This is a useless node

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

Hash slice from a hashref, into a hashref

ARGH. I want to delete this node; I just realized the question is useless. But I can't figure out how to delete it.

There's another question titled "Re: Hash slice from a hashref". However, it's actually asking for a slice of the values of a hash ref. I want a slice of an actual hash. Like this:

use strict; use warnings;
my %h=(a=>1,b=>2,c=>3);
my @x=('a','c');
my $hr=\%h;
my %g=%$hr{@x};      # plain old %g = %h{@x} also works
my $gr = \%g;
print $gr->{a}       # prints "1"
print $gr->{b}       # prints "Use of uninitialized value in print"
[download]

But I want to load the slice directly into a hashref, not a hash, because my %h is really big and I don't have enough RAM to make a copy of it. I want to do what you might think this would do:

my %h=(a=>1,b=>2,c=>3);
my @x=('a','c');
my $hr = \%h;
my $gr = $hr->{@x};
print $gr->{a}
[download]

Or maybe one of these:

my $gr = $hr{@x};
my $gr = $$hr->{@x};
my $gr = \%{$hr}{@x}
[download]

Alas, none of these work. Is there a way to get a slice of a hash (not of its values, but of the hash) without making a copy of the hash? Preferably from a hashref to the original hash, since that's what I've got.

I asked a question, then realized that there was no point in asking what I was asking.

So I have deleted it.

Perlmonks.org does not allow deletion of nodes, so here we are.

Comment on This is a useless node Select or Download Code

Replies are listed 'Best First'.
Re: This is a useless node by Marshall (Canon) on Apr 25, 2018 at 05:01 UTC
Update your original post. You can't delete it, but you can modify it and add stuff to it. Everybody here occasionally makes mistakes. Not a big deal.	[reply]
Re^2: This is a useless node by mxb (Pilgrim) on Apr 25, 2018 at 09:09 UTC
Marshall hits the nail on the head here. While you cannot delete your original node, you can edit it. If you originally asked a question for which you have found the answer, just add the answer into the question with an edit. This will enable others to find the question (and answer!) should they be searching for answers to the same (or a similar) question.	[reply]
Re: Hash slice from a hashref, into a hashref by haukex (Archbishop) on Apr 25, 2018 at 17:42 UTC
I just realized the question is useless. I'm not sure why you think so, how to take a slice of a hash is a valid question, maybe you could explain what resolution you found for your question? Anyway, there are several ways to make a (shallow) copy of a part of a hash, here are a few: Using map: `%slice = map { $_ => $hash{$_} } @keys;` Using Foreach Loops (in this case as a statement modifier): `$slice{$_} = $hash{$_} for @keys;` Using Slices: `@slice{@keys} = @hash{@keys};` Using Key/Value Hash Slices, new in Perl 5.20: `%slice = %hash{@keys};` my %h is really big and I don't have enough RAM to make a copy of it This makes the question interesting. It turns out that, at least on my machine with Perl 5.26, from the options above: `map` takes the most memory, about 180% as much as a regular `@` slice, a regular `@` slice takes about as much memory as a key/value `%` slice, a key/value `%` slice takes about as much memory as a regular `@` slice, and the `for` loop variant takes the least amount of memory, taking only about 88% of a regular `@` slice. Read more... Code for testing (828 Bytes) I want to load the slice directly into a hashref As it turns out, changing `my %slice` into `my $slice` and changing the syntaxes into the following doesn't really make a big difference: `$slice = { map { $_ => $hash{$_} } @keys };` - about 6% more memory `@{$slice}{@keys} = @hash{@keys};` - about the same amount of memory `$slice = {%hash{@keys}};` - about the same amount of memory `$slice->{$_} = $hash{$_} for @keys;` - about the same amount of memory On the other hand, if for example the values in your hash are huge, then perhaps a different approach is needed - for example, in your second hash you could store references to the values of the first hash. Or, you may not even need to create a second hash in the first place, perhaps simply keeping the list of keys around to select values from the first hash is enough.	[reply] [d/l] [select]
Re: This is a useless node by Your Mother (Archbishop) on Apr 25, 2018 at 04:56 UTC
Just about every question whether it seems pointless or silly or whatever contributes to the site and often sparks interesting replies so is considered worth keeping. It's the ethos of the place that things are transparent so deletions or silent redaction are frowned upon, sometimes openly blocked. I don't have a dog in the no deleted nodes fight but don't mind going with the flow either.	[reply]
Re: This is a useless node by haukex (Archbishop) on Apr 25, 2018 at 09:13 UTC
I asked a question, then realized that there was no point in asking what I was asking. Don't worry, just about everyone here has had moments like that. How do I change/delete my post? explains a bit more about what the others have said, that it's best to either edit your original post (or answer your own question), so that others who may have the same question can learn from it.	[reply]
Re: This is a useless node by virtualsue (Vicar) on Apr 25, 2018 at 11:08 UTC
I'm so tempted to frontpage this.	[reply]
Re^2: This is a useless node by Anonymous Monk on Apr 25, 2018 at 12:23 UTC
Because?	[reply]
Re^3: This is a useless node by Anonymous Monk on Apr 25, 2018 at 17:01 UTC
Streisand effect.	[reply]
Re^3: This is a useless node by virtualsue (Vicar) on Apr 26, 2018 at 19:04 UTC
Because not allowing users to delete nodes is frankly silly, not matter what tales are told about why it isn't allowed.	[reply]
Re^4: This is a useless node by Your Mother (Archbishop) on Apr 27, 2018 at 00:10 UTC
Re^4: This is a useless node by Anonymous Monk on Apr 26, 2018 at 20:23 UTC