Re: Trying to understand sort and anonymous sub
From: Paul Johnson (paul_at_pjcj.net)
Date: 07/24/04
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Date: Sat, 24 Jul 2004 15:39:39 +0200 To: Dalton Marris <dmarris@charter.net>
On Sat, Jul 24, 2004 at 08:57:56AM -0400, Dalton Marris wrote:
> I am trying to understand the use of "sub" in the second "foreach
> $daynum" loop in the code shown below. I know it's an anonymous sub,
> but can't figure out why it's needed, if it indeed is. I have read the
> perlref and perlsub documents, and googled for information, to no avail.
>
> Some things I have noticed regarding the first and second "foreach
> $daynum" loops:
>
> When I step through the first "foreach $daynum" loop with Komodo, it
> seems to hang on the "foreach" as each combination of $a and $b values
> is sorted. Stepping continues after everything is sorted. The
> resultant output file contains the following (as expected):
>
> 946684800|Holiday 1|Holiday 2
> 950486400|Holiday 3
> 954547200|Holiday 5
> 977702400|Holiday 7|Holiday 8
>
> When stepping through the second "foreach $daynum" loop, it only takes
> one step before it continues on, so it appears to be faster. However,
> it also results in an extra key, for some reason. The resultant output
> file contains the following:
>
> 946684800|Holiday 1|Holiday 2
> 950486400|Holiday 3
> 954547200|Holiday 5
> 977702400|Holiday 7|Holiday 8
> CODE(0x198f04c)
>
> My questions:
>
> (1) What should the "sub" be doing?
It is a bug. It shouldn't be there.
> (2) Where is the "CODE(0x198f04c)" coming from?
The first sort is using the specified subroutine {$a<=>$b} to sort
numerically. The second sort is using the default sort subroutine
({$a cmp $b }), but also sorting the sub {$a<=>$b} along with what you
want to sort. "CODE(0x198f04c)" is what you are getting when the sub is
stringified.
In your case the <=> and cmp sorts are the same, but try changing adding
99 and 1000000000 as keys and you will see the problem.
> #!/usr/bin/perl -w
> use strict;
>
> my $dayentry;
> my $daynum;
> my $dbrow;
>
> my %dayinfo = (
> 977702400 => [ "Holiday 7",
> "Holiday 8" ],
> 950486400 => [ "Holiday 3" ],
> 946684800 => [ "Holiday 1",
> "Holiday 2" ],
> 954547200 => [ "Holiday 5" ],
> );
>
> open( DBFILE, ">dbfile1.txt" ) or die("Error: could not open
> \"dbfile1.txt\"\n" );
>
> foreach $daynum (sort{$a<=>$b} keys %dayinfo ) {
> $dbrow = "$daynum";
> foreach $dayentry (@{$dayinfo{$daynum}}) {
> $dbrow = join('|',$dbrow,$dayentry);
> }
> # write new data to output file
> print( DBFILE "$dbrow\n" );
> }
> close( DBFILE );
>
> open( DBFILE, ">dbfile2.txt" ) or die("Error: could not open
> \"dbfile2.txt\"\n" );
>
> foreach $daynum (sort(sub{$a<=>$b}, keys %dayinfo)) {
> $dbrow = "$daynum";
> foreach $dayentry (@{$dayinfo{$daynum}}) {
> $dbrow = join('|',$dbrow,$dayentry);
> }
> # write new data to output file
> print( DBFILE "$dbrow\n" );
> }
> close( DBFILE );
-- Paul Johnson - paul@pjcj.net http://www.pjcj.net
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