Ruby, Shifting arrays in an array and creating a new array where the shifted values are added?

Question

I need to create a method whose arguments are an array of subarrays that are of equal size and a shift value. The output needs to be a new array where the shifted values add up to each other.

Example

array[[1,2,3,4,5,6],[7,8,9,10,11,12],[13,14,15,16,17,18]]
shift_value = 4

#shift explained below

[1,2,3,4,5,6]
    [7,8,9,10,11,12]
        [13,14,15,16,17,18]

#desired output
[1,2,10,12,27,30,26,28,17,18]

Is there a method in ruby to do this? I spent some time making loops finding the index numbers that need to be added and forming a new array, but got lost.

You say shift value is 4, and yet you move them over two spots? — tadman
– tadman, Commented Feb 15, 2017 at 22:45

tadman · Accepted Answer · 2017-02-15 22:57:07Z

3

This seems to do what you want even though the shift value is off by a factor of two:

arrays = [ [1,2,3,4,5,6],[7,8,9,10,11,12],[13,14,15,16,17,18] ]
shift = 2

arrays.map.with_index do |a, i|
  # Zero pad the arrays to shift them over
  [ 0 ] * i * shift + a
end.reduce do |a, b|
  # Zip to combine and derive sums
  b.zip(a).map { |x,y| x + (y || 0) }
end

# => [1, 2, 10, 12, 27, 30, 26, 28, 17, 18]

answered Feb 15, 2017 at 22:57

tadman

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Comments

Cary Swoveland · Accepted Answer · 2017-02-15 23:45:10Z

0

This solution employs recursion.

def shifty(arr, n)
  return arr.first if arr.size == 1
  first, *rest = arr
  prepend = [0]*n
  shifty(rest.map { |a| prepend + a }, n).map { |m| m + first.shift.to_i }
end

arr = [[ 1,  2,  3,  4,  5,  6],
               [ 7,  8,  9, 10, 11, 12],
                       [13, 14, 15, 16, 17, 18]]

shifty(arr, 2)
  #=>  [ 1,  2, 10, 12, 27, 30, 26, 28, 17, 18]

When first.shift #=> nil, first.shift.to_i #=> 0.

edited Feb 15, 2017 at 23:45

answered Feb 15, 2017 at 23:27

Cary Swoveland

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