Adding all relevant elements in array by a given element

Question

I'm for some reason hitting a wall here. I need help figuring this out please. I have the following type of 2D array:

[["Bob", "Car", 25000],
["Bob", "TV", 5000],
["Bob", "dog", 1000],
["Sue", "Cat", 1000],
["Sue", "Car", 10000],
["Bob", "shoes", 100],
["Carol", "car", 20000]]

And I need to generate an array that is the sum of each of these people's total from the third element of each sub array. ie:

[["Bob", 31100],
["Sue", 11000],
["Carol", 20000]]

Right now I have a complicated and contrived solution using two loops that iterate through the entire array for each of its own elements. Is there an easier more streamlined way to do this? Especially since my data sets will be quite large. I know ruby has a bunch of awesome enumerables that seem like would fit here, but I can't quite think of how to fit them in.

87, when you include an example in your question (a good thing), it's best to assign a variable to each input object. Here that might be arr = [['Bob', 'Car', 25000],...]. That way, readers can refer to arr in answers and comments without having to define it. — Cary Swoveland
– Cary Swoveland, Commented Oct 23, 2015 at 18:54

Cary Swoveland · Accepted Answer · 2015-10-23 18:41:38Z

4

You can use a counting hash:

arr = [["Bob", "Car", 25000],
       ["Bob", "TV", 5000],
       ["Bob", "dog", 1000],
       ["Sue", "Cat", 1000],
       ["Sue", "Car", 10000],
       ["Bob", "shoes", 100],
       ["Carol", "car", 20000]]

arr.each_with_object(Hash.new(0)) { |(name,_,total),h| h[name] += total }
  #=> {"Bob"=>31100, "Sue"=>11000, "Carol"=>20000}

Hash::new is used to create a hash with a default value of zero. That means that if the hash h does not have a key name, h[name] returns zero. Since:

h[name] += total

expands to:

h[name] = h[name] + total

h[name] on the right side of the equality returns zero when h does not have a key name.

edited Oct 23, 2015 at 18:41

answered Oct 23, 2015 at 18:36

Cary Swoveland

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Comments

Hyung Cho · Accepted Answer · 2015-10-23 18:39:13Z

1

arr = [["Bob", "Car", 25000],
["Bob", "TV", 5000],
["Bob", "dog", 1000],
["Sue", "Cat", 1000],
["Sue", "Car", 10000],
["Bob", "shoes", 100],
["Carol", "car", 20000]]

hash_result = Hash.new(0)

arr.each do |record|
  hash_result[record[0]] += record[2]
end

This will give you a hash result. If you want an array, just call to_a on hash_result.

answered Oct 23, 2015 at 18:39

Hyung Cho

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2 Comments

Cary Swoveland Over a year ago

each returns arr, so you need hash_result on a line by itself after the each block. I'll delete this comment when you've seen it, so no reply is req'd.

MS-87 Over a year ago

Thanks! This is a very simple and streamlined solution. Perfect

Schwern · Accepted Answer · 2015-10-23 18:38:39Z

1

You can iterate through the array of purchases and store the totals in a hash.

purchases = [
  ["Bob", "Car", 25000],
  ["Bob", "TV", 5000],
  ["Bob", "dog", 1000],
  ["Sue", "Cat", 1000],
  ["Sue", "Car", 10000],
  ["Bob", "shoes", 100],
  ["Carol", "car", 20000]
]

totals = Hash.new(0)  # initialize the values to 0
purchases.each { |purchase|
  totals[purchase[0]] += purchase[2]
}

answered Oct 23, 2015 at 18:38

Schwern

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Comments

Nakilon · Accepted Answer · 2015-11-01 16:13:05Z

1

.group_by(&:first).map do |who, group|
  [who, group.map(&:last).inject(:+)]
end

answered Nov 1, 2015 at 16:13

Nakilon

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