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

200k23 gold badges252 silver badges321 bronze badges

Sign up to request clarification or add additional context in comments.

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

3,7924 gold badges22 silver badges45 bronze badges

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

1

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.

edited Jun 23, 2017 at 17:26

answered Jun 23, 2017 at 8:59

Cary Swoveland

111k6 gold badges69 silver badges105 bronze badges

Collectives™ on Stack Overflow

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

3 Answers 3

3 Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related