perldsc.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.

.PP
.Vb 4
\&    for $i (1..10) {
\&        @array = 0 .. $i;
\&        @{$AoA[$i]} = @array;
\&    }
.Ve
.PP
Is it the same?  Well, maybe so\*(--and maybe not.  The subtle difference
is that when you assign something in square brackets, you know for sure
it's always a brand new reference with a new \fIcopy\fR of the data.
Something else could be going on in this new case with the \f(CW\*(C`@{$AoA[$i]}\*(C'\fR
dereference on the left-hand-side of the assignment.  It all depends on
whether \f(CW$AoA[$i]\fR had been undefined to start with, or whether it
already contained a reference.  If you had already populated \f(CW@AoA\fR with
references, as in
.PP
.Vb 1
\&    $AoA[3] = \e@another_array;
.Ve
.PP
Then the assignment with the indirection on the left-hand-side would
use the existing reference that was already there:
.PP
.Vb 1
\&    @{$AoA[3]} = @array;
.Ve
.PP
Of course, this \fIwould\fR have the \*(L"interesting\*(R" effect of clobbering
\&\f(CW@another_array\fR.  (Have you ever noticed how when a programmer says
something is \*(L"interesting\*(R", that rather than meaning \*(L"intriguing\*(R",
they're disturbingly more apt to mean that it's \*(L"annoying\*(R",
\&\*(L"difficult\*(R", or both?  :\-)
.PP
So just remember always to use the array or hash constructors with \f(CW\*(C`[]\*(C'\fR
or \f(CW\*(C`{}\*(C'\fR, and you'll be fine, although it's not always optimally
efficient.
.PP
Surprisingly, the following dangerous-looking construct will
actually work out fine:
.PP
.Vb 4
\&    for $i (1..10) {
\&        my @array = somefunc($i);
\&        $AoA[$i] = \e@array;
\&    }
.Ve
.PP
That's because \fImy()\fR is more of a run-time statement than it is a
compile-time declaration \fIper se\fR.  This means that the \fImy()\fR variable is
remade afresh each time through the loop.  So even though it \fIlooks\fR as
though you stored the same variable reference each time, you actually did
not!  This is a subtle distinction that can produce more efficient code at
the risk of misleading all but the most experienced of programmers.  So I
usually advise against teaching it to beginners.  In fact, except for
passing arguments to functions, I seldom like to see the gimme-a-reference
operator (backslash) used much at all in code.  Instead, I advise
beginners that they (and most of the rest of us) should try to use the
much more easily understood constructors \f(CW\*(C`[]\*(C'\fR and \f(CW\*(C`{}\*(C'\fR instead of
relying upon lexical (or dynamic) scoping and hidden reference-counting to
do the right thing behind the scenes.
.PP
In summary:
.PP
.Vb 3
\&    $AoA[$i] = [ @array ];      # usually best
\&    $AoA[$i] = \e@array;         # perilous; just how my() was that array?
\&    @{ $AoA[$i] } = @array;     # way too tricky for most programmers
.Ve
.SH "CAVEAT ON PRECEDENCE"
.IX Xref "dereference, precedence dereferencing, precedence"
.IX Header "CAVEAT ON PRECEDENCE"
Speaking of things like \f(CW\*(C`@{$AoA[$i]}\*(C'\fR, the following are actually the
same thing:
.IX Xref "->"
.PP
.Vb 2
\&    $aref\->[2][2]       # clear
\&    $$aref[2][2]        # confusing
.Ve
.PP
That's because Perl's precedence rules on its five prefix dereferencers
(which look like someone swearing: \f(CW\*(C`$ @ * % &\*(C'\fR) make them bind more
tightly than the postfix subscripting brackets or braces!  This will no
doubt come as a great shock to the C or \*(C+ programmer, who is quite
accustomed to using \f(CW*a[i]\fR to mean what's pointed to by the \fIi'th\fR
element of \f(CW\*(C`a\*(C'\fR.  That is, they first take the subscript, and only then
dereference the thing at that subscript.  That's fine in C, but this isn't C.
.PP
The seemingly equivalent construct in Perl, \f(CW$$aref[$i]\fR first does
the deref of \f(CW$aref\fR, making it take \f(CW$aref\fR as a reference to an
array, and then dereference that, and finally tell you the \fIi'th\fR value
of the array pointed to by \f(CW$AoA\fR. If you wanted the C notion, you'd have to
write \f(CW\*(C`${$AoA[$i]}\*(C'\fR to force the \f(CW$AoA[$i]\fR to get evaluated first
before the leading \f(CW\*(C`$\*(C'\fR dereferencer.
