I'm trying to get a Ruby Array of Arrays and group it in order to count its values.
The Array has a month, and a boolean:
array = [["June", false], ["June", false], ["June", false], ["October", false]]
I will like to end with a new Array that let me know how many false vs trues in each month.
Example: (where the first item in the Array is month, the second is the count of false and the third is the count of true)
new_array = [["June", 3, 0]], ["October", 1, 0]]
Using Enumerable#group_by:
Array.group_by { |month, b| month }.map{ |month, xs|
[month,
xs.count {|_, b| !b}, # false
xs.count {|_, b| b}] # true
}
# => [["June", 3, 0], ["October", 1, 0]]
Enumerable#group_by returns a Hash, not an Array.
map call. (and need further processing to count true, falses)
array.map(&:dup) # we’ll mutate it
.group_by(&:shift)
.flat_map { |k, v| [k, v.partition { |e| e }.map(&:count)] }
#⇒ [ ['June', [0, 3] ], ['October', [0, 1] ] ]
Enumerable#group_by is a useful tool, but in this case it's not the best one.
If we start with a Hash whose default value is [0, 0] we can do (almost) everything—look up the month (or get the default value of it's a new month) and add 1 to the appropriate index (0 for false and 1 for true)—in a single step:
array = [["June", false], ["June", false], ["June", false], ["October", false]]
hsh = Hash.new {|h,k| h[k] = [0,0] }
array.each {|mo, bool| hsh[mo][bool ? 1 : 0] += 1 }
p hsh.map(&:flatten)
# => [["June", 3, 0], ["October", 1, 0]]
group_by we have to iterate over the data four times: Once to do the grouping, once to count the trues, once more for the falses, and then again to turn the Hash into an Array. With the above, we only have to iterate twice: Once to do the counting and once to turn it into an Array.I think this would make a good use case for a reduce operation. You would do something like:
arr = [["June", false], ["June", false], ["June", false], ["October", false]]
logical_group = Hash.new{|h, k| h[k] = Hash.new(0) }
arr.reduce(logical_group) do |red, ele|
red[ele.first][:false_count] += 1 unless ele.last
red[ele.first][:true_count] += 1 if ele.last
red
end.reduce([]) do |red, (key, value)|
red << [key, value[:false_count], value[:true_count]]
red
end
Array.group_by {|(Array, boolean)| Array }.map {|Array, match| [Array, match.count] }
