# float and 1's

Discussion in 'C Programming' started by Vijay Kumar R Zanvar, Jan 2, 2004.

1. ### Vijay Kumar R ZanvarGuest

Hi,
Just like counting the number of bits set to
1 in an integral variable, can we count the same
in a float?

I have two solutions in my mind:

1.
union {
float f;
int i;
}u = { 10 };

Now, count the required number from u.i.
Will this work?

2.
float f;
unsigned char uc[sizeof (float)];

memcpy ( uc, &f, sizeof (float) );

Season's Greetings!

--
Vijay Kumar R Zanvar

Vijay Kumar R Zanvar, Jan 2, 2004

2. ### lallousGuest

"Vijay Kumar R Zanvar" <> wrote in message
news:bt3kt1\$30ugj\$-berlin.de...
> Hi,
> Just like counting the number of bits set to
> 1 in an integral variable, can we count the same
> in a float?
>
> I have two solutions in my mind:
>
> 1.
> union {
> float f;
> int i;
> }u = { 10 };
>
> Now, count the required number from u.i.
> Will this work?
>
>
> 2.
> float f;
> unsigned char uc[sizeof (float)];
>
> memcpy ( uc, &f, sizeof (float) );
>
>
>
> Season's Greetings!
>
> --
> Vijay Kumar R Zanvar
>
>

Hello,

It is enough to declare the float then take a char* to it as:
float f;
char *p = &f;
// now do the bit counting in 'p' till sizeof(f) , making at most
sizeof(f)*8 bits

--
Elias

lallous, Jan 2, 2004

3. ### Thomas StegenGuest

lallous wrote:

> Hello,
>
> It is enough to declare the float then take a char* to it as:
> float f;
> char *p = &f;

You need an explicit cast here. And it should be unsigned char*
probably.

unsigned char *p = (unsigned char *)&f;

> // now do the bit counting in 'p' till sizeof(f) , making at most
> sizeof(f)*8 bits

Note that this will also count any padding bits if there are any.

--
Thomas.

Thomas Stegen, Jan 2, 2004
4. ### Keith ThompsonGuest

"Vijay Kumar R Zanvar" <> writes:
> Just like counting the number of bits set to
> 1 in an integral variable, can we count the same
> in a float?
>
> I have two solutions in my mind:
>
> 1.
> union {
> float f;
> int i;
> }u = { 10 };
>
> Now, count the required number from u.i.
> Will this work?

Not reliably. Note that float and int may or may not be the same
size. (As a matter of style, I'd use "10.0", or even "10.0f", rather
than "10" in the initialization to make it clear that you're
initializing the float member.)

> 2.
> float f;
> unsigned char uc[sizeof (float)];
>
> memcpy ( uc, &f, sizeof (float) );
>

Yes, that should work, though others have pointed out that you don't
really need to copy f to an array.

BTW, I can't think of any use for the number of 1 bits in a float
other than idle curiosity -- not that there's anything wrong with idle
curiosity.

--
Keith Thompson (The_Other_Keith) <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://www.sdsc.edu/~kst>
Schroedinger does Shakespeare: "To be *and* not to be"

Keith Thompson, Jan 2, 2004
5. ### Jack KleinGuest

On Fri, 2 Jan 2004 13:49:15 +0200, "lallous" <> wrote
in comp.lang.c:

> "Vijay Kumar R Zanvar" <> wrote in message
> news:bt3kt1\$30ugj\$-berlin.de...
> > Hi,
> > Just like counting the number of bits set to
> > 1 in an integral variable, can we count the same
> > in a float?
> >
> > I have two solutions in my mind:
> >
> > 1.
> > union {
> > float f;
> > int i;
> > }u = { 10 };
> >
> > Now, count the required number from u.i.
> > Will this work?
> >
> >
> > 2.
> > float f;
> > unsigned char uc[sizeof (float)];
> >
> > memcpy ( uc, &f, sizeof (float) );
> >
> >
> >
> > Season's Greetings!
> >
> > --
> > Vijay Kumar R Zanvar
> >
> >

> Hello,
>
> It is enough to declare the float then take a char* to it as:
> float f;
> char *p = &f;
> // now do the bit counting in 'p' till sizeof(f) , making at most
> sizeof(f)*8 bits

Aside from what others have said, your very last sentence is
completely wrong. I am working on a platform right now where CHAR_BIT
is 16. sizeof(float) is 2, and a float contains 32 bits.

It is definitely possible for the binary representation of a float to
have more than sizeof(float)*8 1 bits. In fact it could have as many
as sizeof(float)*16 1 bits.

But it will never, ever have more than sizeof(float)*CHAR_BIT 1 bits.

--
Jack Klein
Home: http://JK-Technology.Com
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Jack Klein, Jan 2, 2004
6. ### Vijay Kumar R ZanvarGuest

[..]

>
> BTW, I can't think of any use for the number of 1 bits in a float
> other than idle curiosity -- not that there's anything wrong with idle
> curiosity.
>

You are right. It was only my curiosity.

Thanks
vijay-z.

Vijay Kumar R Zanvar, Jan 4, 2004