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]

Re: how to specify power of number

Yanb <Yanb@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]

Re: how to specify power of number


"Yanb" <Yanb@whatever. invalidwrote in message
news:fu9q89$kem $1@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]

Re: how to specify power of number

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]

Re: how to specify power of number

Ben Bacarisse <ben.usenet@bsb .me.ukwrote in
news:87od877aoh .fsf@bsb.me.uk:

Yanb <Yanb@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]

Re: how to specify power of number


Yanb <Yanb@whatever. invalidwrote:

Ben Bacarisse <ben.usenet@bsb .me.ukwrote in
news:87od877aoh .fsf@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 ___ jt@toerring.de
\______________ ____________ http://toerring.de

[Yanb]

Re: how to specify power of number

Antoninus Twink <nospam@nospam. invalidwrote in
news:slrng0h483 .166.nospam@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]

Re: how to specify power of number

jt@toerring.de (Jens Thoms Toerring) wrote in news:66rlpcF2lt sj2U1@mid.uni-berlin.de:

[snip]

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

[Noob]

Re: how to specify power of number

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/online/pages/man3/inet_addr.3.html

[Antoninus Twink]

Re: how to specify power of number

On 18 Apr 2008 at 14:05, Yanb wrote:

Antoninus Twink <nospam@nospam. invalidwrote:

>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 :-)

Well, it's overkill for exponents small enough not to overflow a
unit64_t, but the idea is this: naively, to raise a to the power b takes
b integer multiplies (a*a*...*a, b times). But actually, you can do this
using more like log(b) multiplies by repeatedly squaring to compute a^2,
a^4, a^8, ... and then multiplying those terms corresponding to
exponents of 2 that occur in the binary expansion of b. For example, if
b=7, then a^7 = a^(1+2+4) = a * a^2 * a^4.

[user923005]

Re: how to specify power of number

On Apr 18, 2:23 pm, Antoninus Twink <nos...@nospam. invalidwrote:

On 18 Apr 2008 at 14:05, Yanb wrote:
>
>
>
>
>

Antoninus Twink <nos...@nospam. invalidwrote:

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 :-)

>
Well, it's overkill for exponents small enough not to overflow a
unit64_t, but the idea is this: naively, to raise a to the power b takes
b integer multiplies (a*a*...*a, b times). But actually, you can do this
using more like log(b) multiplies by repeatedly squaring to compute a^2,
a^4, a^8, ...  and then multiplying those terms corresponding to
exponents of 2 that occur in the binary expansion of b. For example, if
b=7, then a^7 = a^(1+2+4) = a * a^2 * a^4.

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[2])*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] ;

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.

IMO-YMMV.

To the O.P.:
You can't go wrong with "Unix Network Programming" By W. Richard
Stevens (the same ideas work everywhere so don't be concerned if you
are doing Windows TCP/IP.

I don't know of any good reference works for IPv6 (I just use RFCs for
that).

[Antoninus Twink]

Re: how to specify power of number

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.

[Dann Corbit]

Re: how to specify power of number

"Antoninus Twink" <nospam@nospam. invalidwrote in message
news:slrng0i6b9 .7i7.nospam@nos pam.invalid...

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[2])*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.

Because math coprocessors are awfully fast now, I guess that pow(2.0,k) is
nearly as fast as bit shifts and additions, but there is a bigger problem
with using pow(2,x) to find exact powers of 2. It should not be unexpected
for pow(2.0,24) to return 16777215.999999 999 and if you assign that to an
integer, it probably won't be what is wanted unless you are clever enough to
round it -- ceil() won't help either because we might also have seen
16777215.000000 1 or something like that. The Cephes collection of math
functions {for instance} uses a ratio of a cubic polynomial divided by a
quartic polynomial to form the approximation.

Likely, many implementations will actually recognize integral inputs (my C++
compiler definitely does this -- even sometimes complaining about ambiguity
over argument type) but there is no reason to expect that to happen.

So I guess I am really agreeing with you. If you want small and exact
powers of 2, then bit shifting of unsigned integers is the most sensible
method. It will be fast (probably faster than pow()) and also exact. The
meaning of either construct is clear to any C programmer and so the argument
of writing the code that is most clear does not come into play here.

>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.

Well look here, it appears that we agree on everything. At least in this
post.
;-)


** Posted from http://www.teranews.com **

[Dann Corbit]

Re: how to specify power of number

"Dann Corbit" <dcorbit@connx. comwrote in message
news:14653$4809 20eb$8464@news. teranews.com...
[snip]

Because math coprocessors are awfully fast now, I guess that pow(2.0,k) is
nearly as fast as bit shifts and additions, but there is a bigger problem
with using pow(2,x) to find exact powers of 2. It should not be
unexpected for pow(2.0,24) to return 16777215.999999 999 and if you assign
that to an integer, it probably won't be what is wanted unless you are
clever enough to round it -- ceil() won't help either because we might
also have seen 16777215.000000 1 or something like that. The Cephes
collection of math

Of course I meant 16777216.000000 1 -- I would be pretty upset with a double
math library that can't even give me back 8 significant digits of precision.

functions {for instance} uses a ratio of a cubic polynomial divided by a
quartic polynomial to form the approximation.

[snip]


** Posted from http://www.teranews.com **

[Tim Prince]

Re: how to specify power of number

Because math coprocessors are awfully fast now, I guess that pow(2.0,k) is
nearly as fast as bit shifts and additions, but there is a bigger problem
with using pow(2,x) to find exact powers of 2. It should not be unexpected
for pow(2.0,24) to return 16777215.999999 999 and if you assign that to an
integer, it probably won't be what is wanted unless you are clever enough to
round it -- ceil() won't help either because we might also have seen
16777215.000000 1 or something like that. The Cephes collection of math
functions {for instance} uses a ratio of a cubic polynomial divided by a
quartic polynomial to form the approximation.
>
Likely, many implementations will actually recognize integral inputs (my C++
compiler definitely does this -- even sometimes complaining about ambiguity
over argument type) but there is no reason to expect that to happen.
>

It's almost obligatory for an implementation of pow() to special case
integral exponents, so as to handle negative operands, as well as provide
accuracy. I thought Cephes did this.
In case this doesn't happen, it doesn't seem too far fetched to specify
functions such as lrint(), supplying your own code if your position is
that C99 isn't portable enough.