perlfaq4.pod

MSYS在windows下模拟了一个类unix的终端

	my $lastpos   = 1;
	for my $place (@positions) {
	    $template .= "A" . ($place - $lastpos) . " "; 
	    $lastpos   = $place;
	}
	$template .= "A*";
	return $template;
    }

=head2 How do I find the soundex value of a string?

Use the standard Text::Soundex module distributed with Perl.
Before you do so, you may want to determine whether `soundex' is in
fact what you think it is.  Knuth's soundex algorithm compresses words
into a small space, and so it does not necessarily distinguish between
two words which you might want to appear separately.  For example, the
last names `Knuth' and `Kant' are both mapped to the soundex code K530.
If Text::Soundex does not do what you are looking for, you might want
to consider the String::Approx module available at CPAN.

=head2 How can I expand variables in text strings?

Let's assume that you have a string like:

    $text = 'this has a $foo in it and a $bar';

If those were both global variables, then this would
suffice:

    $text =~ s/\$(\w+)/${$1}/g;  # no /e needed

But since they are probably lexicals, or at least, they could
be, you'd have to do this:

    $text =~ s/(\$\w+)/$1/eeg;
    die if $@;			# needed /ee, not /e

It's probably better in the general case to treat those
variables as entries in some special hash.  For example:

    %user_defs = ( 
	foo  => 23,
	bar  => 19,
    );
    $text =~ s/\$(\w+)/$user_defs{$1}/g;

See also ``How do I expand function calls in a string?'' in this section
of the FAQ.

=head2 What's wrong with always quoting "$vars"?

The problem is that those double-quotes force stringification--
coercing numbers and references into strings--even when you
don't want them to be strings.  Think of it this way: double-quote
expansion is used to produce new strings.  If you already 
have a string, why do you need more?

If you get used to writing odd things like these:

    print "$var";   	# BAD
    $new = "$old";   	# BAD
    somefunc("$var");	# BAD

You'll be in trouble.  Those should (in 99.8% of the cases) be
the simpler and more direct:

    print $var;
    $new = $old;
    somefunc($var);

Otherwise, besides slowing you down, you're going to break code when
the thing in the scalar is actually neither a string nor a number, but
a reference:

    func(\@array);
    sub func {
	my $aref = shift;
	my $oref = "$aref";  # WRONG
    }

You can also get into subtle problems on those few operations in Perl
that actually do care about the difference between a string and a
number, such as the magical C<++> autoincrement operator or the
syscall() function.

Stringification also destroys arrays.  

    @lines = `command`;
    print "@lines";		# WRONG - extra blanks
    print @lines;		# right

=head2 Why don't my <<HERE documents work?

Check for these three things:

=over 4

=item 1. There must be no space after the << part.

=item 2. There (probably) should be a semicolon at the end.

=item 3. You can't (easily) have any space in front of the tag.

=back

If you want to indent the text in the here document, you 
can do this:

    # all in one
    ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
        your text
        goes here
    HERE_TARGET

But the HERE_TARGET must still be flush against the margin.
If you want that indented also, you'll have to quote 
in the indentation.

    ($quote = <<'    FINIS') =~ s/^\s+//gm;
            ...we will have peace, when you and all your works have
            perished--and the works of your dark master to whom you
            would deliver us. You are a liar, Saruman, and a corrupter
            of men's hearts.  --Theoden in /usr/src/perl/taint.c
        FINIS
    $quote =~ s/\s*--/\n--/;

A nice general-purpose fixer-upper function for indented here documents
follows.  It expects to be called with a here document as its argument.
It looks to see whether each line begins with a common substring, and
if so, strips that substring off.  Otherwise, it takes the amount of leading
whitespace found on the first line and removes that much off each
subsequent line.

    sub fix {
        local $_ = shift;
        my ($white, $leader);  # common whitespace and common leading string
        if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
            ($white, $leader) = ($2, quotemeta($1));
        } else {
            ($white, $leader) = (/^(\s+)/, '');
        }
        s/^\s*?$leader(?:$white)?//gm;
        return $_;
    }

This works with leading special strings, dynamically determined:

    $remember_the_main = fix<<'    MAIN_INTERPRETER_LOOP';
	@@@ int
	@@@ runops() {
	@@@     SAVEI32(runlevel);
	@@@     runlevel++;
	@@@     while ( op = (*op->op_ppaddr)() );
	@@@     TAINT_NOT;
	@@@     return 0;
	@@@ }
    MAIN_INTERPRETER_LOOP

Or with a fixed amount of leading whitespace, with remaining
indentation correctly preserved:

    $poem = fix<<EVER_ON_AND_ON;
       Now far ahead the Road has gone,
	  And I must follow, if I can,
       Pursuing it with eager feet,
	  Until it joins some larger way
       Where many paths and errands meet.
	  And whither then? I cannot say.
		--Bilbo in /usr/src/perl/pp_ctl.c
    EVER_ON_AND_ON

=head1 Data: Arrays

=head2 What is the difference between a list and an array?

An array has a changeable length.  A list does not.  An array is something
you can push or pop, while a list is a set of values.  Some people make
the distinction that a list is a value while an array is a variable.
Subroutines are passed and return lists, you put things into list
context, you initialize arrays with lists, and you foreach() across
a list.  C<@> variables are arrays, anonymous arrays are arrays, arrays
in scalar context behave like the number of elements in them, subroutines
access their arguments through the array C<@_>, and push/pop/shift only work
on arrays.

