Sum multiple attributes in array of objects

Question

I understand that in ruby we can sum the elements in an array like this

[1,2,3].sum 
[<Object a: 1>, <Object a:2>].sum(&:a)

How about running a sum on multiple attributes in the array of objects. For example:

arr = [<Object a:1, b:2, ..., n:10>, <Object a:1, b:2, ..., n:10>, ... <nth Object>]

What is the most efficient way to get the sum of multiple attributes a, b, ... n?

I have tried these methods:

sum_a = arr.sum(&:a)
sum_b = arr.sum(&:b)

a, b = 0, 0
arr.each do |obj|
  a += obj.a
  b += obj.b
end

I am wondering if there is a better way to go about this.

engineersmnky · Accepted Answer · 2020-05-04 19:25:19Z

2

You could create your own method for this:

def sum_multiple(arr, *keys)
  arr.each_with_object(keys.zip([0] * keys.size).to_h) do |o,obj|
    keys.each {|k| obj[k] += o.public_send(k)}
  end
end

Usage as:

class A 
  attr_reader :a,:b,:c 

  def initialize(a,b,c)
    @a,@b,@c = a,b,c
  end 
end 

arr = 10.times.map {|i| A.new(i,i*2,i*3)}

sum_multiple(arr,:a)
#=> {:a=>45}
sum_multiple(arr,:a,:b)
#=> {:a=>45, :b=>90}
sum_multiple(arr,:a,:b,:c)
#=> {:a=>45, :b=>90, :c=>135}
"Sum of everything: #{sum_multiple(arr,:a,:b,:c).sum(&:last)}" 
#=> "Sum of everything: 270"

Example

Note: If this is rails then you should look into combinations of group by and sum and put this on the database.

edited May 4, 2020 at 19:25

answered May 4, 2020 at 17:20

engineersmnky

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2 Comments

lacostenycoder Over a year ago

how have you defined the A class here?

engineersmnky Over a year ago

@lacostenycoder while the definition of the class isn't all that important since the methods #a, #b, and #c could be antyhing it does lend to reproducibility I have added the definition

lacostenycoder · Accepted Answer · 2020-05-04 19:25:33Z

1

If you want to sum only some of the key values you can do:

[{a: 1, b: 2, c: 3}, {a: 4, b: 5, c: 6}].flat_map{|h| h.slice(:a, :b).values}.sum

Or if you want to sum all values then just

[{a: 1, b: 2, c: 3}, {a: 4, b: 5, c: 6}].flat_map(&:values).sum

For objects who might not be hashes

arr.sum {|o| [:a, :b, :c].sum{ |key| o.send key} }

edited May 4, 2020 at 19:25

answered May 4, 2020 at 16:31

lacostenycoder

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6 Comments

engineersmnky Over a year ago

I think the OP is looking for something more like a.reduce {|memo,h| memo.merge(h) { |_,o,n| o + n }}

Hazhir Derakhshi Over a year ago

But every object does not have merge method.

engineersmnky Over a year ago

@HazhirDerakhshi not every object has a values method either

Hazhir Derakhshi Over a year ago

@engineersmnky Exactly.

engineersmnky Over a year ago

@lacostenycoder yeah it is a bit unclear because this "How about running a sum on multiple attributes in the array of objects" can have multiple interpretations including an interim per pass sum, but the code examples seem more like a sum per attribute either way +1 for the effort.

|

Cary Swoveland · Accepted Answer · 2020-05-04 22:32:51Z

Let's begin by creating an array of instances.

class C
  def initialize(a,b,c)
    @a, @b, @c = a, b, c
  end
end

arr = [[1,2,3], [4,5,6], [7,8,9]].map { |a| C.new(*a) }
  #=> [#<C:0x00005a4282c2d980 @a=1, @b=2, @c=3>,
  #    #<C:0x00005a4282c2d5c0 @a=4, @b=5, @c=6>,
  #    #<C:0x00005a4282c2d3b8 @a=7, @b=8, @c=9>]

then

ivs = arr.first.instance_variables
  #=> [:@a, :@b, :@c]
arr.map { |i| ivs.map { |v| i.instance_variable_get(v) } }. 
    transpose.
    map(&:sum)
 #=> [12, 15, 18]

The steps are as follows.

a = arr.map { |i| ivs.map { |v| i.instance_variable_get(v) } }
  #=> [[1, 2, 3], [4, 5, 6], [7, 8, 9]] 
b = a.transpose
  #=> [[1, 4, 7], [2, 5, 8], [3, 6, 9]] 
c = b.map(&:sum) 
  #=> [12, 15, 18]

If labels are desired:

ivs = arr.first.instance_variables
ivs.zip(arr.map {|i| ivs.map {|v| i.instance_variable_get(v)}}. 
            transpose.
            map(&:sum)).
    to_h
  #=> {:@a=>12, :@b=>15, :@c=>18}

If only sums of some instance variables were desired, say @a and @c, just change ivs:

ivs = [:@a, :@c]
ivs.zip(arr.map {|i| ivs.map {|v| i.instance_variable_get(v)}}. 
            transpose.
            map(&:sum)).
    to_h
  #=> {:@a=>12, :@c=>18}

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