If Not Replace

Inexistence has asked for the wisdom of the Perl Monks concerning the following question:

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Re: If Not Replace by ikegami (Patriarch) on Sep 10, 2011 at 15:00 UTC
Nothing ("`()`") in scalar context evaluates to `undef`, so `if (!$myarray[$arrayKey[$i]]) { $myarray[$arrayKey[$i]]=() }` [download] is really `if (!$myarray[$arrayKey[$i]]) { $myarray[$arrayKey[$i]] = undef; }` [download] If `$myarray[$arrayKey[$i]]` is non-existent, 0, the empty string, "0" or one of the few other things that are false, undef will be assigned to it. Unless you are trying to micro-optimise memory usage (undef takes less memory than the string "`0`"), it sounds like a rather useless thing to do. Maybe the author was trying to stretch the size of the array, but that's probably a useless thing to do too. As the statement demonstrates, using and assigning to non-existent elements is not a problem.	[reply] [d/l] [select]
Re: If Not Replace by derby (Abbot) on Sep 10, 2011 at 14:18 UTC
You probably do not need this at all. You can rely on autovivification when setting the value and defined when accessing. -derby	[reply]
Re^2: If Not Replace by tchrist (Pilgrim) on Sep 10, 2011 at 14:31 UTC
There’s no autoviv involved here. Autovivication is the implicit allocation of references of the proper type when undefined lvalues are dereferenced. There is no dereferencing here.	[reply]
Re^3: If Not Replace by derby (Abbot) on Sep 12, 2011 at 11:46 UTC
Ah ... sorry I was wrongly thinking perl's auto extending of arrays is the same thing as autoviv. -derby	[reply]
Re^4: If Not Replace by tchrist (Pilgrim) on Sep 12, 2011 at 15:13 UTC
Re: If Not Replace by Anonymous Monk on Sep 10, 2011 at 14:31 UTC
Update: After being asked what I think this does... Its suppose to define(create) a table within the parent if it doesn't already exist. I can't assume that the table will be properly defined ... Are you using strict? Read this if you want to cut your development time in half! `$ perl -e " use strict; use warnings; my %f; $f[0]=3; " Global symbol "@f" requires explicit package name at -e line 1. Execution of -e aborted due to compilation errors.` [download] You should not write `%myarray` because that is a hash, and `$myarray[0 ]` is refering to @myarray, whereas `$myarray{0}` is refering to %myarray; perlintro	[reply] [d/l] [select]
Re^2: If Not Replace by Inexistence (Acolyte) on Sep 10, 2011 at 14:51 UTC
You should not write %myarray because that is a hash, and $myarray0 is refering to @myarray, whereas $myarray{0} is refering to %myarray I was following examples from 'http://oreilly.com/catalog/advperl/excerpt/ch01.html' but I no doubt got them wrong. Thank-you muchly for guiding me re the above variable information. Associative arrays are what I believe i'm use to using in LUA, the hash naming just confuses me atm bow	[reply]
Re^3: If Not Replace by davido (Cardinal) on Sep 10, 2011 at 16:43 UTC
That's a good book, but having read it (and having re-read the excerpt you pointed to in the link above), I am not seeing quite how you came up with what you did. But that's not important. The important thing is that you've asked a few good questions, and have had thick enough skin to stick around for the answers. I ++ed both of your nodes in this thread for that reason, particularly after seeing the updates. So many times we see someone "fly by" with a question that seems like it's coming from left field, and then after we answer or request further clarification, never see the individual again. Kudos for sticking with it. I recommend putting the Advanced Perl Programming book off for now. It's a great book but it doesn't deal with advanced Perl syntax so much as advanced topics addressed by Perl. It does get into symbol table tricks to some degree, but for the most part it's about implementing solutions with Perl. Mastering Perl is in a similar category; another great book that sort of takes on random topics related to implementing real-world solutions with Perl. But Learning Perl and Intermediate Perl are books dedicated to Perl itself; Perl syntax, Perl data types, Perl objects, Perl modules, Perl operators, etc. Dave	[reply]
Re^4: If Not Replace by Inexistence (Acolyte) on Sep 10, 2011 at 18:05 UTC
Re^5: If Not Replace by chromatic (Archbishop) on Sep 10, 2011 at 19:18 UTC
Some notes below your chosen depth have not been shown here
Re^5: If Not Replace by davido (Cardinal) on Sep 10, 2011 at 19:59 UTC
Re: If Not Replace by Anonymous Monk on Sep 10, 2011 at 14:07 UTC
What do you think that code does?	[reply]