how to specify power of number

[Yanb]

Hi, I'm new to c, so please excuse, if this is silly ;-)

I made a C function, which takes IP adress from string and converts to unsigned long int. Everything
works, but I found that the "counting" part part works only with this:

numericip=atoi( textip[0])*256*256*256+a toi(textip[1])*256*256+atoi( textip[2])*256+atoi(text ip[3]);

When I wrote *(256^3), I got fake results. I'd like to use something more sophisticated, instead of 256*256
*256, which does look silly ;-) (Just curious, what if I'd ever need to make 100th power of 256 ;-) )

Thank you in advance.

[Ben Bacarisse]

Yanb <Ya**@whatever. invalidwrites:

Hi, I'm new to c, so please excuse, if this is silly ;-)

Well, not silly, but I am surprised that you start a new language
without consulting some sort of reference.

I made a C function, which takes IP adress from string and converts to unsigned long int. Everything
works, but I found that the "counting" part part works only with this:

numericip=atoi( textip[0])*256*256*256+a toi(textip[1])*256*256+atoi( textip[2])*256+atoi(text ip[3]);

When I wrote *(256^3), I got fake results. I'd like to use something
more sophisticated, instead of 256*256

^ is exclusive or in C. C does not have an exponentiation operator.
Most C programmers would use a shift (<<) by 24, 16 or 8 bits to
compute the value you want.

--
Ben.

[Bartc]

"Yanb" <Ya**@whatever. invalidwrote in message
news:fu******** **@aioe.org...

Hi, I'm new to c, so please excuse, if this is silly ;-)

I made a C function, which takes IP adress from string and converts to
unsigned long int. Everything
works, but I found that the "counting" part part works only with this:

numericip=atoi( textip[0])*256*256*256+a toi(textip[1])*256*256+atoi( textip[2])*256+atoi(text ip[3]);

When I wrote *(256^3), I got fake results. I'd like to use something more
sophisticated, instead of 256*256
*256, which does look silly ;-) (Just curious, what if I'd ever need to
make 100th power of 256 ;-) )

You would have to use pow(256,3) which uses floating point arithmetic -- not
recommended, even though the compiler is likely to optimise.

Your code is fine as it is, the compiler will optimise 256*256*256 to
*16777216 or <<24. Or just write those in yourself.

--
Bart

[Antoninus Twink]

On 18 Apr 2008 at 11:01, Bartc wrote:

You would have to use pow(256,3) which uses floating point arithmetic -- not
recommended, even though the compiler is likely to optimise.

Your code is fine as it is, the compiler will optimise 256*256*256 to
*16777216 or <<24. Or just write those in yourself.

For general integer powers, a simple square-and-multiply algorithm is a
good bet, e.g.

unsigned long long power(unsigned a, unsigned b)
{
unsigned long long r, bit, pow;
for(bit=r=1, pow=a; bit<=b; bit<<=1, pow *= pow)
if(b & bit)
r*=pow;
return r;
}

The hardest part is checking for overflow - left as an exercise for the
reader...

[Yanb]

Ben Bacarisse <be********@bsb .me.ukwrote in
news:87******** ****@bsb.me.uk:

Yanb <Ya**@whatever. invalidwrites:

>Hi, I'm new to c, so please excuse, if this is silly ;-)

Well, not silly, but I am surprised that you start a new language
without consulting some sort of reference.

Hi, and thank you for reply. I have read some very basics, but usually I need a language for a specific
thing and as soon as I know enough to complete the task, I leave learning till later :-/

Do you have some tip for a newbie where to start? Of course I can google out load of pages about c,
but no search engine can compete with reference from a human ;-)

>

>I made a C function, which takes IP adress from string and converts
to unsigned long int. Everything works, but I found that the
"counting" part part works only with this:

numericip=atoi (textip[0])*256*256*256+a toi(textip[1])*256*256+atoi( tex
tip[2])*256+atoi(text ip[3]);

When I wrote *(256^3), I got fake results. I'd like to use something
more sophisticated, instead of 256*256

^ is exclusive or in C. C does not have an exponentiation operator.
Most C programmers would use a shift (<<) by 24, 16 or 8 bits to
compute the value you want.

