Get previous value of an array and unshift it in the next array-CodePudding

I'm working on geographic web app with postgis and postgresql

@slopes = Slope.order(:id)
@array_slopes = @slopes.chunk_while { |i, j| i.average_slope == j.average_slope }.to_a

I get an array of arrays of slopes (with geo coordinates) like :

[[slope_1, slope_2, slope3][slope_4, slope_5, slope_6, slope_7]][[slope_8, slope_9]....]

I m create a geojson linestring with @array_slopes, but get a broken line because I should have :

[[slope_1, slope_2, slope3][slope3, slope_4, slope_5, slope_6, slope_7]] [[slope_7 slope_8, slope_9]...]

I need to get the last slope from each array en push it in the next array for generate à continuous line with my geojson builder :

..slope3] => [slope3..
or
..slope_7]] => [[slope_7..

Something like this (but in my controller) :

<% @array_slopes.each_with_index do |array, index| %>
    <% array.each do |slope| %>
        <% @array_slopes[index  1 ].unshift(array.last) %>
    <% end %>
<% end %>

How can I do that ?

CodePudding user response：

This is a straightforward way of doing that.

arr =[[1, 2, 3], [4, 5, 6], [7, 8, 9]]

arr.each_index.map do |i|
  if i.zero?
    arr[0]
  else
    [arr[i-1].last]   arr[i]
  end
end
  #=> [[1, 2, 3], [3, 4, 5, 6], [6, 7, 8, 9]]

CodePudding user response：

I have found another solution :

@array_slopes.each_with_index do |group, i|
  next if group.empty? || i == @array.length - 1
  last_value = group.last
  @array[i 1].unshift(last_value)
end