how to check which index $x is in an hash

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

I have a very small image viewer script which has a database full of id numbers from 1 - infinity. I delete a few of them from the database and my script knows how to ++ or -- to the next eligible entry in case there's a gap in ids.

I wrote the code quite a few years ago and now I'm thinking there's GOT to be a better way to do this. Here's a sample of my script finding the next available image

  # determine next eligible image
  my $next_cnt = 0;
  while ($lowest)
  {  
     $next_cnt++;
     if (exists $upload{$next_cnt})
     { 
        last;
     }

  } 
  $next_cnt = $next_cnt + 2; # jump one ahead
[download]

I want to know if there's a way to determine what index a certain number is, inside my hash. Now I know indexes are used with arrays, but is there a similar way to do this in hashes if the keys are numbers?

Ie:

1,2,3,5,9,10
[download]

We are on image 5 and want to find out what the previous and next number in sequence would be..

my $prev = $db{$picid}..
[download]

There just has to be a way to automatically determine, in a sorted hash, what the previous and next items would be. Right?

Comment on how to check which index $x is in an hash Select or Download Code

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Re: how to check which index $x is in an hash by GrandFather (Saint) on May 21, 2007 at 21:40 UTC
If you use the lookup a lot then it's worth making a lookup table. Consider use strict; use warnings; my %upload = (1 => 'one', 2 => 'two', 3 => 'three', 5 => 'five', 9 => +'nine', 10 => 'ten'); my @keys = sort {$a <=> $b} keys %upload; my %lookup = map { $keys[$_] => { prev => $_ ? $keys[$_ - 1] : undef, next => $_ != $#keys ? $keys[$_ + 1] : undef, } } 0 .. $#keys; for my $key (1, 5, 10) { if (defined $lookup{$key}{next}) { print "$lookup{$key}{next} follows $key\n"; } else { print "$key is the last item\n"; } if (defined $lookup{$key}{prev}) { print "$lookup{$key}{prev} precedes $key\n"; } else { print "$key is the first item\n"; } } [download] Prints: `2 follows 1 1 is the first item 9 follows 5 3 precedes 5 10 is the last item 9 precedes 10` [download] The lookup creation is a little more succinct if you make use of autovivification: `my %lookup = map {$keys[$_] => {prev => $keys[$_ - 1], next => $keys[$ +_ + 1]}} 0 .. $#keys;` [download] Update: see johngg's reply below! DWIM is Perl's answer to Gödel	[reply] [d/l] [select]
Re^2: how to check which index $x is in an hash by johngg (Canon) on May 21, 2007 at 22:13 UTC
I think your more succinct version will have a problem with the first key. This is because `$_` going into the `map` is going to be 0 thus `prev => $keys[0 - 1]` will result in `prev => 10`. At the other end, I think that `next => $keys[$#keys + 1]` will do the right thing as pointing off the end of the array should result in `undef`. Cheers, JohnGG	[reply] [d/l] [select]
Re^3: how to check which index $x is in an hash by GrandFather (Saint) on May 21, 2007 at 22:19 UTC
Argh, you are quite right! I "checked" it, honest, I just didn't notice the line that says "10 precedes 1"! DWIM is Perl's answer to Gödel	[reply]
Re: how to check which index $x is in an hash by shmem (Chancellor) on May 21, 2007 at 21:31 UTC
`my @sorted_indices = sort {$a <=> $b} keys %upload;` [download] Then iterate over `@sorted_indices`. --shmem _($_=" "x(1<<5)."?\n".q·/)Oo. G°\ / /\_¯/(q / ---------------------------- \__(m.====·.(_("always off the crowd"))."· ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}	[reply] [d/l] [select]
Re^2: how to check which index $x is in an hash by Anonymous Monk on May 21, 2007 at 21:50 UTC
Right, that's my question though. I have an array of hash keys. If I have an array with 1,2,5,7,10, how do I determine what index 5 is? I know it's 2, but how can the script know?	[reply]
Re^3: how to check which index $x is in an hash by shmem (Chancellor) on May 22, 2007 at 05:16 UTC
how can the script know? The script doesn't know anything, ever. It's you who knows, or doesn't, and the script reflects that knowledge. If I have an array with 1,2,5,7,10, how do I determine what index 5 is? I know it's 2 You make another hash, keyed with the contents of your array slots, whose values are their indices: `# (merging previous post) my @sorted_indices = sort {$a <=> $b} keys %upload; my %lookup_hash; @lookup_hash{@sorted_indices} = 0 .. $#sorted_indices;` [download] Now you lookup e.g. 5 in %lookup_hash: `my $i = $lookup_hash{5}; print $x,"\n"; # prints 2 # get the next element from the hash my $next = $sorted_indices[$x + 1]; # $next holds 7 # get your image my $image = $uploads{$next};` [download] --shmem _($_=" "x(1<<5)."?\n".q·/)Oo. G°\ / /\_¯/(q / ---------------------------- \__(m.====·.(_("always off the crowd"))."· ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}	[reply] [d/l] [select]
Re: how to check which index $x is in an hash by Tux (Canon) on May 21, 2007 at 21:31 UTC
like in `my @image_ids = sort { $a <=> $b } keys %db;` [download] ? Enjoy, Have FUN! H.Merijn	[reply] [d/l]
Re: how to check which index $x is in an hash by Anonymous Monk on May 21, 2007 at 21:28 UTC
Or heck, I could make an array full of hash keys. Is there a way to detect previous/next index in arrays?	[reply]