Re: how to specify power of number

"Antoninus Twink" <nospam@xxxxxxxxxxxxxx> wrote in message
news:slrng0i6b9.7i7.nospam@xxxxxxxxxxxxxxxxx
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+atoi(textip[1])*256*256+atoi(textip[2­])*256+atoi(textip[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 16777215.999999999 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
16777215.0000001 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 WSAStringToAddress() 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.
;-)


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