How to count bits in a unsigned int?

Discussion in 'C Programming' started by tfeldman21@gmail.com, Feb 16, 2007.

1. Guest

On 32-bit platform,
I am working on getting how many bits equal to 1 without an if loop.

--
Regards.
Terrence Feldman
Email:

, Feb 16, 2007

2. =?utf-8?B?SGFyYWxkIHZhbiBExLNr?=Guest

wrote:
> On 32-bit platform,
> I am working on getting how many bits equal to 1 without an if loop.

Use a for loop or a while loop, testing each bit until you have no
more bits to test. I'm not sure what an if loop would be like, unless
you're using goto.

=?utf-8?B?SGFyYWxkIHZhbiBExLNr?=, Feb 16, 2007

3. Keith ThompsonGuest

writes:
> On 32-bit platform,
> I am working on getting how many bits equal to 1 without an if loop.

"if loop", so you won't have any trouble avoiding it.

The most important step in solving any problem is defining exactly
what the problem is. You haven't done that.

But questions of the form "How can I do X without using feature Y?"
are almost always homework assignments. We won't do your homework for
you (unless you're willing to pay our exhorbitant consulting fees
*and* let us submit our solutions directly to your instructor).

we can't do that unless you write some and post it.

--
Keith Thompson (The_Other_Keith) <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

Keith Thompson, Feb 16, 2007
4. CBFalconerGuest

Harald van D?k wrote:
> wrote:
>
>> On 32-bit platform, I am working on getting how many bits equal
>> to 1 without an if loop.

>
> Use a for loop or a while loop, testing each bit until you have
> no more bits to test. I'm not sure what an if loop would be like,
> unless you're using goto.

I have no idea what an 'if loop' is.

unsigned int value, count;

value = whatever;
count = 0;

/* loop invariant: count + bitsinvalue = bitsinwhatever */
while (value) {
count++;
value = (value - 1) & value;
}
/* bitsinvalue = 0 */

which doesn't care how wide an int is.

--
<http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfocus.com/columnists/423>

"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews

CBFalconer, Feb 16, 2007
5. Malcolm McLeanGuest

<> wrote in message n
> On 32-bit platform,
> I am working on getting how many bits equal to 1 without an if loop.
>

On the lowest bit can equal one. The rest equal 2, 4, 8 ... etc.

Tell your professor he means the number of set bits.

Malcolm McLean, Feb 16, 2007
6. =?utf-8?B?SGFyYWxkIHZhbiBExLNr?=Guest

CBFalconer wrote:
> Harald van D?k wrote:
> > wrote:
> >
> >> On 32-bit platform, I am working on getting how many bits equal
> >> to 1 without an if loop.

> >
> > Use a for loop or a while loop, testing each bit until you have
> > no more bits to test. I'm not sure what an if loop would be like,
> > unless you're using goto.

>
> I have no idea what an 'if loop' is.
> [example]
> which doesn't care how wide an int is.

And the way I mentioned does?

count = value < 0;
for (bit = 1; bit < INT_MAX / 2; bit *= 2)
if (value & bit)
count++;

It accepts ints of any width.

=?utf-8?B?SGFyYWxkIHZhbiBExLNr?=, Feb 16, 2007
7. =?utf-8?B?SGFyYWxkIHZhbiBExLNr?=Guest

Harald van DÄ³k wrote:
> CBFalconer wrote:
> > Harald van D?k wrote:
> > > wrote:
> > >
> > >> On 32-bit platform, I am working on getting how many bits equal
> > >> to 1 without an if loop.
> > >
> > > Use a for loop or a while loop, testing each bit until you have
> > > no more bits to test. I'm not sure what an if loop would be like,
> > > unless you're using goto.

> >
> > I have no idea what an 'if loop' is.
> > [example]
> > which doesn't care how wide an int is.

>
> And the way I mentioned does?
>
> count = value < 0;

count = 0;

> for (bit = 1; bit < INT_MAX / 2; bit *= 2)

for (bit = 1; bit != 0; bit *= 2)

> if (value & bit)
> count++;
>
> It accepts ints of any width.

The question asked for unsigned ints, but I posted an example for ints.

=?utf-8?B?SGFyYWxkIHZhbiBExLNr?=, Feb 16, 2007
8. =?utf-8?B?SGFyYWxkIHZhbiBExLNr?=Guest

Harald van DÄ³k wrote:
> Harald van DÄ³k wrote:
> > CBFalconer wrote:
> > > Harald van D?k wrote:
> > > > wrote:
> > > >
> > > >> On 32-bit platform, I am working on getting how many bits equal
> > > >> to 1 without an if loop.
> > > >
> > > > Use a for loop or a while loop, testing each bit until you have
> > > > no more bits to test. I'm not sure what an if loop would be like,
> > > > unless you're using goto.
> > >
> > > I have no idea what an 'if loop' is.
> > > [example]
> > > which doesn't care how wide an int is.

