Yes, Perl has grep and map but is missing this idiom, which is why I added it to Algorithm::Loops as Filter(). But I still use grep to remove selected lines, which makes the map w/ "local $_= $_;" version appealing. 

use Algorithm::Loops qw( Filter );
my @lines = grep defined, Filter {
    s/^\s+//;
    if( /^#/ ) {
        undef $_;
    } else {
        s/\s+$//;
    }
} <$in>;

[download]

It isn't terribly hard to do "filtering" directly with grep or map, but it is terribly easy to only get filtering almost right with them and the difference is usually subtle but can still lead to real problems.

A pragma to make the aliases that map and grep provide in $_ always be read-only would be something I would use... I wonder if Perl6 lets you specify "read-only aliases" vs. "read-write copies" vs. "read-write aliases" for map / grep like I know it will for function arguments.

A concise alternative: 

s/^\s+//, s/\s+$//
    for my @lines= grep ! /^\s*#/, <$in>;

[download]

- tye        

In reply to Re: A perverse use of grep (filtering) by tye
in thread A perverse use of grep by eyepopslikeamosquito

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.