perlfaq4.1

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

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.\" ========================================================================
.\"
.IX Title "PERLFAQ4 1"
.TH PERLFAQ4 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH "NAME"
perlfaq4 \- Data Manipulation ($Revision: 10394 $)
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This section of the \s-1FAQ\s0 answers questions related to manipulating
numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
.SH "Data: Numbers"
.IX Header "Data: Numbers"
.Sh "Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?"
.IX Subsection "Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?"
Internally, your computer represents floating-point numbers in binary.
Digital (as in powers of two) computers cannot store all numbers
exactly.  Some real numbers lose precision in the process.  This is a
problem with how computers store numbers and affects all computer
languages, not just Perl.
.PP
perlnumber shows the gory details of number representations and
conversions.
.PP
To limit the number of decimal places in your numbers, you can use the
printf or sprintf function.  See the \*(L"Floating Point
Arithmetic\*(R" for more details.
.PP
.Vb 1
\&        printf "%.2f", 10/3;
\&
\&        my $number = sprintf "%.2f", 10/3;
.Ve
.Sh "Why is \fIint()\fP broken?"
.IX Subsection "Why is int() broken?"
Your \f(CW\*(C`int()\*(C'\fR is most probably working just fine.  It's the numbers that
aren't quite what you think.
.PP
First, see the answer to \*(L"Why am I getting long decimals
(eg, 19.9499999999999) instead of the numbers I should be getting
(eg, 19.95)?\*(R".
.PP
For example, this
.PP
.Vb 1
\&        print int(0.6/0.2\-2), "\en";
.Ve
.PP
will in most computers print 0, not 1, because even such simple
numbers as 0.6 and 0.2 cannot be presented exactly by floating-point
numbers.  What you think in the above as 'three' is really more like
2.9999999999999995559.
.Sh "Why isn't my octal data interpreted correctly?"
.IX Subsection "Why isn't my octal data interpreted correctly?"
Perl only understands octal and hex numbers as such when they occur as
literals in your program.  Octal literals in perl must start with a
leading \f(CW0\fR and hexadecimal literals must start with a leading \f(CW\*(C`0x\*(C'\fR.
If they are read in from somewhere and assigned, no automatic
conversion takes place.  You must explicitly use \f(CW\*(C`oct()\*(C'\fR or \f(CW\*(C`hex()\*(C'\fR if you
want the values converted to decimal.  \f(CW\*(C`oct()\*(C'\fR interprets hexadecimal (\f(CW0x350\fR),
octal (\f(CW0350\fR or even without the leading \f(CW0\fR, like \f(CW377\fR) and binary
(\f(CW\*(C`0b1010\*(C'\fR) numbers, while \f(CW\*(C`hex()\*(C'\fR only converts hexadecimal ones, with
or without a leading \f(CW\*(C`0x\*(C'\fR, such as \f(CW0x255\fR, \f(CW\*(C`3A\*(C'\fR, \f(CW\*(C`ff\*(C'\fR, or \f(CW\*(C`deadbeef\*(C'\fR.
The inverse mapping from decimal to octal can be done with either the
<%o> or \f(CW%O\fR \f(CW\*(C`sprintf()\*(C'\fR formats.
.PP
This problem shows up most often when people try using \f(CW\*(C`chmod()\*(C'\fR,
\&\f(CW\*(C`mkdir()\*(C'\fR, \f(CW\*(C`umask()\*(C'\fR, or \f(CW\*(C`sysopen()\*(C'\fR, which by widespread tradition
typically take permissions in octal.
.PP
.Vb 2
\&        chmod(644,  $file);   # WRONG
\&        chmod(0644, $file);   # right
.Ve
.PP
Note the mistake in the first line was specifying the decimal literal
\&\f(CW644\fR, rather than the intended octal literal \f(CW0644\fR.  The problem can
be seen with:
.PP
.Vb 1
\&        printf("%#o",644);   # prints 01204
.Ve
.PP
Surely you had not intended \f(CW\*(C`chmod(01204, $file);\*(C'\fR \- did you?  If you
want to use numeric literals as arguments to \fIchmod()\fR et al. then please
try to express them as octal constants, that is with a leading zero and
with the following digits restricted to the set \f(CW0..7\fR.
.Sh "Does Perl have a \fIround()\fP function?  What about \fIceil()\fP and \fIfloor()\fP?  Trig functions?"
.IX Subsection "Does Perl have a round() function?  What about ceil() and floor()?  Trig functions?"
Remember that \f(CW\*(C`int()\*(C'\fR merely truncates toward 0.  For rounding to a
certain number of digits, \f(CW\*(C`sprintf()\*(C'\fR or \f(CW\*(C`printf()\*(C'\fR is usually the
easiest route.
