Dynamically creating a hash based on a variable

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

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Re: Dynamically creating a hash based on a variable by Joost (Canon) on Apr 15, 2007 at 13:54 UTC
This question gets asked a lot, and the answer is almost always "don't do that". It's better to use a hash-of-hashes instead: `# set up the base hash my %hashes = (); # in the loop: my $date = $4.$3.$2; if (my $hashref = $hashes{$date}) { print "<TD class=table_font>$date - Hash exists</TD>\n"; # play with hash } else { my $hashref = {}; # create a new empty hash $hashes{$date} = $hashref; store the hashref in the main hash print "<TD class=table_font>$date - Hash doesn't exist</TD>\n"; # play with hash }` [download] See also perlreftut and perldata. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l]
Re^2: Dynamically creating a hash based on a variable by beethamd (Initiate) on Apr 15, 2007 at 14:02 UTC
Joost - That's wonderful. Thank you very much for the reply.	[reply]
Re: Dynamically creating a hash based on a variable by cdarke (Prior) on Apr 15, 2007 at 13:54 UTC
I hope I have interpreted your question correctly, if not apologies. You can find out if the hash exists with: if defined. However, you might consider another way of doing this. I suggest you create a single hash where the keys are the dates. The value to each key would be a reference to an anonymous hash, like this: `$hash{$date} = {};` [download] To "play with the hash", adding values would be: `$hash{$date}->{key} = value;` [download] To see if the hash exists, use exists: `if (exists $hash{$date}) {...}` [download]	[reply] [d/l] [select]
Re^2: Dynamically creating a hash based on a variable by beethamd (Initiate) on Apr 15, 2007 at 14:13 UTC
Thank you - Seems I need to read about hashes tonight.	[reply]
Re: Dynamically creating a hash based on a variable by GrandFather (Saint) on Apr 15, 2007 at 13:57 UTC
It may be that you want something like: `use strict; use warnings; my %dates; while (<DATA>) { next unless /(.)(..)(..)(.{4})/; my $date = "$4$3$2"; if (exists $dates{$date}) { print "<TD class=table_font>$date - Hash exists</TD>\n"; # play with hash } else { print "<TD class=table_font>$date - Hash doesn't exist</TD>\n" +; # create the hash $dates{$date} = undef; # play with hash } } __DATA__ x11112007 y11122007 p11112007 x11132007` [download] Prints: `<TD class=table_font>20071111 - Hash doesn't exist</TD> <TD class=table_font>20071211 - Hash doesn't exist</TD> <TD class=table_font>20071111 - Hash exists</TD> <TD class=table_font>20071311 - Hash doesn't exist</TD>` [download] Note that it helps a lot to understand what you are trying to do if you provide a little data and tell us what you expect to see and what you actually see. DWIM is Perl's answer to Gödel	[reply] [d/l] [select]
Re: Dynamically creating a hash based on a variable by naikonta (Curate) on Apr 15, 2007 at 16:36 UTC
`my $state = $hash{$date}++ ? 'exists' : "doesn't exist"; print qq(<TD class="table_font">$date - Hash $state</TD>\n);` [download] Or, if you really want to play with it, `if ($hash{$date}++) { print qq(<TD class="table_font">$date - Hash exists</TD>\n); } else { print qq(<TD class="table_font">$date - Hash doesn't exist</TD>\n); }` [download] Open source softwares? Share and enjoy. Make profit from them if you can. Yet, share and enjoy!	[reply] [d/l] [select]