Ruby inject with index and brackets - ruby

Question

I try to clean my Code. The first Version uses each_with_index. In the second version I tried to compact the code with the Enumerable.inject_with_index-construct, that I found here.
It works now, but seems to me as obscure as the first code.
Add even worse I don't understand the brackets around element,index in
.. .inject(groups) do |group_container, (element,index)|
but they are necessary
What is the use of these brackets?
How can I make the code clear and readable?
FIRST VERSION -- WITH "each_with_index"
class Array
# splits as good as possible to groups of same size
# elements are sorted. I.e. low elements go to the first group,
# and high elements to the last group
#
# the default for number_of_groups is 4
# because the intended use case is
# splitting statistic data in 4 quartiles
#
# a = [1, 8, 7, 5, 4, 2, 3, 8]
# a.sorted_in_groups(3) # => [[1, 2, 3], [4, 5, 7], [8, 8]]
#
# b = [[7, 8, 9], [4, 5, 7], [2, 8]]
# b.sorted_in_groups(2) {|sub_ary| sub_ary.sum } # => [ [[2, 8], [4, 5, 7]], [[7, 8, 9]] ]
def sorted_in_groups(number_of_groups = 4)
groups = Array.new(number_of_groups) { Array.new }
return groups if size == 0
average_group_size = size.to_f / number_of_groups.to_f
sorted = block_given? ? self.sort_by {|element| yield(element)} : self.sort
sorted.each_with_index do |element, index|
group_number = (index.to_f / average_group_size).floor
groups[group_number] << element
end
groups
end
end
SECOND VERSION -- WITH "inject" AND index
class Array
def sorted_in_groups(number_of_groups = 4)
groups = Array.new(number_of_groups) { Array.new }
return groups if size == 0
average_group_size = size.to_f / number_of_groups.to_f
sorted = block_given? ? self.sort_by {|element| yield(element)} : self.sort
sorted.each_with_index.inject(groups) do |group_container, (element,index)|
group_number = (index.to_f / average_group_size).floor
group_container[group_number] << element
group_container
end
end
end

Answer 1

What is the use of these brackets?
It's a very nice feature of ruby. I call it "destructuring array assignment", but it probably has an official name too.
Here's how it works. Let's say you have an array
arr = [1, 2, 3]
Then you assign this array to a list of names, like this:
a, b, c = arr
a # => 1
b # => 2
c # => 3
You see, the array was "destructured" into its individual elements. Now, to the each_with_index. As you know, it's like a regular each, but also returns an index. inject doesn't care about all this, it takes input elements and passes them to its block as is. If input element is an array (elem/index pair from each_with_index), then we can either take it apart in the block body
sorted.each_with_index.inject(groups) do |group_container, pair|
element, index = pair
# or
# element = pair[0]
# index = pair[1]
# rest of your code
end
Or destructure that array right in the block signature. Parentheses there are necessary to give ruby a hint that this is a single parameter that needs to be split in several.
Hope this helps.

Answer 2

lines = %w(a b c)
indexes = lines.each_with_index.inject([]) do |acc, (el, ind)|
acc << ind - 1 if el == "b"
acc
end
indexes # => [0]

Answer 3

What is the use of these brackets?
To understand the brackets, first you need to understand how destruction works in ruby. The simplest example I can think of this this:
1.8.7 :001 > [[1,3],[2,4]].each do |a,b|
1.8.7 :002 > puts a, b
1.8.7 :003?> end
1
3
2
4
You should know how each function works, and that the block receives one parameter. So what happens when you pass two parameters? It takes the first element [1,3] and try to split (destruct) it in two, and the result is a=1 and b=3.
Now, inject takes two arguments in the block parameter, so it is usually looks like |a,b|. So passing a parameter like |group_container, (element,index)| we are in fact taking the first one as any other, and destructing the second in two others (so, if the second parameter is [1,3], element=1 and index=3). The parenthesis are needed because if we used |group_container, element, index| we would never know if we are destructing the first or the second parameter, so the parenthesis there works as disambiguation.
9In fact, things works a bit different in the bottom end, but lets hide this for this given question.)

Answer 4

Seems like there already some answers given with good explanation. I want to add some information regards the clear and readable.
Instead of the solution you chose, it is also a possibility to extend Enumerable and add this functionality.
module Enumerable
# The block parameter is not needed but creates more readable code.
def inject_with_index(memo = self.first, &block)
skip = memo.equal?(self.first)
index = 0
self.each_entry do |entry|
if skip
skip = false
else
memo = yield(memo, index, entry)
end
index += 1
end
memo
end
end
This way you can call inject_with_index like so:
# m = memo, i = index, e = entry
(1..3).inject_with_index(0) do |m, i, e|
puts "m: #{m}, i: #{i}, e: #{e}"
m + i + e
end
#=> 9
If you not pass an initial value the first element will be used, thus not executing the block for the first element.

Answer 5

In case, someone is here from 2013+ year, you have each_with_object and with_index for your needs:
records.each_with_object({}).with_index do |(record, memo), index|
memo[record.uid] = "#{index} in collection}"
end