.PP
.Vb 1
\&        printf("%.3f", 3.1415926535);   # prints 3.142
.Ve
.PP
The \f(CW\*(C`POSIX\*(C'\fR module (part of the standard Perl distribution)
implements \f(CW\*(C`ceil()\*(C'\fR, \f(CW\*(C`floor()\*(C'\fR, and a number of other mathematical
and trigonometric functions.
.PP
.Vb 3
\&        use POSIX;
\&        $ceil   = ceil(3.5);   # 4
\&        $floor  = floor(3.5);  # 3
.Ve
.PP
In 5.000 to 5.003 perls, trigonometry was done in the \f(CW\*(C`Math::Complex\*(C'\fR
module.  With 5.004, the \f(CW\*(C`Math::Trig\*(C'\fR module (part of the standard Perl
distribution) implements the trigonometric functions. Internally it
uses the \f(CW\*(C`Math::Complex\*(C'\fR module and some functions can break out from
the real axis into the complex plane, for example the inverse sine of
2.
.PP
Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely.  In these
cases, it probably pays not to trust whichever system rounding is
being used by Perl, but to instead implement the rounding function you
need yourself.
.PP
To see why, notice how you'll still have an issue on half-way-point
alternation:
.PP
.Vb 1
\&        for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
\&
\&        0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
\&        0.8 0.8 0.9 0.9 1.0 1.0
.Ve
.PP
Don't blame Perl.  It's the same as in C.  \s-1IEEE\s0 says we have to do
this. Perl numbers whose absolute values are integers under 2**31 (on
32 bit machines) will work pretty much like mathematical integers.
Other numbers are not guaranteed.
.Sh "How do I convert between numeric representations/bases/radixes?"
.IX Subsection "How do I convert between numeric representations/bases/radixes?"
As always with Perl there is more than one way to do it.  Below are a
few examples of approaches to making common conversions between number
representations.  This is intended to be representational rather than
exhaustive.
.PP
Some of the examples later in perlfaq4 use the \f(CW\*(C`Bit::Vector\*(C'\fR
module from \s-1CPAN\s0. The reason you might choose \f(CW\*(C`Bit::Vector\*(C'\fR over the
perl built in functions is that it works with numbers of \s-1ANY\s0 size,
that it is optimized for speed on some operations, and for at least
some programmers the notation might be familiar.
.IP "How do I convert hexadecimal into decimal" 4
.IX Item "How do I convert hexadecimal into decimal"
Using perl's built in conversion of \f(CW\*(C`0x\*(C'\fR notation:
.Sp
.Vb 1
\&        $dec = 0xDEADBEEF;
.Ve
.Sp
Using the \f(CW\*(C`hex\*(C'\fR function:
.Sp
.Vb 1
\&        $dec = hex("DEADBEEF");
.Ve
.Sp
Using \f(CW\*(C`pack\*(C'\fR:
.Sp
.Vb 1
\&        $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", \-8)));
.Ve
.Sp
Using the \s-1CPAN\s0 module \f(CW\*(C`Bit::Vector\*(C'\fR:
.Sp
.Vb 3
\&        use Bit::Vector;
\&        $vec = Bit::Vector\->new_Hex(32, "DEADBEEF");
\&        $dec = $vec\->to_Dec();
.Ve
.IP "How do I convert from decimal to hexadecimal" 4
.IX Item "How do I convert from decimal to hexadecimal"
Using \f(CW\*(C`sprintf\*(C'\fR:
.Sp
.Vb 2
\&        $hex = sprintf("%X", 3735928559); # upper case A\-F
\&        $hex = sprintf("%x", 3735928559); # lower case a\-f
.Ve
.Sp
Using \f(CW\*(C`unpack\*(C'\fR:
.Sp
.Vb 1
\&        $hex = unpack("H*", pack("N", 3735928559));
.Ve
.Sp
Using \f(CW\*(C`Bit::Vector\*(C'\fR:
.Sp
.Vb 3
\&        use Bit::Vector;
\&        $vec = Bit::Vector\->new_Dec(32, \-559038737);
\&        $hex = $vec\->to_Hex();
.Ve
.Sp
And \f(CW\*(C`Bit::Vector\*(C'\fR supports odd bit counts:
.Sp
.Vb 4
\&        use Bit::Vector;
\&        $vec = Bit::Vector\->new_Dec(33, 3735928559);
\&        $vec\->Resize(32); # suppress leading 0 if unwanted
\&        $hex = $vec\->to_Hex();
.Ve
.IP "How do I convert from octal to decimal" 4
.IX Item "How do I convert from octal to decimal"
Using Perl's built in conversion of numbers with leading zeros:
.Sp
.Vb 1
\&        $dec = 033653337357; # note the leading 0!
.Ve
.Sp
Using the \f(CW\*(C`oct\*(C'\fR function:
.Sp
.Vb 1
\&        $dec = oct("33653337357");
.Ve