Question

I have a sparse array, for example:

rare = [[0,1], [2,3], [4,5], [7,8]]

I want to plot a chart with these data, each pair are point coordinates.
As you can see I don’t have points for x=1, x=3 , x=5, x=6

I want to fill the array with the previous values, so for the above example I will get:

filled = [[0,1], [1,1], [2,3], [3,3], [4,5], [5,5], [6,5], [7,8]

As you can see, for calculating the y value, I simply take the last y value I used.

What is the best aproach to accomplish this ?

Answer 1

Range.new(*rare.transpose.first.sort.values_at(0,-1)).inject([]){|a,i|
  a<<[i, Hash[rare][i] || a.last.last]
}

Step-by-step explanation:

1. rare.transpose.first.sort.values_at(0,-1) finds min and max x ([0,7] in your example)
2. Range.new() makes a range out of it (0..7)
3. inject iterates through the range and for every x returns pair [x,y], where y is:
   1. y from input array, where defined
   2. y from previously evaluated pair, where not

Note: here are some other ways of finding min and max x:

[:min,:max].map{|m| Hash[rare].keys.send m}
rare.map{|el| el.first}.minmax # Ruby 1.9, by steenslag