[ruby] How to return a part of an array in Ruby?

With a list in Python I can return a part of it using the following code:

foo = [1,2,3,4,5,6]
bar = [10,20,30,40,50,60]
half = len(foo) / 2
foobar = foo[:half] + bar[half:]

Since Ruby does everything in arrays I wonder if there is something similar to that.

Ruby 2.6 Beginless/Endless Ranges

(..1)
# or
(...1)

(1..)
# or
(1...)

[1,2,3,4,5,6][..3]
=> [1, 2, 3, 4]

[1,2,3,4,5,6][...3]
 => [1, 2, 3]

ROLES = %w[superadmin manager admin contact user]
ROLES[ROLES.index('admin')..]
=> ["admin", "contact", "user"]

another way is to use the range method

foo = [1,2,3,4,5,6]
bar = [10,20,30,40,50,60]
a = foo[0...3]
b = bar[3...6]

print a + b 
=> [1, 2, 3, 40, 50 , 60]

If you want to split/cut the array on an index i,

arr = arr.drop(i)

> arr = [1,2,3,4,5]
 => [1, 2, 3, 4, 5] 
> arr.drop(2)
 => [3, 4, 5] 

You can use slice() for this:

>> foo = [1,2,3,4,5,6]
=> [1, 2, 3, 4, 5, 6]
>> bar = [10,20,30,40,50,60]
=> [10, 20, 30, 40, 50, 60]
>> half = foo.length / 2
=> 3
>> foobar = foo.slice(0, half) + bar.slice(half, foo.length)
=> [1, 2, 3, 40, 50, 60]

By the way, to the best of my knowledge, Python "lists" are just efficiently implemented dynamically growing arrays. Insertion at the beginning is in O(n), insertion at the end is amortized O(1), random access is O(1).

I like ranges for this:

def first_half(list)
  list[0...(list.length / 2)]
end

def last_half(list)
  list[(list.length / 2)..list.length]
end

However, be very careful about whether the endpoint is included in your range. This becomes critical on an odd-length list where you need to choose where you're going to break the middle. Otherwise you'll end up double-counting the middle element.

The above example will consistently put the middle element in the last half.