.ie n .SH "WHY YOU SHOULD ALWAYS ""use strict"""
.el .SH "WHY YOU SHOULD ALWAYS \f(CWuse strict\fP"
.IX Header "WHY YOU SHOULD ALWAYS use strict"
If this is starting to sound scarier than it's worth, relax.  Perl has
some features to help you avoid its most common pitfalls.  The best
way to avoid getting confused is to start every program like this:
.PP
.Vb 2
\&    #!/usr/bin/perl \-w
\&    use strict;
.Ve
.PP
This way, you'll be forced to declare all your variables with \fImy()\fR and
also disallow accidental \*(L"symbolic dereferencing\*(R".  Therefore if you'd done
this:
.PP
.Vb 5
\&    my $aref = [
\&        [ "fred", "barney", "pebbles", "bambam", "dino", ],
\&        [ "homer", "bart", "marge", "maggie", ],
\&        [ "george", "jane", "elroy", "judy", ],
\&    ];
\&
\&    print $aref[2][2];
.Ve
.PP
The compiler would immediately flag that as an error \fIat compile time\fR,
because you were accidentally accessing \f(CW@aref\fR, an undeclared
variable, and it would thereby remind you to write instead:
.PP
.Vb 1
\&    print $aref\->[2][2]
.Ve
.SH "DEBUGGING"
.IX Xref "data structure, debugging complex data structure, debugging AoA, debugging HoA, debugging AoH, debugging HoH, debugging array of arrays, debugging hash of arrays, debugging array of hashes, debugging hash of hashes, debugging"
.IX Header "DEBUGGING"
Before version 5.002, the standard Perl debugger didn't do a very nice job of
printing out complex data structures.  With 5.002 or above, the
debugger includes several new features, including command line editing as
well as the \f(CW\*(C`x\*(C'\fR command to dump out complex data structures.  For
example, given the assignment to \f(CW$AoA\fR above, here's the debugger output:
.PP
.Vb 10
\&    DB<1> x $AoA
\&    $AoA = ARRAY(0x13b5a0)
\&       0  ARRAY(0x1f0a24)
\&          0  \*(Aqfred\*(Aq
\&          1  \*(Aqbarney\*(Aq
\&          2  \*(Aqpebbles\*(Aq
\&          3  \*(Aqbambam\*(Aq
\&          4  \*(Aqdino\*(Aq
\&       1  ARRAY(0x13b558)
\&          0  \*(Aqhomer\*(Aq
\&          1  \*(Aqbart\*(Aq
\&          2  \*(Aqmarge\*(Aq
\&          3  \*(Aqmaggie\*(Aq
\&       2  ARRAY(0x13b540)
\&          0  \*(Aqgeorge\*(Aq
\&          1  \*(Aqjane\*(Aq
\&          2  \*(Aqelroy\*(Aq
\&          3  \*(Aqjudy\*(Aq
.Ve
.SH "CODE EXAMPLES"
.IX Header "CODE EXAMPLES"
Presented with little comment (these will get their own manpages someday)
here are short code examples illustrating access of various
types of data structures.
.SH "ARRAYS OF ARRAYS"
.IX Xref "array of arrays AoA"
.IX Header "ARRAYS OF ARRAYS"
.Sh "Declaration of an \s-1ARRAY\s0 \s-1OF\s0 \s-1ARRAYS\s0"
.IX Subsection "Declaration of an ARRAY OF ARRAYS"
.Vb 5
\& @AoA = (
\&        [ "fred", "barney" ],
\&        [ "george", "jane", "elroy" ],
\&        [ "homer", "marge", "bart" ],
\&      );
.Ve
.Sh "Generation of an \s-1ARRAY\s0 \s-1OF\s0 \s-1ARRAYS\s0"
.IX Subsection "Generation of an ARRAY OF ARRAYS"
.Vb 4
\& # reading from file
\& while ( <> ) {
\&     push @AoA, [ split ];
\& }
\&
\& # calling a function
\& for $i ( 1 .. 10 ) {
\&     $AoA[$i] = [ somefunc($i) ];
\& }
\&
\& # using temp vars
\& for $i ( 1 .. 10 ) {
\&     @tmp = somefunc($i);
\&     $AoA[$i] = [ @tmp ];
\& }
\&
\& # add to an existing row
\& push @{ $AoA[0] }, "wilma", "betty";
.Ve
.Sh "Access and Printing of an \s-1ARRAY\s0 \s-1OF\s0 \s-1ARRAYS\s0"
.IX Subsection "Access and Printing of an ARRAY OF ARRAYS"
.Vb 2
\& # one element
\& $AoA[0][0] = "Fred";
\&
\& # another element
\& $AoA[1][1] =~ s/(\ew)/\eu$1/;
\&
\& # print the whole thing with refs
\& for $aref ( @AoA ) {
\&     print "\et [ @$aref ],\en";
\& }
\&
\& # print the whole thing with indices