As a side note, there's no such thing as a list in scalar context.
When you say

    $scalar = (2, 5, 7, 9);

you're using the comma operator in scalar context, so it uses the scalar
comma operator.  There never was a list there at all!  This causes the
last value to be returned: 9.

=head2 What is the difference between $array[1] and @array[1]?

The former is a scalar value; the latter an array slice, making
it a list with one (scalar) value.  You should use $ when you want a
scalar value (most of the time) and @ when you want a list with one
scalar value in it (very, very rarely; nearly never, in fact).

Sometimes it doesn't make a difference, but sometimes it does.
For example, compare:

    $good[0] = `some program that outputs several lines`;

with

    @bad[0]  = `same program that outputs several lines`;

The C<use warnings> pragma and the B<-w> flag will warn you about these 
matters.

=head2 How can I remove duplicate elements from a list or array?

There are several possible ways, depending on whether the array is
ordered and whether you wish to preserve the ordering.

=over 4

=item a)

If @in is sorted, and you want @out to be sorted:
(this assumes all true values in the array)

    $prev = "not equal to $in[0]";
    @out = grep($_ ne $prev && ($prev = $_, 1), @in);

This is nice in that it doesn't use much extra memory, simulating
uniq(1)'s behavior of removing only adjacent duplicates.  The ", 1"
guarantees that the expression is true (so that grep picks it up)
even if the $_ is 0, "", or undef.

=item b)

If you don't know whether @in is sorted:

    undef %saw;
    @out = grep(!$saw{$_}++, @in);

=item c)

Like (b), but @in contains only small integers:

    @out = grep(!$saw[$_]++, @in);

=item d)

A way to do (b) without any loops or greps:

    undef %saw;
    @saw{@in} = ();
    @out = sort keys %saw;  # remove sort if undesired

=item e)

Like (d), but @in contains only small positive integers:

    undef @ary;
    @ary[@in] = @in;
    @out = grep {defined} @ary;

=back

But perhaps you should have been using a hash all along, eh?

=head2 How can I tell whether a list or array contains a certain element?

Hearing the word "in" is an I<in>dication that you probably should have
used a hash, not a list or array, to store your data.  Hashes are
designed to answer this question quickly and efficiently.  Arrays aren't.

That being said, there are several ways to approach this.  If you
are going to make this query many times over arbitrary string values,
the fastest way is probably to invert the original array and keep an
associative array lying about whose keys are the first array's values.

    @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
    undef %is_blue;
    for (@blues) { $is_blue{$_} = 1 }

Now you can check whether $is_blue{$some_color}.  It might have been a
good idea to keep the blues all in a hash in the first place.

If the values are all small integers, you could use a simple indexed
array.  This kind of an array will take up less space:

    @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
    undef @is_tiny_prime;
    for (@primes) { $is_tiny_prime[$_] = 1 }
    # or simply  @istiny_prime[@primes] = (1) x @primes;

Now you check whether $is_tiny_prime[$some_number].

If the values in question are integers instead of strings, you can save
quite a lot of space by using bit strings instead:

    @articles = ( 1..10, 150..2000, 2017 );
    undef $read;
    for (@articles) { vec($read,$_,1) = 1 }

Now check whether C<vec($read,$n,1)> is true for some C<$n>.

Please do not use

    ($is_there) = grep $_ eq $whatever, @array;

or worse yet

    ($is_there) = grep /$whatever/, @array;

These are slow (checks every element even if the first matches),
inefficient (same reason), and potentially buggy (what if there are
regex characters in $whatever?).  If you're only testing once, then
use:

    $is_there = 0;
    foreach $elt (@array) {
	if ($elt eq $elt_to_find) {
	    $is_there = 1;
	    last;
	}
    }
    if ($is_there) { ... }

=head2 How do I compute the difference of two arrays?  How do I compute the intersection of two arrays?

Use a hash.  Here's code to do both and more.  It assumes that
each element is unique in a given array:

    @union = @intersection = @difference = ();
    %count = ();
    foreach $element (@array1, @array2) { $count{$element}++ }
    foreach $element (keys %count) {
	push @union, $element;
	push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
    }

Note that this is the I<symmetric difference>, that is, all elements in
either A or in B but not in both.  Think of it as an xor operation.

=head2 How do I test whether two arrays or hashes are equal?

The following code works for single-level arrays.  It uses a stringwise
comparison, and does not distinguish defined versus undefined empty
strings.  Modify if you have other needs.

    $are_equal = compare_arrays(\@frogs, \@toads);

    sub compare_arrays {
	my ($first, $second) = @_;
	no warnings;  # silence spurious -w undef complaints
	return 0 unless @$first == @$second;
	for (my $i = 0; $i < @$first; $i++) {
	    return 0 if $first->[$i] ne $second->[$i];
	}
	return 1;
    }

For multilevel structures, you may wish to use an approach more
like this one.  It uses the CPAN module FreezeThaw:

    use FreezeThaw qw(cmpStr);
    @a = @b = ( "this", "that", [ "more", "stuff" ] );