From: Ilya Zakharevich <ilya@math.ohio-state.edu>
Date: Thu, 1 Apr 1999 03:29:25 -0500 (EST)
Message-Id: <199904010829.DAA16282@monk.mps.ohio-state.edu>
To: mjd@plover.com
X-Also-Posted-To: comp.lang.perl.moderated
Subject: Re: Monthly Post:  Infrequently Asked Questions about Perl
References: <19990401052410.23923.qmail@plover.com>
Organization: Department of Mathematics, The Ohio State University
X-Newsreader: trn 4.0-test68 (29 August 1998)

[A complimentary Cc of this posting was sent to Mark-Jason Dominus 
<mjd@plover.com>],
who wrote in article <19990401052410.23923.qmail@plover.com>:
> How can I find the creation date of a file?
> 
>     Use this function:
> 
>             sub creation_date {
>               use FileHandle;
>               my $filename = shift or die "Usage: &creation_date(filename)\n";
>               my $fh1 = new FileHandle;
>               open $fh1, "< $filename" or return undef;
>               unlink $filename or return undef;
>               my $fh2 = new FileHandle;
>               open $fh2, "> $filename" or return undef;
>               print $fh2 <$fh1>;
>               time;
>             }

This is not portable, since unlink(open_file()) will fail on DOSISH
machines.  Why not use an already cooked solution: rename() and File::Copy?

> How can I tell if a string is a number?
> 
>     The simplest method is:
> 
>             if ($string == "$string") { 
>               # It is a number
>             }
> 
> 
>     Note the use of the `==' operator to compare the string to its numeric
>     value. However, this approach is dangerous because the `$string' might
>     contain arbitrary code such as C<@{[system "rm -rf /"]}> which would be
>     executed as a result of the interpolation process. For safety, use this
>     regular expression:
> 
>       if ($var =~
> /(?=.)M{0,3}(C[MD]|D?C{0,3})(X[CL]|L?X{0,3})(I[XV]|V?I{0,3})/) {
>         print "$var contains a number.\b";
>       }

Why not use ==~ which is created exactly for this purpose?

    if ($string ==~ "$string") {

> How do I find the largest element in an array?
> 
>     Write a `foreach' loop to scan the elements one at a time, and stop when
>     you get to the largest one.

This will not work with tied arrays referencing /dev/random, as in the
implementation of Net::WWW.

> Someone told me that `tr/x//d' would be faster than `s/x//g'. 
> Is this true?  Why?
> 
>     `tr///' is faster on multiprocessor machines because it can be
>     vectorized, so that each `x' is removed by a different processor.
>     `s///', however, must always be performed sequentially.

Beware that future enhancement to REx engine may make this advice wrong.

> 
> How do I get the length of a variable?
> 
>     Use
> 
>             length('$variable');

This may get you into problems if the variable contains "'".  Better
to use HERE-docs, and strip the final newline

   $l = length chomp <<'EOF';
$variable
EOF

> How do I differentiate my Object Oriented Perl scripts from other perl scripts?
> 
>     Just as C programmers use `.cpp' as an extension for their object
>     oriented C programs, many perl programmers use `.ppp' for their object
>     oriented Perl programs.

This advice is very obsolete.  It is a long-standing tradition to use
the extension .oops for object oriented Perl scripts.

> How do I sort a hash?
> 
>             %sorted = sort %hash;

This mixes keys and values.  The proper way is to repeat the hash, so
both the keys and values are seen by sort:

     (%sorted, %sorted) = sort %hash, %hash;

> What's a regular expression to replace `09:23:53 08-OCT-98' with 
> `Thu Oct  8 09:23:53 1998'?
> 
>             s{.*}{Thu Oct  8 09:23:53 1998}

Append 'l' to make this locale-conscient.

> How do I block warnings?
> 
>     The simplest way is to do
> 
>             close STDERR;

Seasoned Perl programmers know that one can save 1 char:

	 exit STDERR;

Hope this helps,
Ilya


----------------------------------------------------------------

[A complimentary Cc of this posting was sent to Mark-Jason Dominus 
<mjd@plover.com>],
who wrote in article <19990401052410.23923.qmail@plover.com>:
> How do I get tomorrow's date?
> 
>     Use this function:
> 
>             sub tomorrow_date {
>               sleep 86_400; 
>               return localtime();
>             }

In addition to other remarks: this implementation is very pessimal
speed-wise.  Here is an important optimization:

        sub tomorrow_date {
          sleep 86_400;
          return localtime();
          sleep -86_400;
        }

Ilya