Double/float to long integer

bastl · 06-03-2006, 08:24 AM

Yes that's a nice idea, Thank you. - Yes it works. - O.K. in my case it than should work with + 0.0001 because of 5.9999 see above. Yes it works. - now why does ceil() not work? Ups, it works... why not yesterday? I had a restart by turning on my computer this morning. Now I get exactly the results of - graemef, Yesterday, 07:41 PM -. But what was wrong: I did some tests:

Code:

 
#include <math.h> 
#include <fenv.h> 
void test( double d )
 {
 long int x;
 fesetround( FE_UPWARD ); // you have FE_TONEAREST, FE_DOWNWARD, FE_UPWARD, FE_TOWARDZERO on x87 FPUs
 x = long( d );
 KMessageBox::messageBox ( 0, KMessageBox::Information, QString("x = %1\nd = %2").arg(x).arg(d), "Info");
 x = floor( d );
 KMessageBox::messageBox ( 0, KMessageBox::Information, QString("x = %1\nd = %2").arg(x).arg(d), "Info");
 x = ceil( d );
 KMessageBox::messageBox ( 0, KMessageBox::Information, QString("x = %1\nd = %2").arg(x).arg(d), "Info");
 x = round( d );
 KMessageBox::messageBox ( 0, KMessageBox::Information, QString("x = %1\nd = %2").arg(x).arg(d), "Info");
 }

The result is, that fesetround() has no effect on round() in my case, but on int(), long(), floor(), ceil(). And with fesetround(FE_TONEAREST) it really don't did what it should do with 5.9999. In my case only fesetround(FE_UPWARD) has the behavior I wanted. I think it's the best thing to use round(). I thought round(), int() and long() does the same - no, they don't !
And sorry to all, that my answers came as late, but I did tests and reading in the www.
Now to you -graemef-. No, I only read that int() uses this FPU instruction, I use only int() and long() like in my function test() without inline things, so no assembler code. Only pure gcc code.

graemef · 06-03-2006, 08:47 AM

int() and long() are not functions as such but are type casts and so they essentially ignore the fractional part.

One observation is that you should check on the return from fesetround() non zero measn it failed.

lreason · 06-03-2006, 08:50 AM

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Quote:

Originally Posted by bastl

Yes that's a nice idea, Thank you.
- Yes it works.
- O.K. in my case it than should work with + 0.0001 because of 5.9999 see above. Yes it works.
- now why does ceil() not work? Ups, it works... why not yesterday?
I had a restart by turning on my computer this morning.
Now I get exactly the results of - graemef, Yesterday, 07:41 PM -.

But what was wrong:
I did some tests:

Code:

#include <math.h>
#include <fenv.h>

void test( double d )
  {
  long int x;

  fesetround( FE_UPWARD ); // you have FE_TONEAREST, FE_DOWNWARD, FE_UPWARD, FE_TOWARDZERO on x87 FPUs
  
  x = long( d );
  KMessageBox::messageBox ( 0, KMessageBox::Information, QString("x =  %1\nd =  %2").arg(x).arg(d), "Info");
  x = floor( d );
  KMessageBox::messageBox ( 0, KMessageBox::Information, QString("x =  %1\nd =  %2").arg(x).arg(d), "Info");
  x = ceil( d );
  KMessageBox::messageBox ( 0, KMessageBox::Information, QString("x =  %1\nd =  %2").arg(x).arg(d), "Info");
  x = round( d );
  KMessageBox::messageBox ( 0, KMessageBox::Information, QString("x =  %1\nd =  %2").arg(x).arg(d), "Info");
  }

The result is, that fesetround() has no effect on round() in my case, but on int(), long(), floor(), ceil(). And with fesetround(FE_TONEAREST) it really don't did what it should do with 5.9999. In my case only fesetround(FE_UPWARD) has the behavior I wanted.
I think it's the best thing to use round().
I thought round(), int() and long() does the same - no, they don't !

bastl · 06-03-2006, 09:24 AM

I observed now the function fesetround(), it gives back always 0.
And the behavior of int() and long() changes on executing fesetround() with different values.
But it seems that only the binary deviation of the floating point numbers are handled with.