
> One step: 1202690506
> One step: 785052894
> Reset
> One step: 1889402444
> One step: 1202690506
> One step: 785052894
> 
> So the first time, the sequence is different, all following ones are
> always the same.
> 
> If i do not call srand in the constructor but instead start the
> sequence with the reset-button,
> the first sequence of random numbers is the same as the following.
> 
> So my question is: does gtkmm call random numbers somewhere in its code?
> If yes, at what points?
> If no, how can this strange behaviour be explained?
> 
> Thank You
>  Jody
> 
> 
> Here is the code
> 
> ///// gtkrandwin.h ------------------------------------------------
> #ifndef __GTKRANDWIN_H__
> #define __GTKRANDWIN_H__
> 
> #include <gtkmm/window.h>
> #include <gtkmm/button.h>
> #include <gtkmm/box.h>
> 
> class gtkrandwin : public Gtk::Window {
> public:
>    gtkrandwin();
> 
> protected:
> 
>    Gtk::HBox    m_HBox0;
>    Gtk::Button  m_butReset;
>    Gtk::Button  m_butStep;
>    virtual void reset_action();
>    virtual void step_action();
> 
> };
> 
> #endif
> 
> 
> ///// gtkrandwin.cpp ------------------------------------------------
> #include <gtkmm/window.h>
> #include <gtkmm/button.h>
> #include <gtkmm/box.h>
> 
> #include "gtkrandwin.h"
> 
> #define CAPTION_RESET  "Reset"
> #define CAPTION_STEP   "Step"
> 
> //-------------------------------
> // constructor
> //
> gtkrandwin::gtkrandwin()
>    : m_HBox0(false, 5),
>      m_butReset(CAPTION_RESET),
>      m_butStep(CAPTION_STEP) {
> 
> 
>    m_butStep.signal_clicked().connect(sigc::mem_fun(*this,
> &gtkrandwin::step_action) );
>    m_butReset.signal_clicked().connect(sigc::mem_fun(*this,
> &gtkrandwin::reset_action) );
> 
>    m_HBox0.pack_start(m_butReset,   Gtk::PACK_EXPAND_WIDGET,5);
>    m_HBox0.pack_start(m_butStep,   Gtk::PACK_EXPAND_WIDGET,5);
>    add(m_HBox0);
> 
>    reset_action();
>    show_all_children();
> }
> 
> 
> //-----------------------------------------------------------------------------
> // step_action
> //   callback for reset action
> //
> void gtkrandwin::reset_action() {
>    // one step
>    srand(12343271);
>    printf("Reset\n");
> }
> 
> //-----------------------------------------------------------------------------
> // step_action
> //   callback for reset action
> //
> void gtkrandwin::step_action() {
>    // one step
>    printf("One step: %d\n", rand());
> }
> 
> 
> ///// gtkrand.cpp ------------------------------------------------
> #include <glibmm.h>
> #include <gtkmm/main.h>
> 
> #include "gtkrandwin.h"
> 
> 
> int main(int iArgC, char *apArgV[]) {
>    Gtk::Main kit(iArgC, apArgV);
> 
>    gtkrandwin m_randwin;
> 
>    Gtk::Main::run(m_randwin);
> 
>    return 0;
> }
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