Ruby Array of hash, and map function

Question

I have an array of hashes, like this:

my_array = [{foo:1,bar:"hello",baz:3},{foo:2,bar:"hello2",baz:495,foo_baz:"some_string"},...] 
#there can be arbitrary many in this list.
#There can also be arbitrary many keys on the hashes.

I want to create a new array that is a copy of the last array, except that I remove any :bar entries.

my_array2 = [{foo:1,baz:3},{foo:2,baz:495,foo_baz:"some_string"},...]

I can get the my_array2 by doing this:

my_array2 = my_array.map{|h| h.delete(:bar)}

However, this changes the original my_array, which I want to stay the same.

Is there a way of doing this without having to duplicate my_array first?

@Cort3z - Just one more note: You will end up with a new array with new hashes regardless of whether you use dup or not. Those are however a shallow copies, so they won't use too much additional memory. — BroiSatse
– BroiSatse, Commented May 8, 2014 at 21:25
Yeah, they should be reference only, which is ok for me. The reason why I don't want to do deep copy is that the stuff in my hash is potentially very big (strings of length 1000+). — Automatico
– Automatico, Commented May 8, 2014 at 21:32

Philipp Antar · Accepted Answer · 2014-05-08 14:07:30Z

5

one of many ways to accomplish this:

my_array2 = my_array.map{|h| h.reject{|k,v| k == :bar}}

answered May 8, 2014 at 14:07

Philipp Antar

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6 Comments

Santhosh Over a year ago

beat me by 21 secs :)

Cary Swoveland Over a year ago

I would say Hash#reject is purpose-built for this. One tiny detail: I would make reject's block variables |k,_| to emphasize the fact you are only using the key (as I see @Santosh has done).

Automatico Over a year ago

@CarySwoveland: Two small questions: Why is reject better than select? Is it faster, or is it simply readability? And: same question for the _. Faster or just pretty?

Cary Swoveland Over a year ago

Sorry for misleading you, Cortt3z. reject and select are more-or-less equivalent here, as both must check all the elements. I was poking fun at @Santosh, suggesting he chose select only because Philipp had used reject.

Automatico Over a year ago

Ahh.. hehe, get it. Funny that I selected the reject ^^ But really tough, the _. I remember reading something about that a long time ago. Is it faster, or just pretty? I would guess it is a bit faster if it does not have to allocate any variables for each element.

|

Santhosh · Accepted Answer · 2014-05-08 14:07:52Z

2

my_array.map {|h| h.select{|k, _| k != :bar} }
# => [{:foo=>1, :baz=>3}, {:foo=>2, :baz=>495, :foo_baz=>"some_string"}]

answered May 8, 2014 at 14:07

Santhosh

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1 Comment

Cary Swoveland Over a year ago

I would say 21 seconds counts as a tie. +1 for both of you. reject is so much better though. :-)

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