list::util.3

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 "List::Util 3"
.TH List::Util 3 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
List::Util \- A selection of general\-utility list subroutines
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\&    use List::Util qw(first max maxstr min minstr reduce shuffle sum);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\f(CW\*(C`List::Util\*(C'\fR contains a selection of subroutines that people have
expressed would be nice to have in the perl core, but the usage would
not really be high enough to warrant the use of a keyword, and the size
so small such that being individual extensions would be wasteful.
.PP
By default \f(CW\*(C`List::Util\*(C'\fR does not export any subroutines. The
subroutines defined are
.IP "first \s-1BLOCK\s0 \s-1LIST\s0" 4
.IX Item "first BLOCK LIST"
Similar to \f(CW\*(C`grep\*(C'\fR in that it evaluates \s-1BLOCK\s0 setting \f(CW$_\fR to each element
of \s-1LIST\s0 in turn. \f(CW\*(C`first\*(C'\fR returns the first element where the result from
\&\s-1BLOCK\s0 is a true value. If \s-1BLOCK\s0 never returns true or \s-1LIST\s0 was empty then
\&\f(CW\*(C`undef\*(C'\fR is returned.
.Sp
.Vb 3
\&    $foo = first { defined($_) } @list    # first defined value in @list
\&    $foo = first { $_ > $value } @list    # first value in @list which
\&                                          # is greater than $value
.Ve
.Sp
This function could be implemented using \f(CW\*(C`reduce\*(C'\fR like this
.Sp
.Vb 1
\&    $foo = reduce { defined($a) ? $a : wanted($b) ? $b : undef } undef, @list
.Ve
.Sp
for example \fIwanted()\fR could be \fIdefined()\fR which would return the first
defined value in \f(CW@list\fR
.IP "max \s-1LIST\s0" 4
.IX Item "max LIST"
Returns the entry in the list with the highest numerical value. If the
list is empty then \f(CW\*(C`undef\*(C'\fR is returned.
.Sp
.Vb 3
\&    $foo = max 1..10                # 10
\&    $foo = max 3,9,12               # 12
\&    $foo = max @bar, @baz           # whatever
.Ve
.Sp
This function could be implemented using \f(CW\*(C`reduce\*(C'\fR like this
.Sp
.Vb 1
\&    $foo = reduce { $a > $b ? $a : $b } 1..10
.Ve
.IP "maxstr \s-1LIST\s0" 4
.IX Item "maxstr LIST"
Similar to \f(CW\*(C`max\*(C'\fR, but treats all the entries in the list as strings
and returns the highest string as defined by the \f(CW\*(C`gt\*(C'\fR operator.
If the list is empty then \f(CW\*(C`undef\*(C'\fR is returned.
.Sp
.Vb 3
\&    $foo = maxstr \*(AqA\*(Aq..\*(AqZ\*(Aq          # \*(AqZ\*(Aq
\&    $foo = maxstr "hello","world"   # "world"
\&    $foo = maxstr @bar, @baz        # whatever
.Ve
.Sp
This function could be implemented using \f(CW\*(C`reduce\*(C'\fR like this
.Sp
.Vb 1
\&    $foo = reduce { $a gt $b ? $a : $b } \*(AqA\*(Aq..\*(AqZ\*(Aq
.Ve
.IP "min \s-1LIST\s0" 4
.IX Item "min LIST"
Similar to \f(CW\*(C`max\*(C'\fR but returns the entry in the list with the lowest
numerical value. If the list is empty then \f(CW\*(C`undef\*(C'\fR is returned.
.Sp
.Vb 3
\&    $foo = min 1..10                # 1
\&    $foo = min 3,9,12               # 3
\&    $foo = min @bar, @baz           # whatever
.Ve
.Sp
This function could be implemented using \f(CW\*(C`reduce\*(C'\fR like this
.Sp
.Vb 1
\&    $foo = reduce { $a < $b ? $a : $b } 1..10
.Ve
.IP "minstr \s-1LIST\s0" 4
.IX Item "minstr LIST"
Similar to \f(CW\*(C`min\*(C'\fR, but treats all the entries in the list as strings
and returns the lowest string as defined by the \f(CW\*(C`lt\*(C'\fR operator.
