TLatex affects random number generator after TCanvas::Print

ROOT
1

I’ve come across a very strange problem involving TLatex, which affects the standard C++ random number generator. (I’m using Root 5.34.32).

When drawing a TLatex object to a canvas, and then printing that canvas, the state of the C++ random number generator is somehow affected. Here is a minimal program to demonstrate what happens:

#include <iostream>
#include "TCanvas.h"
#include "TLatex.h"

using namespace std;

int main()
{
  for(int i=0;i<10;++i){
    cout << rand() << endl;
  }
  TCanvas c("c","",100,100);
  TLatex l(0,0,"hello");
  l.Draw();
  c.Print("tst.png","png");
  cout << "-" << endl;
  for(int i=0;i<10;++i){
    cout << rand() << endl;
  }

  return(0);
}

If I run that multiple times, I get different random numbers in the second lot of 10 each time I run the program.

This behaviour only seems to occur for some print options (I’ve tested PDF, JPG and PNG files). If print out a root macro (.C), the sequence of random numbers is not affected.

What is going on here?

2

That sounds really weird. Have you tried to use the random number generator ROOT provide ?
I also see that you do not set the seed using srand…

3

Hi,

I filled a bug report:
sft.its.cern.ch/jira/browse/ROOT-7461

It seems to be fine w/ the current major version of ROOT, i.e. ROOT 6. It is fine as well when using ROOT in batch mode. Maybe that helps circumventing your problem for now.

Cheers,
Benedikt

4

I tried your example on Mac


{
   for (int i=0;i<10;++i) cout << rand() << endl;
////
//  TCanvas c("c","",100,100);
//   TLatex l(0,0,"hello");
//   l.Draw();
//   c.Print("tst.png","png");
/// 
   cout << "-" << endl;
   for (int i=0;i<10;++i) cout << rand() << endl;
}

Without or with the graphics commands between the 2 loops I get the same output with all ROOT versions.

16807
282475249
1622650073
984943658
1144108930
470211272
101027544
1457850878
1458777923
2007237709
-
823564440
1115438165
1784484492
74243042
114807987
1137522503
1441282327
16531729
823378840
143542612
5

Not sure if this helps, but there is no guarantee that rand() isn’t itself called within your C++ standard library between your explicit calls to it:

http://en.cppreference.com/w/cpp/numeric/random/rand

From my reading of that page, if you require reproducible random numbers with library calls in between, you can’t rely on std::rand(). There are other options that have better guarantees: http://en.cppreference.com/w/cpp/numeric/random

Disclaimer: This is just after a few minutes of googling about this problem, I am not an official C++ expert. = )
Jean-François