How to use one array as a map for another array in ruby and then join them?

Question

arr1 = [{2=>7}, {2=>3, 8=>1}, {2=>2, 5=>2}]
arr2 = [{2=>9.95, 5=>16.95, 8=>24.95}]

expected result [ 69.65 , 54.8 , 53.8 ]

Here is the calculation:

So arr1 is kind of the map for the second array.

So I need to loop through the array 1 and array 2 then match the key of the array 1 and array 2 then grab the value of array 1 and multiple by the value of array 2.

Now in the array 1 second index there are 2 key value pairs which it means I have to do what I have explained and then then sum up the findings in that hash.

Hope it make sense.

I can do it through lots of loops. Just want to see if there are better ways like using map, etc to be more efficient.

Thank you for your help.

arr2 is not an array. Either it is badly named or wrong. Can you clarify? — Jörg W Mittag
– Jörg W Mittag, Commented Jun 23, 2017 at 8:13
You are correct it doesn't need to be an array. I got it wrong. However I better to change it to hash as @vamsi already answer to both of possibilites. — Mr H
– Mr H, Commented Jun 23, 2017 at 8:22

Amadan · Accepted Answer · 2017-06-23 08:34:57Z

4

Assuming your arr2 is actually {2=>9.95, 5=>16.95, 8=>24.95} (because it makes no sense for it to be an array) and renaming it as hash2:

arr1.map { |hash|
  hash.inject(0) { |accu, (key, multiplier)|
    accu + hash2[key] * multiplier
  }
}

EDIT: maybe a bit more understandable (but will only work on Ruby 2.4+):

arr1.map { |hash|
  hash.sum { |key, multiplier|
    hash2[key] * multiplier
  }
}

edited Jun 23, 2017 at 8:34

answered Jun 23, 2017 at 7:56

Amadan

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3 Comments

Mr H Over a year ago

Thanks mate. Your solution is very good however @vamsi solution is easier to understand. Thanks again for your help

Stefan Over a year ago

sum can be used with a block, i.e. hash.sum { |k, m| hash2[k] * m }

Amadan Over a year ago

Thanks again, @Stefan. I really should read those new docs more carefully :)

Vamsi Krishna · Accepted Answer · 2017-06-23 08:06:26Z

3

As per your question,

arr1.map{|a1| a1.keys.map{|a1k| arr2[0][a1k]*a1[a1k]}.inject(:+)}

If arr2 is just a simple hash:

arr1.map{|a1| a1.keys.map{|a1k| arr2[a1k]*a1[a1k]}.inject(:+)}

answered Jun 23, 2017 at 8:06

Vamsi Krishna

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1 Comment

Mr H Over a year ago

Thank mate that was simple. I should have thought outside the box and map the keys. Did not think like that. Thanks

Cary Swoveland · Accepted Answer · 2017-06-23 17:26:20Z

As we need to compute one inner product for each element of arr1, we can use matrix multiplication.

arr = [{2=>7}, {2=>3, 8=>1}, {2=>2, 5=>2}]
h = {2=>9.95, 5=>16.95, 8=>24.95}

require 'matrix'

keys, values = h.to_a.transpose
  #=> [[2, 5, 8], [9.95, 16.95, 24.95]]

(Matrix[*arr.map { |g| keys.map { |k| g.fetch(k,0) } }] *
  Matrix.column_vector(values)).to_a.flatten
  #=> [69.64999999999999, 54.8, 53.8]

Note that

arr.map { |g| keys.map { |k| g.fetch(k,0) } }
  #=> [[7, 0, 0], [3, 0, 1], [2, 2, 0]]

See Matrix and Hash#fetch.

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