> >
> > And the way I mentioned does?
> >
> > count = value < 0;

>
> count = 0;
>
> > for (bit = 1; bit < INT_MAX / 2; bit *= 2)

>
> for (bit = 1; bit != 0; bit *= 2)
>
> > if (value & bit)
> > count++;
> >
> > It accepts ints of any width.

>
> The question asked for unsigned ints, but I posted an example for ints.

Sorry for replying to myself so much, but the example for ints was
wrong anyway. After the loop exited, I needed to check value & bit
once more, to see if the highest value bit was set.

=?utf-8?B?SGFyYWxkIHZhbiBExLNr?=, Feb 16, 2007
9. KsitamiGuest

static inline ulong bit_count(ulong x) {
// Return number of bits set
x = (0x55555555 & x) + (0x55555555 & (x>> 1)); // 0-2 in 2 bits
x = (0x33333333 & x) + (0x33333333 & (x>> 2)); // 0-4 in 4 bits
x = (0x0f0f0f0f & x) + (0x0f0f0f0f & (x>> 4)); // 0-8 in 8 bits
x = (0x00ff00ff & x) + (0x00ff00ff & (x>> 8)); // 0-16 in 16
bits
x = (0x0000ffff & x) + (0x0000ffff & (x>>16)); // 0-31 in 32
bits
return x;
}

//Algorithms for programmers ideas and source code
//by J¨org Arndt,

Ksitami, Feb 16, 2007
10. CoLGuest

You also try recursion for the same....

~col

On Feb 16, 3:03Â pm, "Harald van DÄ³k" <> wrote:
> CBFalconer wrote:
> > Harald van D?k wrote:
> > > wrote:

>
> > >> On 32-bit platform, I am working on getting how many bits equal
> > >> to 1 without an if loop.

>
> > > Use a for loop or a while loop, testing each bit until you have
> > > no more bits to test. I'm not sure what an if loop would be like,
> > > unless you're using goto.

>
> > I have no idea what an 'if loop' is.
> > [example]
> > which doesn't care how wide an int is.

>
> And the way I mentioned does?
>
> count = value < 0;
> for (bit = 1; bit < INT_MAX / 2; bit *= 2)
> Â  Â  if (value & bit)
> Â  Â  Â  Â  count++;
>
> It accepts ints of any width.

CoL, Feb 16, 2007
11. Christopher LayneGuest

CoL wrote:

> You also try recursion for the same....
>
> ~col

To count bits? You're not in the business of performance eh?

Christopher Layne, Feb 16, 2007
12. CBFalconerGuest

Harald van D?k wrote:
>
> CBFalconer wrote:
> > Harald van D?k wrote:
> > > wrote:
> > >
> > >> On 32-bit platform, I am working on getting how many bits equal
> > >> to 1 without an if loop.
> > >
> > > Use a for loop or a while loop, testing each bit until you have
> > > no more bits to test. I'm not sure what an if loop would be like,
> > > unless you're using goto.

> >
> > I have no idea what an 'if loop' is.
> > [example]
> > which doesn't care how wide an int is.

>
> And the way I mentioned does?
>
> count = value < 0;
> for (bit = 1; bit < INT_MAX / 2; bit *= 2)
> if (value & bit)
> count++;
>
> It accepts ints of any width.

But it needs to know the precise type. My version will work when
fed bytes, longs, shorts, etc. (with matching changes in the type
of value, but making value an unsigned long will cover them all,
except long long which may or may not be available).

I find code fragments that need no editing more useful.
--
<http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfocus.com/columnists/423>

"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews

CBFalconer, Feb 16, 2007
13. Christopher LayneGuest

CBFalconer wrote:

> But it needs to know the precise type. My version will work when
> fed bytes, longs, shorts, etc. (with matching changes in the type
> of value, but making value an unsigned long will cover them all,
> except long long which may or may not be available).
>
> I find code fragments that need no editing more useful.

What's wrong with the typical straightforward way?

int bs(unsigned long v)
{
int c;

for (c = 0; v; v >>= 1)
c += v & 1;

return c;
}

Christopher Layne, Feb 16, 2007
14. =?utf-8?B?SGFyYWxkIHZhbiBExLNr?=Guest

CBFalconer wrote:
> Harald van D?k wrote:
> >
> > CBFalconer wrote:
> > > Harald van D?k wrote:
> > > > wrote:
> > > >
> > > >> On 32-bit platform, I am working on getting how many bits equal
> > > >> to 1 without an if loop.
> > > >
> > > > Use a for loop or a while loop, testing each bit until you have
> > > > no more bits to test. I'm not sure what an if loop would be like,
> > > > unless you're using goto.
> > >
> > > I have no idea what an 'if loop' is.
> > > [example]
> > > which doesn't care how wide an int is.

