A C Question.

[Jack Young]

Hi:
There is question about C programming:
Use just one C statement to check whether a variable is the power of 2. No
loops allowed.
Waiting for your answer.I have not slept well a few days for this.

[Eric Sosman]

Re: A C Question.

Jack Young wrote:[color=blue]
> Hi:
> There is question about C programming:
> Use just one C statement to check whether a variable is the power of 2. No
> loops allowed.
> Waiting for your answer.I have not slept well a few days for this.[/color]

int is_power_of_two (double x) {
return x > 0.0; /* one statement, no loops */
}

Sweet dreams!

--
Eric Sosman
esosman@acm-dot-org.invalid

[jmazzi]

Re: A C Question.

if ( var % 2 == 0 ) {
printf("yep\n") ;
}

[Robert Gamble]

Re: A C Question.

Jack Young wrote:[color=blue]
> Hi:
> There is question about C programming:
> Use just one C statement to check whether a variable is the power of 2. No
> loops allowed.
> Waiting for your answer.I have not slept well a few days for this.[/color]

puts(x & (x-1) || x==0 ? "not a power of two" : "power of two");

Robert Gamble

[Robert Gamble]

Re: A C Question.

PLEASE read how to properly post via Google Groups at
http://cfaj.freeshell.org/google/ before attempting to do so again.

jmazzi wrote:[color=blue]
> if ( var % 2 == 0 ) {
> printf("yep\n") ;
> }[/color]

The OP asked how to determine if a variable was a *power* of two, not a
*multiple* of two.

Robert Gamble

[jmazzi]

Re: A C Question.

my bad ;)

[Keith Thompson]

Re: A C Question.

"jmazzi" <jmazzi@gmail.c om> writes:[color=blue]
> my bad ;)[/color]

Please read <http://cfaj.freeshell. org/google/>. Thanks.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <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.

[Logan Shaw]

Re: A C Question.

Eric Sosman wrote:[color=blue]
> Jack Young wrote:[color=green]
>> There is question about C programming:
>> Use just one C statement to check whether a variable is the power of
>> 2. No loops allowed.
>> Waiting for your answer.I have not slept well a few days for this.[/color][/color]
[color=blue]
> int is_power_of_two (double x) {
> return x > 0.0; /* one statement, no loops */
> }[/color]

Floating point numbers (at least IEEE and similar ones) can be expressed
as a fraction whose denominator is a power of two. But that is not the
same thing as the number itself being a power of two. For instance,
3.0 is not a power of two.

- Logan

[Keith Thompson]

Re: A C Question.

Logan Shaw <lshaw-usenet@austin.r r.com> writes:[color=blue]
> Eric Sosman wrote:[color=green]
>> Jack Young wrote:[color=darkred]
>>> There is question about C programming:
>>> Use just one C statement to check whether a variable is the power
>>> of 2. No loops allowed.
>>> Waiting for your answer.I have not slept well a few days for
>>> this.[/color][/color]
>[color=green]
>> int is_power_of_two (double x) {
>> return x > 0.0; /* one statement, no loops */
>> }[/color]
>
> Floating point numbers (at least IEEE and similar ones) can be expressed
> as a fraction whose denominator is a power of two. But that is not the
> same thing as the number itself being a power of two. For instance,
> 3.0 is not a power of two.[/color]

Sure it is; the exponent is approximately 1.58496.

The OP is expecting us to do his homework for him without even
providing a precise statement of the problem.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <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.

[pete]

Re: A C Question.

Jack Young wrote:[color=blue]
>
> Hi:
> There is question about C programming:
> Use just one C statement to check whether
> a variable is the power of 2.
> No loops allowed.
> Waiting for your answer.I have not slept well a few days for this.[/color]

int n_is_Power_of_t wo(long unsigned n)
{
return (n & n - 1) == 0 && n != 0;
}

int n_is_Power_of_f our(long unsigned n)
{
return (n & n - 1) == 0 && n % 3 == 1;
}

int n_is_Power_of_e ight(long unsigned n)
{
return (n & n - 1) == 0 && n % 7 == 1;
}

--
pete

[jat]

Re: A C Question.

modf(log(a)/log(2),&b)?0:1;

i have tried this on C++. I m not sure whether the functions are
available in C.
but the logic is that if we get a integer after taking the log of given
number with respect to 2(or any number), it is divisible.

n power x = y
x log(n) = log(y)
x = log(y)/log(n)

it may seem abstruse , but it's just another solution.

[Ian Collins]

Re: A C Question.

jat wrote:[color=blue]
> modf(log(a)/log(2),&b)?0:1;
>
> i have tried this on C++. I m not sure whether the functions are
> available in C.
> but the logic is that if we get a integer after taking the log of given
> number with respect to 2(or any number), it is divisible.
>
> n power x = y
> x log(n) = log(y)
> x = log(y)/log(n)
>
> it may seem abstruse , but it's just another solution.
>[/color]
Imagine waiting for this to run on an 8 bit microcontroller !

--
Ian Collins.

[Keith Thompson]

Re: A C Question.

"jat" <contactravidal al@gmail.com> writes:[color=blue]
> modf(log(a)/log(2),&b)?0:1;
>
> i have tried this on C++. I m not sure whether the functions are
> available in C.
> but the logic is that if we get a integer after taking the log of given
> number with respect to 2(or any number), it is divisible.
>
> n power x = y
> x log(n) = log(y)
> x = log(y)/log(n)
>
> it may seem abstruse , but it's just another solution.[/color]

Another solution to what? The question was about determing whether a
number is a power of two, but we can't necessarily see the article to
which you're replying. Please read <http://cfaj.freeshell. org/google/>.

The method may be mathematically valid, but since floating-point
numbers are inexact, it's likely to fail in many cases.

But since the original poster was probably asking us to do his
homework for him, giving him *correct* answers is a bad idea.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <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.

[Jordan Abel]

Re: A C Question.

On 2006-01-18, Logan Shaw <lshaw-usenet@austin.r r.com> wrote:[color=blue]
> Eric Sosman wrote:[color=green]
>> Jack Young wrote:[color=darkred]
>>> There is question about C programming:
>>> Use just one C statement to check whether a variable is the power of
>>> 2. No loops allowed.
>>> Waiting for your answer.I have not slept well a few days for this.[/color][/color]
>[color=green]
>> int is_power_of_two (double x) {
>> return x > 0.0; /* one statement, no loops */
>> }[/color]
>
> Floating point numbers (at least IEEE and similar ones) can be expressed
> as a fraction whose denominator is a power of two. But that is not the
> same thing as the number itself being a power of two. For instance,
> 3.0 is not a power of two.[/color]

Sure it is.

2^1.58496250072 115618146

oh, you meant an integer power of two?

[Nelu]

Re: A C Question.

jmazzi wrote:[color=blue]
> if ( var % 2 == 0 ) {
> printf("yep\n") ;
> }
>[/color]
Since when is 6 a power of 2?
If you're allowed to have real powers than 7 is a power of 2 and doesn't
satisfy your condition. Either way, you're either catching too many numbers
or not enough :-).
Shorter: an even number is not necessarily a power of 2.

--
Ioan - Ciprian Tandau
tandau _at_ freeshell _dot_ org (hope it's not too late)
(... and that it still works...)