Oh yes, that's it, thanks, shifting I know shifts from php ;-) This way will need less cpu cycles than
multiplication.

Ok, so there no way to write mathematic "power of" in c?

[Jens Thoms Toerring]

Yanb <Ya**@whatever. invalidwrote:

Ben Bacarisse <be********@bsb .me.ukwrote in
news:87******** ****@bsb.me.uk:

^ is exclusive or in C. C does not have an exponentiation operator.
Most C programmers would use a shift (<<) by 24, 16 or 8 bits to
compute the value you want.

Oh yes, that's it, thanks, shifting I know shifts from php ;-) This way
will need less cpu cycles than multiplication.

I wouldn't bet on that, a good compiler probably will "see" if
a multiplication can be replaced by a shift operation and will
use that if it's faster on the machine you're compiling for.

Ok, so there no way to write mathematic "power of" in c?

While there's no "power of" operator like for example in FORTRAN
there's the pow() function from the math part of the standard
C library, but which takes double arguments and returns a double.

Regards, Jens
--
\ Jens Thoms Toerring ___ [e-mail]
\______________ ____________ http://toerring.de

[Yanb]

Antoninus Twink <no****@nospam. invalidwrote in
news:sl******** ***********@nos pam.invalid:

On 18 Apr 2008 at 11:01, Bartc wrote:

>You would have to use pow(256,3) which uses floating point arithmetic
-- not recommended, even though the compiler is likely to optimise.

Your code is fine as it is, the compiler will optimise 256*256*256 to
*16777216 or <<24. Or just write those in yourself.

For general integer powers, a simple square-and-multiply algorithm is
a good bet, e.g.

unsigned long long power(unsigned a, unsigned b)
{
unsigned long long r, bit, pow;
for(bit=r=1, pow=a; bit<=b; bit<<=1, pow *= pow)
if(b & bit)
r*=pow;
return r;
}

The hardest part is checking for overflow - left as an exercise for
the reader...

Thank you both. I must admit, that the code Is almost a mystery for me :-)

[Yanb]

[e-mail] (Jens Thoms Toerring) wrote in news:66******** *****@mid.uni-berlin.de:

[snip]

Thank you for a good advices, Jens. Good to know it.

[Noob]

Yanb wrote:

I made a C function, which takes IP adress from string and converts
to unsigned long int.

Your function sounds like inet_aton().

(comp.unix.prog rammer is a better place to discuss such a function.)

http://www.kernel.org/doc/man-pages/...et_addr.3.html

[Antoninus Twink]

On 18 Apr 2008 at 21:46, user923005 wrote:

If we look at his original post (and knowing he is just turning these
TCP/IP addresses into 4 byte integers):
"
numericip=atoi( textip[0])*256*256*256+a toi(textip[1])*256*256+atoi( textip[2Â*])*256+atoi(text ip[3]);"

It could clearly be done as {assuming textip[] is an array of unsigned
char} as:
numericip = (unsigned long)textip[0] << 24 +
(unsigned long)textip[1] << 16 +
(unsigned long)textip[2] << 8 +
(unsigned long)textip[3] ;

True, and someone had already suggested this. But the discussion
broadened, and someone claimed that using pow() was the best way to
raise integers to (positive) integer powers: it was this assertion that
I was responding to.

Or {better yet} simply use inet_aton() {which is also fine for
Winsock, though WSAStringToAddr ess() is an alternative}. That is what
would be normally done to retrieve a TCP/IP address from a dotted
address string.

HOWEVER!

The OP should keep in mind that inet_aton() does NOT support IPv6 and
that getnameinfo() should be used instead for IPv4/v6 dual stack
support.

Good advice.

To the O.P.:
You can't go wrong with "Unix Network Programming" By W. Richard
Stevens

Agreed.