# Re: using MFC VC++ - which is more efficient - float or double?

From:
=?Utf-8?B?SlJHbGlkZQ==?= <JRGlide@discussions.microsoft.com>
Newsgroups:
microsoft.public.vc.mfc
Date:
Wed, 22 Jul 2009 13:58:02 -0700
Message-ID:
<82E1CFF7-238F-4B1E-A997-6BFA253D85D2@microsoft.com>
I guess you are right about the speed. I just did my own test and then
I was right about one thing - multiplies are still much faster than divides.
I did a loop of 2 billion calculations. The divides took about 24.9 seconds
while the multiplies took 3.9 seconds. Which is why you are better off to do

a = b * 0.5;
rather than
a = b/2.0;

If speed is important, that is. For whatever its worth, here's the code I
used. Just run it without optimizations or it takes zero seconds!

#include <stdio.h>
#include "Timer.h"

#define CYCLES 2000000000

int main ()
{
CTimer timer;

float a,b,c;
double x,y,z;
int i;

a = 2.0F;
b = 3.0F;
x = 2.0;
y = 3.0;

timer.startTimer();
for (i=0; i<CYCLES; i++)
c = a/b;

printf ("Float Time: %f Seconds\n", timer.stopTimer());

timer.startTimer();
for (i=0; i<CYCLES; i++)
z = x/y;

printf ("Double Time: %f Seconds\n", timer.stopTimer());
return 1;
}

#include <sys\timeb.h>

class CTimer
{
public:

void startTimer (void)
{
_ftime (&t1);
}

float stopTimer (void)
{
float delta;

_ftime (&t2);

delta = ( (float) (t2.time) + (float) (t2.millitm) / 1000.0F) -
( (float) (t1.time) + (float) (t1.millitm) / 1000.0F);

return delta;
}

private:

struct _timeb t1, t2;
};

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