Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!cs.utexas.edu!uunet!convex!usenet
From: tchrist@convex.COM (Tom Christiansen)
Newsgroups: comp.lang.perl
Subject: Re: Sortin based on part of an associative array?
Message-ID: <1991Apr11.022531.16242@convex.com>
Date: 11 Apr 91 02:25:31 GMT
References: <17524@venera.isi.edu>
Sender: usenet@convex.com (news access account)
Reply-To: tchrist@convex.COM (Tom Christiansen)
Distribution: comp
Organization: CONVEX Software Development, Richardson, TX
Lines: 57
Nntp-Posting-Host: pixel.convex.com

From the keyboard of jas@ISI.EDU (Jeff Sullivan):
:
:I have an situation where I'm storing a "structure" of information
:about various files in a SPLITtable string using an associative array.
:For example:
:
:$Images{"TEST.IMG"} = "120#55#4#GIF"
:
:which might mean that the file TEST.IMG is 120x55 pixels, 4 bitplanes
:deep, and is a GIF format file.
:
:What I want to do is write a sort routine that lets me sort the
:%Images elements by image length or width.  Here's what I have:
:
:sub bylen {
:
:local ($alen, $awid, $adep, $afmt, $blen, $bwid, $bdep, $bfmt);
:
:	($alen, $awid, $adep, $afmt) = split('#', $Images{$a});
:	($blen, $bwid, $bdep, $bfmt) = split('#', $Images{$b});
:
:	$alen cmp $blen;
:
:}
:
:
:However, when I try to do this (on MS-PERL PL 18), I get a syntax
:error at the line containing "$alen cmp $blen;"
:
:Why?

Because you're running PL 18.  The cmp operator wasn't in the language
then.  You could use ($alen - $blen) instead of ($alen cmp $blen).

:Can't I do this?  Is there a better way?

Yes: only run one split per element, instead of two per compare.
Something like this (untested):

    sub sort_by_len { &sort_by_field(*Images, $[); } 
    sub sort_by_wid { &sort_by_field(*Images, $[+1); } 
    sub bynum { $a - $b; }

    sub sort_by_field { 
	local(*table, $field) = @_;
	local($_, @keys);
	for (keys %table) {
	    push(@keys, (split(/#/, $table{$_}))[$field]);
	} 
	sort bynum @keys;
    } 

@Images{&sort_by_len} is all the elements in length order; 
@Images{&sort_by_wid} is all the elements in width order.


--tom