\& for $i ( 0 .. $#AoA ) {
\&     print "\et [ @{$AoA[$i]} ],\en";
\& }
\&
\& # print the whole thing one at a time
\& for $i ( 0 .. $#AoA ) {
\&     for $j ( 0 .. $#{ $AoA[$i] } ) {
\&         print "elt $i $j is $AoA[$i][$j]\en";
\&     }
\& }
.Ve
.SH "HASHES OF ARRAYS"
.IX Xref "hash of arrays HoA"
.IX Header "HASHES OF ARRAYS"
.Sh "Declaration of a \s-1HASH\s0 \s-1OF\s0 \s-1ARRAYS\s0"
.IX Subsection "Declaration of a HASH OF ARRAYS"
.Vb 5
\& %HoA = (
\&        flintstones        => [ "fred", "barney" ],
\&        jetsons            => [ "george", "jane", "elroy" ],
\&        simpsons           => [ "homer", "marge", "bart" ],
\&      );
.Ve
.Sh "Generation of a \s-1HASH\s0 \s-1OF\s0 \s-1ARRAYS\s0"
.IX Subsection "Generation of a HASH OF ARRAYS"
.Vb 6
\& # reading from file
\& # flintstones: fred barney wilma dino
\& while ( <> ) {
\&     next unless s/^(.*?):\es*//;
\&     $HoA{$1} = [ split ];
\& }
\&
\& # reading from file; more temps
\& # flintstones: fred barney wilma dino
\& while ( $line = <> ) {
\&     ($who, $rest) = split /:\es*/, $line, 2;
\&     @fields = split \*(Aq \*(Aq, $rest;
\&     $HoA{$who} = [ @fields ];
\& }
\&
\& # calling a function that returns a list
\& for $group ( "simpsons", "jetsons", "flintstones" ) {
\&     $HoA{$group} = [ get_family($group) ];
\& }
\&
\& # likewise, but using temps
\& for $group ( "simpsons", "jetsons", "flintstones" ) {
\&     @members = get_family($group);
\&     $HoA{$group} = [ @members ];
\& }
\&
\& # append new members to an existing family
\& push @{ $HoA{"flintstones"} }, "wilma", "betty";
.Ve
.Sh "Access and Printing of a \s-1HASH\s0 \s-1OF\s0 \s-1ARRAYS\s0"
.IX Subsection "Access and Printing of a HASH OF ARRAYS"
.Vb 2
\& # one element
\& $HoA{flintstones}[0] = "Fred";
\&
\& # another element
\& $HoA{simpsons}[1] =~ s/(\ew)/\eu$1/;
\&
\& # print the whole thing
\& foreach $family ( keys %HoA ) {
\&     print "$family: @{ $HoA{$family} }\en"
\& }
\&
\& # print the whole thing with indices
\& foreach $family ( keys %HoA ) {
\&     print "family: ";
\&     foreach $i ( 0 .. $#{ $HoA{$family} } ) {
\&         print " $i = $HoA{$family}[$i]";
\&     }
\&     print "\en";
\& }
\&
\& # print the whole thing sorted by number of members
\& foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) {
\&     print "$family: @{ $HoA{$family} }\en"
\& }
\&
\& # print the whole thing sorted by number of members and name
\& foreach $family ( sort {
\&                            @{$HoA{$b}} <=> @{$HoA{$a}}
\&                                        ||
\&                                    $a cmp $b
\&            } keys %HoA )
\& {
\&     print "$family: ", join(", ", sort @{ $HoA{$family} }), "\en";
\& }
.Ve
.SH "ARRAYS OF HASHES"
.IX Xref "array of hashes AoH"
.IX Header "ARRAYS OF HASHES"
.Sh "Declaration of an \s-1ARRAY\s0 \s-1OF\s0 \s-1HASHES\s0"
.IX Subsection "Declaration of an ARRAY OF HASHES"
.Vb 10
\& @AoH = (
\&        {
\&            Lead     => "fred",
\&            Friend   => "barney",
\&        },
\&        {
\&            Lead     => "george",
\&            Wife     => "jane",
\&            Son      => "elroy",
\&        },
\&        {
\&            Lead     => "homer",
\&            Wife     => "marge",
\&            Son      => "bart",
\&        }
\&  );
.Ve
.Sh "Generation of an \s-1ARRAY\s0 \s-1OF\s0 \s-1HASHES\s0"
.IX Subsection "Generation of an ARRAY OF HASHES"
.Vb 10
\& # reading from file
\& # format: LEAD=fred FRIEND=barney
\& while ( <> ) {
\&     $rec = {};
\&     for $field ( split ) {
\&         ($key, $value) = split /=/, $field;
\&         $rec\->{$key} = $value;
\&     }
\&     push @AoH, $rec;
\& }