perlfaq4.html

《Perl 5教程》及《perl常问问题集》

<HR>
<H3><A NAME="How_do_I_look_up_a_hash_element_">How do I look up a hash element by value?

</A></H3>
Create a reverse hash:


<P>

<PRE>    %by_value = reverse %by_key;
    $key = $by_value{$value};
</PRE>

<P>

That's not particularly efficient. It would be more space-efficient to use:


<P>

<PRE>    while (($key, $value) = each %by_key) {
        $by_value{$value} = $key;
    }
</PRE>

<P>

If your hash could have repeated values, the methods above will only find
one of the associated keys. This may or may not worry you.


<P>

<P>
<HR>
<H3><A NAME="How_can_I_know_how_many_entries_">How can I know how many entries are in a hash?

</A></H3>
If you mean how many keys, then all you have to do is take the scalar sense
of the <CODE>keys()</CODE> function:


<P>

<PRE>    $num_keys = scalar keys %hash;
</PRE>

<P>

In void context it just resets the iterator, which is faster for tied
hashes.


<P>

<P>
<HR>
<H3><A NAME="How_do_I_sort_a_hash_optionally">How do I sort a hash (optionally by value instead of key)?

</A></H3>
Internally, hashes are stored in a way that prevents you from imposing an
order on key-value pairs. Instead, you have to sort a list of the keys or
values:


<P>

<PRE>    @keys = sort keys %hash;    # sorted by key
    @keys = sort {
                    $hash{$a} cmp $hash{$b}
            } keys %hash;       # and by value
</PRE>

<P>

Here we'll do a reverse numeric sort by value, and if two keys are identical, sort by length of key, and if that fails, by straight 
<FONT SIZE=-1>ASCII</FONT> comparison of the keys (well, possibly modified by your locale -- see

<A HREF="../../tppmsgs/msgs1.htm#105" tppabs="http://www.perl.org/CPAN/doc/manual/html/pod/perllocale.html">the perllocale manpage</A>).


<P>

<PRE>    @keys = sort {
                $hash{$b} &lt;=&gt; $hash{$a}
                          ||
                length($b) &lt;=&gt; length($a)
                          ||
                      $a cmp $b
    } keys %hash;
</PRE>

<P>

<P>
<HR>
<H3><A NAME="How_can_I_always_keep_my_hash_so">How can I always keep my hash sorted?

</A></H3>
You can look into using the DB_File module and <CODE>tie()</CODE> using the
<CODE>$DB_BTREE</CODE> hash bindings as documented in <A HREF="../../tppmsgs/msgs1.htm#108" tppabs="http://www.perl.org/CPAN/doc/manual/html/lib/DB_File.html#In_Memory_Databases">In Memory Databases</A>.


<P>

<P>
<HR>
<H3><A NAME="What_s_the_difference_between_d">What's the difference between "delete" and "undef" with hashes?

</A></H3>
Hashes are pairs of scalars: the first is the key, the second is the value.
The key will be coerced to a string, although the value can be any kind of
scalar: string, number, or reference. If a key <CODE>$key</CODE> is present in the array, <CODE>exists($key)</CODE> will return true. The value for a given key can be <CODE>undef</CODE>, in which case <CODE>$array{$key}</CODE> will be
<CODE>undef</CODE> while <CODE>$exists{$key}</CODE> will return true. This corresponds to (<CODE>$key</CODE>, <CODE>undef</CODE>) being in the hash.


<P>

Pictures help... here's the <CODE>%ary</CODE> table:


<P>

<PRE>          keys  values
        +------+------+
        |  a   |  3   |
        |  x   |  7   |
        |  d   |  0   |
        |  e   |  2   |
        +------+------+
</PRE>

<P>

And these conditions hold


<P>

<PRE>        $ary{'a'}                       is true
        $ary{'d'}                       is false
        defined $ary{'d'}               is true
        defined $ary{'a'}               is true
        exists $ary{'a'}                is true (perl5 only)
        grep ($_ eq 'a', keys %ary)     is true
</PRE>

<P>

If you now say


<P>

<PRE>        undef $ary{'a'}
</PRE>

<P>

your table now reads:


<P>

<PRE>          keys  values
        +------+------+
        |  a   | undef|
        |  x   |  7   |
        |  d   |  0   |
        |  e   |  2   |
        +------+------+
</PRE>

<P>

and these conditions now hold; changes in caps:


<P>

<PRE>        $ary{'a'}                       is FALSE
        $ary{'d'}                       is false
        defined $ary{'d'}               is true
        defined $ary{'a'}               is FALSE
        exists $ary{'a'}                is true (perl5 only)
        grep ($_ eq 'a', keys %ary)     is true
</PRE>

<P>

Notice the last two: you have an undef value, but a defined key!


