return specific indexes of multidimensional array without side effects

Question

Given this multidimensional array:

a = [ [0,'abc'], [1,'def'] ]

I want to return just the keys (where keys are index 0s of subarray) and just the values (where values are index 1s of subarray):

a.keys
# => [0,1]

a.values
# => ['abc','def']

My solution to this is to use map to return a collection and use shift and pop to return the correct items:

a.map(&:shift)
=> [0, 1] 
a.map(&:pop)
=> ["abc", "def"]

Great it works, but then I noticed the side effects:

 a
 => [[], []]

Is there a way to get the functionality I have but without the side effects?

Not sure I understand the desired output, but Hash[a] will give you back {0 => "abc", 1 => "def"}. Is that what you're going for? — infused
– infused, Commented Jun 26, 2015 at 0:41
@infused my collection is an array of subarrays, not a hash. If it was a hash, it would be easy. I could just use keys and values methods. But with array there is no such keys and values method. I want all the first positions returned in one array and all the last positions returned in another array, as my example above achieves. The only problem is the side effects. — Donato
– Donato, Commented Jun 26, 2015 at 0:42
That's what converting the array to a Hash will do for you. I just posted a full answer. — infused
– infused, Commented Jun 26, 2015 at 0:45

infused · Accepted Answer · 2015-06-26 00:52:04Z

5

First, convert your array of arrays to a Hash:

a = [ [0,'abc'], [1,'def'] ]
h = Hash[a]

Then get the keys and values:

h.keys #=> [0, 1]
h.values # => ['abc', 'def']

You can also get at the same results more directly (if the sub-arrays always have only 2 elements):

a = [ [0,'abc'], [1,'def'] ]
a.map(&:first) # => [0, 1]
a.map(&:last)  # => ['abc', 'def']

answered Jun 26, 2015 at 0:44

infused

24.5k13 gold badges71 silver badges80 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Cary Swoveland · Accepted Answer · 2015-06-26 01:22:15Z

3

a = [ [0,'abc'], [1,'def'] ]

k,v = a.transpose
  #=> [[0, 1], ["abc", "def"]] 
k #=> [0, 1] 
v #=> ["abc", "def"]

I can't bring myself to write keys and values, as those words are generally reserved for hashes.

answered Jun 26, 2015 at 1:22

Cary Swoveland

111k6 gold badges69 silver badges105 bronze badges