If the list is empty then \f(CW\*(C`undef\*(C'\fR is returned.
.Sp
.Vb 3
\&    $foo = minstr \*(AqA\*(Aq..\*(AqZ\*(Aq          # \*(AqA\*(Aq
\&    $foo = minstr "hello","world"   # "hello"
\&    $foo = minstr @bar, @baz        # whatever
.Ve
.Sp
This function could be implemented using \f(CW\*(C`reduce\*(C'\fR like this
.Sp
.Vb 1
\&    $foo = reduce { $a lt $b ? $a : $b } \*(AqA\*(Aq..\*(AqZ\*(Aq
.Ve
.IP "reduce \s-1BLOCK\s0 \s-1LIST\s0" 4
.IX Item "reduce BLOCK LIST"
Reduces \s-1LIST\s0 by calling \s-1BLOCK\s0, in a scalar context, multiple times,
setting \f(CW$a\fR and \f(CW$b\fR each time. The first call will be with \f(CW$a\fR
and \f(CW$b\fR set to the first two elements of the list, subsequent
calls will be done by setting \f(CW$a\fR to the result of the previous
call and \f(CW$b\fR to the next element in the list.
.Sp
Returns the result of the last call to \s-1BLOCK\s0. If \s-1LIST\s0 is empty then
\&\f(CW\*(C`undef\*(C'\fR is returned. If \s-1LIST\s0 only contains one element then that
element is returned and \s-1BLOCK\s0 is not executed.
.Sp
.Vb 4
\&    $foo = reduce { $a < $b ? $a : $b } 1..10       # min
\&    $foo = reduce { $a lt $b ? $a : $b } \*(Aqaa\*(Aq..\*(Aqzz\*(Aq # minstr
\&    $foo = reduce { $a + $b } 1 .. 10               # sum
\&    $foo = reduce { $a . $b } @bar                  # concat
.Ve
.IP "shuffle \s-1LIST\s0" 4
.IX Item "shuffle LIST"
Returns the elements of \s-1LIST\s0 in a random order
.Sp
.Vb 1
\&    @cards = shuffle 0..51      # 0..51 in a random order
.Ve
.IP "sum \s-1LIST\s0" 4
.IX Item "sum LIST"
Returns the sum of all the elements in \s-1LIST\s0. If \s-1LIST\s0 is empty then
\&\f(CW\*(C`undef\*(C'\fR is returned.
.Sp
.Vb 3
\&    $foo = sum 1..10                # 55
\&    $foo = sum 3,9,12               # 24
\&    $foo = sum @bar, @baz           # whatever
.Ve
.Sp
This function could be implemented using \f(CW\*(C`reduce\*(C'\fR like this
.Sp
.Vb 1
\&    $foo = reduce { $a + $b } 1..10
.Ve
.SH "KNOWN BUGS"
.IX Header "KNOWN BUGS"
With perl versions prior to 5.005 there are some cases where reduce
will return an incorrect result. This will show up as test 7 of
reduce.t failing.
.SH "SUGGESTED ADDITIONS"
.IX Header "SUGGESTED ADDITIONS"
The following are additions that have been requested, but I have been reluctant
to add due to them being very simple to implement in perl
.PP
.Vb 1
\&  # One argument is true
\&
\&  sub any { $_ && return 1 for @_; 0 }
\&
\&  # All arguments are true
\&
\&  sub all { $_ || return 0 for @_; 1 }
\&
\&  # All arguments are false
\&
\&  sub none { $_ && return 0 for @_; 1 }
\&
\&  # One argument is false
\&
\&  sub notall { $_ || return 1 for @_; 0 }
\&
\&  # How many elements are true
\&
\&  sub true { scalar grep { $_ } @_ }
\&
\&  # How many elements are false
\&
\&  sub false { scalar grep { !$_ } @_ }
.Ve
.SH "SEE ALSO"
.IX Header "SEE ALSO"
Scalar::Util, List::MoreUtils
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (c) 1997\-2006 Graham Barr <[e-mail]>. All rights reserved.
This program is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.