Passing elements from one array to another, generating CSV - arrays

Question

I'm trying to write a program in Ruby that allows one array to receive information from another array. Basically, I have a multidimensional array called "student_array" that contains information on a few students
student_array = [["Mike", 13, "American", "male"],
["Grace", 12, "Canadian", "female"],
["Joey", 13, "American", "male"],
["Lily", 13, "American", "female"]
]
I also initialized two other arrays that will count nationalities:
nationality_array = Array.new
nationality_count = Array.new
The purpose of this program is to loop through student array, count the different nationalities of the students, and create a CSV file that will contain the headers of the different nationalities, and a count for each one.
Expected output.csv
American, Canadian
3, 1
Here is the code I have so far
student_array.each do |student|
#pushes the nationality string into the nationality array
nationality_array.push(student[2])
end
so the nationality_array should currently look like this:
nationality_array = ["American", "Canadian", "American", "American"];
nationality_array.uniq = ["American", "Canadian"];
So I will have two headers - "American" and "Canadian"
Now I need a way to loop through the student_array, count up each instance of "American" and "Canadian", and somehow assign it back to the nationality array. I'm having a hard time visualizing how to go about this. This is what I have so far--
american_count = 0;
canadian_count = 0;
student_array.each do |student|
if student[2] = "American"
american_count++
elsif student[2] = "Canadian"
canadian_count++
end
end
nationality_count.push(american_count);
nationality_count.push(canadian_count);
Okay, now I have those counts in the nationality_count array, but how can I pass it to a CSV, making sure that they are assigned to the right headers? I have a feeling that my code is very awkward and could be much more streamlined as well.
It would probably look something like this?
CSV.open("output/redemptions.csv", "wb") do |csv|
csv << [nationality_array]
csv << [nationality_count]
end
Can anyone provide any insight into a cleaner way to go about this?

Answer 1

You could use a Hash to group the counts by nationality instead of different arrays.
nationalities_count = student_array.each_with_object(Hash.new(0)) do |student, hash|
nationality = student[2]
hash[nationality] += 1
end
That will give you a Hash that would look like
{ "American" => 2, "Canadian" => 1 }
You could then use Hash#to_a and Array#transpose like so:
hsh = { "American" => 2, "Canadian" => 1 }
=> {"American"=>2, "Canadian"=>1}
2.4.2 :002 > hsh.to_a
=> [["American", 2], ["Canadian", 1]]
2.4.2 :003 > hsh.to_a.transpose
=> [["American", "Canadian"], [2, 1]]
Finally, to output the CSV file all you need to do is write the arrays into the file
nationalities_with_count = hash.to_a.transpose
CSV.open("output/redemptions.csv", "wb") do |csv|
csv << nationalities_with_count[0]
csv << nationalities_with_count[1]
end

Answer 2

Array#group_by in Ruby core and Hash#transform_values in ActiveSupport are two very versitile methods that can be used here:
require 'active_support/all'
require 'csv'
student_array = [
["Mike", 13, "American", "male"],
["Grace", 12, "Canadian", "female"],
["Joey", 13, "American", "male"],
["Lily", 13, "American", "female"]
]
counts = student_array.group_by { |attrs| attrs[2] }.transform_values(&:length)
# => => {"American"=>3, "Canadian"=>1}
CSV.open("output/redemptions.csv", "wb") do |csv|
csv << counts.keys
csv << counts.values
end
puts File.read "output/redemptions.csv"
# => American,Canadian
# 3,1
.group_by { |attrs| attrs[2] } turns the array into a hash, where keys are the unique values for attrs[2], and values are a list of elements that have that attrs[2]. At this point you can use transform_values to turn those values into numbers representing their length (meaning, how many elements have that specific attrs[2]). The keys and values can then be extracted from the hash as separate arrays.

Answer 3

You even don’t need a CSV tool here:
result =
student_array.
map { |a| a[2] }. # get nationalities
group_by { |e| e }. # hash
map { |n, c| [n, c.count] }. # map values to count
transpose. # put data in rows
map { |row| row.join ',' }. # join values in a row
join($/) # join rows
#⇒ American,Canadian
# 3,1
Now you have a string that is valid CSV, just spit it out to the file.