Subj : Serious problems with std::modf()
To   : borland.public.cpp.borlandcpp
From : hall
Date : Sat Feb 12 2005 04:37 pm

Hi group.
I have a serious problem concerning the function std::modf() from the 
<cmath> header. I get the problem when compiling in BCB 6 Pro, but not 
in g++ under Unix. The program looks as:

//---------------------------------------------------------------------------
#pragma hdrstop
#include <cmath>
#include <iostream>
#pragma argsused

int main(int argc, char* argv[])
{
    for (int i = 0; i<5; ++i)
    {
       float dummy;
       float f = std::modf (1.33, &dummy);
       std::cout << " i =" << i << std::endl;
    }
    return 0;
}
//---------------------------------------------------------------------------

Simple enough, i would expect that the program would output

----
  i =0
  i =1
  i =2
  i =3
  i =4
---

to the screen, but it doesn't, instead it outputs

---
  i =1072693248
---

Can anyone here explain why? Debugging the program shows that the value 
of i changes form 0 to 1072693248 when modf() is executed. I can make 
the program act as expected by doing any of the following:

1) changing the header from <cmath> to the (non-standard?) header <math>
2) changing the type of 'dummy' to double
3) conmmenting out the line containing std::modf()
and finaly
4) compiling the program as it is in g++ under linux

What have I done wrong here? And how can a function call to modf() 
change the value of the variable i?

Best regards.
andreas

.