<P>

Now, consider this:


<P>

<PRE>        delete $ary{'a'}
</PRE>

<P>

your table now reads:


<P>

<PRE>          keys  values
        +------+------+
        |  x   |  7   |
        |  d   |  0   |
        |  e   |  2   |
        +------+------+
</PRE>

<P>

and these conditions now hold; changes in caps:


<P>

<PRE>        $ary{'a'}                       is false
        $ary{'d'}                       is false
        defined $ary{'d'}               is true
        defined $ary{'a'}               is false
        exists $ary{'a'}                is FALSE (perl5 only)
        grep ($_ eq 'a', keys %ary)     is FALSE
</PRE>

<P>

See, the whole entry is gone!


<P>

<P>
<HR>
<H3><A NAME="Why_don_t_my_tied_hashes_make_th">Why don't my tied hashes make the defined/exists distinction?

</A></H3>
They may or may not implement the <CODE>EXISTS()</CODE> and <CODE>DEFINED()</CODE> methods differently. For example, there isn't the concept of undef with hashes that are tied to 
<FONT SIZE=-1>DBM*</FONT> files. This means the true/false tables above will give different results when used on such a hash. It also means that exists and defined do the same thing with a 
<FONT SIZE=-1>DBM*</FONT> file, and what they end up doing is not what they do with ordinary hashes.



<P>

<P>
<HR>
<H3><A NAME="How_do_I_reset_an_each_operati">How do I reset an each() operation part-way through?

</A></H3>
Using <CODE>keys %hash</CODE> in a scalar context returns the number of keys in the hash <EM>and</EM> resets the iterator associated with the hash. You may need to do this if
you use <CODE>last</CODE> to exit a loop early so that when you re-enter it, the hash iterator has
been reset.


<P>

<P>
<HR>
<H3><A NAME="How_can_I_get_the_unique_keys_fr">How can I get the unique keys from two hashes?

</A></H3>
First you extract the keys from the hashes into arrays, and then solve the
uniquifying the array problem described above. For example:


<P>

<PRE>    %seen = ();
    for $element (keys(%foo), keys(%bar)) {
        $seen{$element}++;
    }
    @uniq = keys %seen;
</PRE>

<P>

Or more succinctly:


<P>

<PRE>    @uniq = keys %{{%foo,%bar}};
</PRE>

<P>

Or if you really want to save space:


<P>

<PRE>    %seen = ();
    while (defined ($key = each %foo)) {
        $seen{$key}++;
    }
    while (defined ($key = each %bar)) {
        $seen{$key}++;
    }
    @uniq = keys %seen;
</PRE>

<P>

<P>
<HR>
<H3><A NAME="How_can_I_store_a_multidimension">How can I store a multidimensional array in a DBM file?

</A></H3>
Either stringify the structure yourself (no fun), or else get the 
<FONT SIZE=-1>MLDBM</FONT> (which uses Data::Dumper) module from 
<FONT SIZE=-1>CPAN</FONT> and layer it on top of either DB_File or GDBM_File.



<P>

<P>
<HR>
<H3><A NAME="How_can_I_make_my_hash_remember_">How can I make my hash remember the order I put elements into it?

</A></H3>
Use the Tie::IxHash from 
<FONT SIZE=-1>CPAN.</FONT>


<P>

<PRE>    use Tie::IxHash;
    tie(%myhash, Tie::IxHash);
    for ($i=0; $i&lt;20; $i++) {
        $myhash{$i} = 2*$i;
    }
    @keys = keys %myhash;
    # @keys = (0,1,2,3,...)
</PRE>

<P>

<P>
<HR>
<H3><A NAME="Why_does_passing_a_subroutine_an">Why does passing a subroutine an undefined element in a hash create it?

</A></H3>
If you say something like:


<P>

<PRE>    somefunc($hash{&quot;nonesuch key here&quot;});
</PRE>

<P>

Then that element ``autovivifies''; that is, it springs into existence
whether you store something there or not. That's because functions get
scalars passed in by reference. If <CODE>somefunc()</CODE> modifies <CODE>$_[0]</CODE>, it has to be ready to write it back into the caller's version.


<P>

This has been fixed as of perl5.004.


<P>

Normally, merely accessing a key's value for a nonexistent key does
<EM>not</EM> cause that key to be forever there. This is different than awk's behavior.


<P>

<P>
<HR>
<H3><A NAME="How_can_I_make_the_Perl_equivale">How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?

</A></H3>
Use references (documented in <A HREF="../../tppmsgs/msgs0.htm#59" tppabs="http://www.perl.org/CPAN/doc/manual/html/pod/perlref.html">the perlref manpage</A>). Examples of complex data structures are given in <A HREF="../../tppmsgs/msgs0.htm#64" tppabs="http://www.perl.org/CPAN/doc/manual/html/pod/perldsc.html">the perldsc manpage</A> and <A HREF="../../tppmsgs/msgs0.htm#63" tppabs="http://www.perl.org/CPAN/doc/manual/html/pod/perllol.html">the perllol manpage</A>. Examples of structures and object-oriented classes are in <A HREF="../../tppmsgs/msgs0.htm#57" tppabs="http://www.perl.org/CPAN/doc/manual/html/pod/perltoot.html">the perltoot manpage</A>.


