Beefy Boxes and Bandwidth Generously Provided by pair Networks
Don't ask to ask, just ask

comment on

( #3333=superdoc: print w/replies, xml ) Need Help??

You don't need a eval to do this, and it's easier to do without it. You basically want a higher order function that composes other functions, and all of the functions are going to get the same arguments.

Starting from the finished product, you might have a sort subroutine like this, where the return value is the first thing that returns true (which should be -1 or 1).

sub sortsub { $a cmp $b || other_function( $a, $b ) || ... }

That's the same as writing it as a collection of subroutines that get arguments instead of using globals though:

sub sortsub { string_compare( $a, $b ) || other_function( $a, $b ) || ... }

But I can rewrite that sortsub to take a list of subroutine references as its arguments. Now my sortsub goes through each subroutine and returns when it finds one that returns a true value. If it finds 0 (meaning that sub thought the elements were equal), it goes onto the next subroutine.

sub sortsub { my @subs = @_; foreach my $sub ( @subs ) { my $result = $sub->($a, $b); return $result if $result; } }

I can't use this in sort yet because I can't pass it arguments. I can, however, modify it to return a subroutine reference I can use:

sub make_sort_sub { my @subs = @_; return sub { foreach my $sub ( @subs ) { my $result = $sub->($a, $b); return $result if $result; } }; }

Now, I basically make the reference then use it in my sort block.

my $sort_sub = make_sort_sub( @sub_references ); my @sorted = sort { $sort_sub->() } @stuff;

And here's a full demo, using it with numbers, then strings, then a user-defined sort subroutine. I don't need an eval for any of it.

#!/usr/bin/perl # pre-defined common sort subroutines my %Subs = ( numeric_descending => sub { $b <=> $a }, numeric_ascending => sub { $a <=> $b }, string_descending => sub { $b cmp $a }, string_ascending => sub { $b cmp $a }, case_insensitive => sub { "\L$a" cmp "\L$b" }, ); sub make_sort_sub { my @subs = @_; return sub { foreach my $sub ( @subs ) { my $result = $sub->($a, $b); return $result if $result; } }; } # numbers { my @use_these_subs = map { $Subs{$_} } qw(numeric_descending); my $sort_sub = make_sort_sub( @use_these_subs ); my @sorted = sort { $sort_sub->() } qw( 1 10 11 100 2 12 21 3 31 300 4 5 6 66 7 + 71 ); print "@sorted\n"; } # strings { my @use_these_subs = map { $Subs{$_} } qw(case_insensitive string_asce +nding); my $sort_sub = make_sort_sub( @use_these_subs ); my @sorted = sort { $sort_sub->() } qw( Fred fred FReD Barney barney Betty BETT +Y ); print "@sorted\n"; } # strings by length with user defined subroutine { my @use_these_subs = ( sub { length $a <=> length $b } ); push @use_these_subs, map { $Subs{$_} } qw(string_ascending); my $sort_sub = make_sort_sub( @use_these_subs ); my @sorted = sort { $sort_sub->() } qw( Fred fred FReD Barney barney Betty BETT +Y ); print "@sorted\n"; }
brian d foy <>
Subscribe to The Perl Review

In reply to Re^2: Small examples of string eval by brian_d_foy
in thread Small examples of string eval by spurperl

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, 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, 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.
  • Log In?

    What's my password?
    Create A New User
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others making s'mores by the fire in the courtyard of the Monastery: (3)
    As of 2020-08-14 21:07 GMT
    Find Nodes?
      Voting Booth?
      Which rocket would you take to Mars?

      Results (76 votes). Check out past polls.