||= in Hash Slice

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

I want to do some hash slice assigns.

my $key_fields    = [ qw( trade_ref sec_code lot of other fields ) ];
my $copy_fields   = [ qw( item_period other fields) ];

my $audit = {};

@{$audit}{@$key_fields}    = @{$item}{@$key_fields};
# $item is a hashref with some but maybe not all of the
# above fields defined
$audit->{$_} = $item->{$_} || '' for @$copy_fields;
[download]

Alternatively :

@{$audit}{@$copy_fields} = @{$item}{@$copy_fields};
$audit->{$_} ||= '' for @$copy_fields;
[download]

These work, but I really want to do :

@{$audit}{@$copy_fields} = @{$item}{@$copy_fields} || '';
[download]

to make sure there are no undefs. Can I do the above as a single Hash slice assignment ?

--
Brovnik

Comment on \|\|= in Hash Slice Select or Download Code

Replies are listed 'Best First'.
Re: \|\|= in Hash Slice by ikegami (Patriarch) on Jan 06, 2009 at 12:40 UTC
Three ways: `@{$audit}{@$copy_fields} = map $_ \|\| '', @{$item}{@$copy_fields};` [download] `$_ \|\|= '' for @{$audit}{@$copy_fields} = @{$item}{@$copy_fields};` [download] `no warnings 'uninitialized';` [download] I only recommend the first.	[reply] [d/l] [select]
Re: \|\|= in Hash Slice by Corion (Patriarch) on Jan 06, 2009 at 12:00 UTC
I'm not aware of a convenient per-item combination. I hear that Perl 6 will have "hyper-operators" for that, when it reaches a stable release version >= 1.0. I'd do that with a map, as I always use map when transforming one list into another: `@{$audit}{@$copy_fields} = map { defined $item->{$_} ? $item->{$_} : ' +' } @$copy_fields;` [download] Your code will happily replace a value of "0" by "", and I guess that's not intended.	[reply] [d/l]
Re^2: \|\|= in Hash Slice by Brovnik (Hermit) on Jan 06, 2009 at 12:15 UTC
Yup, I guess that works, but I was hoping for something a bit more elegant. `@{$audit}{@$copy_fields} = map { $item->{$_} \|\| '' } @$copy_fields;` [download] Is probably the nearest I'll get. P.S. The '' vs. 0 difference is irrelevant here in my case, I just want avoid undefs. -- Brovnik	[reply] [d/l]
Re^3: \|\|= in Hash Slice by Bloodnok (Vicar) on Jan 06, 2009 at 17:56 UTC
That may not, probably won't, work in all cases ... consider when the value of `$item->{$_}` is `0`: in this case, the wanted value of `0` will be overwritten by the empty string, since `$item->{$_}` is, from a perl POV, false. Hence Corions suggestion, tho' not quite what you were seeking, is better. A user level that continues to overstate my experience :-))	[reply] [d/l] [select]
Re: \|\|= in Hash Slice by gone2015 (Deacon) on Jan 06, 2009 at 12:08 UTC
I don't think so. But this should work: `@{$audit}{@$copy_fields} = map { defined($_) ? $_ : '' } @{$item}{@$ +copy_fields} ;` [download]	[reply] [d/l]
Re^2: \|\|= in Hash Slice by Brovnik (Hermit) on Jan 06, 2009 at 12:12 UTC
$_ here is the key, which will always be defined, so the defined($item->{$_}) as above is needed. Edit: Oops, misread the reply, apologies. -- Brovnik	[reply]
Re^3: \|\|= in Hash Slice by ikegami (Patriarch) on Jan 06, 2009 at 12:31 UTC
No it's not. It's the value.	[reply]
Re: \|\|= in Hash Slice by JavaFan (Canon) on Jan 06, 2009 at 12:33 UTC
Can I do the above as a single Hash slice assignment ? No. The problem is the '\|\|' operator which provides scalar context to its LHS argument. You'll have to type 'map' if you want a map... ;-)	[reply]


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