<P>

<P>
<HR>
<H3><A NAME="How_can_I_use_a_reference_as_a_h">How can I use a reference as a hash key?

</A></H3>
You can't do this directly, but you could use the standard Tie::Refhash
module distributed with perl.


<P>

<P>
<HR>
<H2><A NAME="Data_Misc">Data: Misc

</A></H2>
<P>
<HR>
<H3><A NAME="How_do_I_handle_binary_data_corr">How do I handle binary data correctly?

</A></H3>
Perl is binary clean, so this shouldn't be a problem. For example, this
works fine (assuming the files are found):


<P>

<PRE>    if (`cat /vmunix` =~ /gzip/) {
        print &quot;Your kernel is GNU-zip enabled!\n&quot;;
    }
</PRE>

<P>

On some systems, however, you have to play tedious games with ``text''
versus ``binary'' files. See <A HREF="../../tppmsgs/msgs0.htm#68" tppabs="http://www.perl.org/CPAN/doc/manual/html/pod/perlfunc.html#binmode">binmode</A>.


<P>

If you're concerned about 8-bit 
<FONT SIZE=-1>ASCII</FONT> data, then see <A HREF="../../tppmsgs/msgs1.htm#105" tppabs="http://www.perl.org/CPAN/doc/manual/html/pod/perllocale.html">the perllocale manpage</A>.


<P>

If you want to deal with multibyte characters, however, there are some
gotchas. See the section on Regular Expressions.


<P>

<P>
<HR>
<H3><A NAME="How_do_I_determine_whether_a_sca">How do I determine whether a scalar is a number/whole/integer/float?

</A></H3>
Assuming that you don't care about 
<FONT SIZE=-1>IEEE</FONT> notations like ``NaN'' or ``Infinity'', you
probably just want to use a regular expression.


<P>

<PRE>   warn &quot;has nondigits&quot;        if     /\D/;
   warn &quot;not a whole number&quot;   unless /^\d+$/;
   warn &quot;not an integer&quot;       unless /^-?\d+$/;  # reject +3
   warn &quot;not an integer&quot;       unless /^[+-]?\d+$/;
   warn &quot;not a decimal number&quot; unless /^-?\d+\.?\d*$/;  # rejects .2
   warn &quot;not a decimal number&quot; unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
   warn &quot;not a C float&quot;
       unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
</PRE>

<P>

Or you could check out <A HREF="../../tppmsgs/msgs1.htm#137" tppabs="http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz">http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz</A> instead. The 
<FONT SIZE=-1>POSIX</FONT> module (part of the standard Perl distribution) provides the
 <CODE>strtol</CODE> and <CODE>strtod</CODE> for converting strings to double and longs, respectively.


<P>

<P>
<HR>
<H3><A NAME="How_do_I_keep_persistent_data_ac">How do I keep persistent data across program calls?

</A></H3>
For some specific applications, you can use one of the 
<FONT SIZE=-1>DBM</FONT> modules. See <A HREF="../../tppmsgs/msgs1.htm#138" tppabs="http://www.perl.org/CPAN/doc/manual/html/lib/AnyDBM_File.html">the AnyDBM_File manpage</A>. More generically, you should consult the FreezeThaw, Storable, or Class::Eroot modules from 
<FONT SIZE=-1>CPAN.</FONT>



<P>

<P>
<HR>
<H3><A NAME="How_do_I_print_out_or_copy_a_rec">How do I print out or copy a recursive data structure?

</A></H3>
The Data::Dumper module on 
<FONT SIZE=-1>CPAN</FONT> is nice for printing out data structures, and
FreezeThaw for copying them. For example:


<P>

<PRE>    use FreezeThaw qw(freeze thaw);
    $new = thaw freeze $old;
</PRE>

<P>

Where <CODE>$old</CODE> can be (a reference to) any kind of data structure
you'd like. It will be deeply copied.


<P>

<P>
<HR>
<H3><A NAME="How_do_I_define_methods_for_ever">How do I define methods for every class/object?

</A></H3>
Use the 
<FONT SIZE=-1>UNIVERSAL</FONT> class (see <A HREF="../../tppmsgs/msgs1.htm#139" tppabs="http://www.perl.org/CPAN/doc/manual/html/lib/UNIVERSAL.html">the UNIVERSAL manpage</A>).


<P>

<P>
<HR>
<H3><A NAME="How_do_I_verify_a_credit_card_ch">How do I verify a credit card checksum?

</A></H3>
Get the Business::CreditCard module from 
<FONT SIZE=-1>CPAN.</FONT>


<P>

<P>
<HR>
<H2><A NAME="AUTHOR_AND_COPYRIGHT">AUTHOR AND COPYRIGHT

</A></H2>
Copyright (c) 1997 Tom Christiansen and Nathan Torkington. All rights
reserved. See <A HREF="perlfaq.html" tppabs="http://202.96.217.5/~xiaoyi/perlfaq.html">the perlfaq manpage</A> for distribution information.


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