> >
> > And the way I mentioned does?
> >
> > count = value < 0;
> > for (bit = 1; bit < INT_MAX / 2; bit *= 2)
> > if (value & bit)
> > count++;
> >
> > It accepts ints of any width.

>
> But it needs to know the precise type. My version will work when
> fed bytes, longs, shorts, etc. (with matching changes in the type
> of value, but making value an unsigned long will cover them all,
> except long long which may or may not be available).
>
> I find code fragments that need no editing more useful.

Your version will not work with signed types, and the version for
unsigned types I posted later does not need editing either, as long as
the type of 'bit' is large enough.

=?utf-8?B?SGFyYWxkIHZhbiBExLNr?=, Feb 16, 2007
15. CBFalconerGuest

Christopher Layne wrote:
> CBFalconer wrote:
>
>> But it needs to know the precise type. My version will work when
>> fed bytes, longs, shorts, etc. (with matching changes in the type
>> of value, but making value an unsigned long will cover them all,
>> except long long which may or may not be available).
>>
>> I find code fragments that need no editing more useful.

>
> What's wrong with the typical straightforward way?
>
> int bs(unsigned long v)
> {
> int c;
>
> for (c = 0; v; v >>= 1)
> c += v & 1;
> return c;
> }

Nothing that I can see. It will take more cycles, for each 0 bit
to the right of any 1 bit.

--
<http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfocus.com/columnists/423>

"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews

CBFalconer, Feb 16, 2007

17. peteGuest

wrote:
>
> On 32-bit platform,
> I am working on getting how many bits equal to 1 without an if loop.

unsigned bit_count(unsigned n)
{
unsigned count;

for (count = 0; n != 0; n &= n - 1) {
++count;
}
return count;
}

--
pete

pete, Feb 16, 2007
18. jxhGuest

On Feb 16, 5:01 am, Christopher Layne <> wrote:
> CoL wrote:
> > You also try recursion for the same....

>
> > ~col

>
> To count bits? You're not in the business of performance eh?

Should perform just as well with proper optimizations:

static int
bits_r (unsigned int u, int count)
{
return !u ? count : bits_r(u & u-1, count+1);
}

int
bits (unsigned int u)
{
return bits_r(u, 0);
}

-- James

jxh, Feb 16, 2007
19. dickGuest

CBFalconer wrote:
> Harald van D?k wrote:
> > wrote:
> >
> >> On 32-bit platform, I am working on getting how many bits equal
> >> to 1 without an if loop.

> >
> > Use a for loop or a while loop, testing each bit until you have
> > no more bits to test. I'm not sure what an if loop would be like,
> > unless you're using goto.

>
> I have no idea what an 'if loop' is.
>
> unsigned int value, count;
>
> value = whatever;
> count = 0;
>
> /* loop invariant: count + bitsinvalue = bitsinwhatever */
> while (value) {
> count++;
> value = (value - 1) & value;
> }
> /* bitsinvalue = 0 */
>
> which doesn't care how wide an int is.

this is an interview question.

i = 0;
while(var)
{
var &= ~(-var);
i++;
}

dick, Feb 17, 2007
20. CBFalconerGuest

dick wrote:
> CBFalconer wrote:
>> Harald van D?k wrote:
>>> wrote:
>>>
>>>> On 32-bit platform, I am working on getting how many bits equal
>>>> to 1 without an if loop.
>>>
>>> Use a for loop or a while loop, testing each bit until you have
>>> no more bits to test. I'm not sure what an if loop would be like,
>>> unless you're using goto.

>>
>> I have no idea what an 'if loop' is.
>>
>> unsigned int value, count;
>>
>> value = whatever;
>> count = 0;
>>
>> /* loop invariant: count + bitsinvalue = bitsinwhatever */
>> while (value) {
>> count++;
>> value = (value - 1) & value;
>> }
>> /* bitsinvalue = 0 */
>>
>> which doesn't care how wide an int is.

>
> this is an interview question.
>
> i = 0;
> while(var)
> {
> var &= ~(-var);
> i++;
> }

Doesn't work. Consider overflow (-var, for var == INT_MIN, 2'
complement), sign-magnitude representation, 1's complement
representation.

--
<http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfocus.com/columnists/423>

"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews

CBFalconer, Feb 17, 2007