from array to hash

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

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Re: from array to hash by ikegami (Patriarch) on Mar 09, 2006 at 15:16 UTC
`%b = map { $_ => 1 } grep defined, @a;` [download] or `%b = map { defined ? ($_ => 1) : () } @a;` [download] or you could avoid getting undef in the first place by using `splice` instead of `delete` or `undef` to delete the element from the array: `@a = qw( a b c d ); # Delete 1 starting at index 2. # What used to be at index 3 is now at index 2. splice(@a, 2, 1); print("\@a has ", scalar(@a), "elements.\n");` [download]	[reply] [d/l] [select]
Re^2: from array to hash by jeanluca (Deacon) on Mar 09, 2006 at 15:44 UTC
Thanks, but what if the hash already has some values, like: `#! /usr/bin/perl @a = qw(a b c d ); $b{x} = 10 ; %b = map { $_, 1} splice(@a, 0, scalar(@a) ); print "$_ --> $b{$_}\n" foreach (keys %b) ;` [download] all values of %b are erased using map..... Furthermore, is scalar(@a) really necessary, or can you as well do @a ? Luca	[reply] [d/l]
Re^3: from array to hash by friedo (Prior) on Mar 09, 2006 at 15:48 UTC
In that case, you can use a loop instead: `$b{$_} = 1 for grep defined, @a;` [download]	[reply] [d/l]
Re^3: from array to hash by ikegami (Patriarch) on Mar 09, 2006 at 17:20 UTC
You've severly misunderstood the use of `splice`. Did you read the docs? `@a = qw(a b c d ); undef $a[2]; $b{x} = 10 ; $b{$_} = 1 foreach grep defined, @a;` [download] or `@a = qw(a b c d ); splice(@a, 2, 1); $b{x} = 10 ; $b{$_} = 1 foreach @a;` [download]	[reply] [d/l] [select]
Re: from array to hash by radiantmatrix (Parson) on Mar 09, 2006 at 16:04 UTC
Maybe map isn't what you want: `my %b; foreach (@a) { $b{$_} = 1 if defined $_ }` [download] Using foreach is probably faster anyhow. FWIW: `use Benchmark ':all'; my @a; push @a, rand(500) for (0..1000); #build array $a[int(rand(1000))] = undef; #delete an element (sort of) $a[int(rand(1000))] = undef; #delete an element (sort of) cmpthese( 1_000, { 'map' => sub { my %b = map{ $_ => 1 } @a }, 'foreach' => sub { my %b; foreach(@a) { $b{$_} = 1 if defined $_ } + }, }); __END__ Rate map foreach map 374/s -- -44% foreach 673/s 80% --` [download] Note: with warnings on, the map is even slower, because a warning is generated every time an undefined value is encountered in the array. I turned warnings off after discovering this, and so the numbers above represent a sort of best-case. <-radiant.matrix-> A collection of thoughts and links from the minds of geeks The Code that can be seen is not the true Code I haven't found a problem yet that can't be solved by a well-placed trebuchet	[reply] [d/l] [select]
Re: from array to hash by linux454 (Pilgrim) on Mar 09, 2006 at 17:16 UTC
Like previous posters have said use splice(@arr, $off, $len) to delete items from your lists, though if you can't use slice then you will have to test for definedness yourself. Example: `my @a = qw( a b c d ); # delete a & c splice(@a, 0, 1) splice(@1, 1, 1)` [download] As for converting the array into a hash while preserving the previous contents something like this will do the job nicely: `my @a = qw( a b c d ); my %b = ( 'e' => 2 ); %b = (%b, map { $_ => 1} grep defined, @a);` [download] This preserves the previous contents of %b while adding keys for @a into %b.	[reply] [d/l] [select]