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Re: Is there a simple syntax to logically slice a hash?

by TomDLux (Vicar)
on Feb 21, 2013 at 16:29 UTC ( #1019976=note: print w/replies, xml ) Need Help??

in reply to Is there a simple syntax to logically slice a hash?

Using the perl debugger, for speed and convenience ...

# Lets create a hash that has some similar keys and some unsimilar key +s %h1 = ( cat => 1, dog=> 2, catalog => 3, doggerel => 4 } x \%h1 0 HASH(0x7f9cb41aaad0) 'cat' => 1 'catalog' => 3 'dog' => 2 'doggerel' => 4 # See ALL the keys ... x keys %h1 0 'cat' 1 'doggerel' 2 'catalog' 3 'dog' # grep allows us to filter that list x grep { /cat/ } keys %h1 0 'cat' 1 'catalog' # You access a single hash element with $h{$key} # You can access multiple elements, called a slice, # using an array or list of keys. In this case you have to put # a '@' sigil in front, rather than a '$', to indicate #you're expecting multiple values x @h1{grep { /cat/ } keys %h1} 0 1 1 3 # So 'cat' -like keys have the values, '1' and '3'. # If you wanted to assign those to the keys in # another hash, I would save the set of keys in a temp: @a = grep { /cat/ } keys %h1 @b{@a} = @h1{@a} DB<14> x \%b 0 HASH(0x7f9cb41bd0e8) 'cat' => 1 'catalog' => 3

The last bit is saying, create a new hash called 'b' ( we know it's a hash because of how it's elements are being referenced with curly braces ), and create elements for each value in @a, assigning the elements extraced from %h1. You COULD repeat the 'grep' part, once in the LHS and once on the RHS of the assignment, to really get a one-line assignment, but that's messy. While you don't know the order of the elements, it would be the same on each side, but if the set is huge, you don't want to go through all that .. and it's a maintainence problem, and hard to verify the two are identical; so I would split it into two parts.

As Occam said: Entia non sunt multiplicanda praeter necessitatem.

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Re^2: Is there a simple syntax to logically slice a hash?
by BrowserUk (Pope) on Feb 21, 2013 at 16:41 UTC

    Ouch!. That creates 5 unneccesary temporary lists and an unneccesary temporary array:

    #T1 L2 L1 @a = grep { /cat/ } keys %h1 # L5 L4 L3 @b{@a} = @h1{@a}

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