perl evil for the week

[Matt Diephouse]

Rather than use map to create the hash, you could just use a hash slice.

#!/usr/bin/perl

@foo = qw( a b c d e f g );
@bar = qw( h i j k l m n );

# here is the hash slice
# notice that it uses @ as the sigil
# but also uses {}'s
@combined{@bar} = (@foo);

foreach my $key ( keys(%combined) ) {
     print "$key => $combined{$key}\n";
}

For more information, go to your terminal and type `perldoc perldata`.

matt

On Monday, July 14, 2003, at 01:13  PM, Al Tobey wrote:

> First off, my little example program shows some new behavior that
> everybody should be aware of: perl 5.8.1+ will explicitly randomize the
> order of your hash keys.  This is supposed to fix some possible DOS
> attacks against perl CGI scripts that guess hash key orders.
>
> Taking two arrays with the same number of elements and putting them 
> into
> a hash in one line.  Inspiration credit goes to Damian Conway during 
> his
> Advanced Object Oriented Perl class at OSCON.  He used map a number of
> times, each time saying something like, "this is evil, so don't do it
> unless you really know what you're doing - and please comment it."
>
> #!/usr/bin/perl
>
> @foo = qw( a b c d e f g );
> @bar = qw( h i j k l m n );
>
> # here is the evil
> %combined = map { $bar[$_] => $foo[$_] } 0..$#foo;
>
> foreach my $key ( keys(%combined) ) {
>     print "$key => $combined{$key}\n";
> }
>
> -------------------------------
> tobeya >perl /tmp/test.pl
> l => e
> n => g
> k => d
> h => a
> m => f
> j => c
> i => b
>
> This was really useful because I was parsing the output of a unix tool
> which had known field names, such as vmstat.  I hard-coded the field
> names, then mapped the output of the command against the field name
> array into a hash.  Very useful in my line of work.
>
> -Al Tobey
>
> ---------------------------------------------------------------
> P.S. Here is a more complex but possibly better example:
>
> # untested, so probably has errors
> @header = qw( proc_r proc_b proc_w swpd free buff cache
>            swap_in swap_out blocks_in blocks_out interrupts
>            context_switches user_cpu system_cpu idle_cpu );
>
> # run vmstat 10 times with 5 seconds between each listing
> open( VMSTAT, "/usr/bin/vmstat -n 5 10 |" )
> 	|| die "could not execute vmstat: $!";
> <VMSTAT>; <VMSTAT>; # skip the headers
>
> my @data = ();
> while ( my $vmstat = <VMSTAT> ) {
>    my @splitstat = split( /\s+/, $vmstat, $#header+1 );
>    my %combined = map { $header[$_] => $splitstat[$_] } 0..$#header;
>    push( @data, \%combined );
> }
> close( VMSTAT );
>
> # do something with @data ...
>
>
>
>
>
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