Matthew Bass

I started building Fuelinator at the last West End Ruby meetup. The motivation behind the project was the lack of a decent system for tracking my business mileage. Existing apps like Fuelly and My Mile Marker make it unnecessarily difficult to enter mileage, and the statistics they produce just aren’t that useful to me.

My initial goal with Fuelinator is to make mileage entry dead simple and to provide some compelling new features… for example, alerts via email or SMS when my vehicle’s mileage changes suddenly. This helps me track down maintenance problems early and gives me valuable information about what does and doesn’t improve my gas mileage. For example, I changed my air filter and inflated my tires last week. If my mileage changes drastically this week, I want to know. Now, Fuelinator will tell me.

The ultimate goal is for Fuelinator to save its users gobs of money on their gas bills. I haven’t made Fuelinator public yet, but if you’d like to participate in the beta program make sure you sign up. It’s going to be a fun ride!

Ruby arrays have this handy method called transpose which takes an existing 2-dimensional array (i.e. a matrix) and flips it on its side:

>> a = [[1,2], [3,4], [5,6]]
>> puts a.transpose.inspect
[[1, 3, 5], [2, 4, 6]]

Each row becomes a column, essentially. This is fine and dandy for polite arrays. If one of the rows in the original array is not as long as the others, though, Ruby chunders thusly:

>> a = [[1,2], [3,4], [5]]
>> a.transpose
IndexError: element size differs (1 should be 2)
	from (irb):3:in `transpose'
	from (irb):3

That ain’t pretty, especially if the intent behind using transpose is to render data in a nice columnar fashion. For example, what if we wanted to render a list of high school courses in columns, one column per semester? Grouping the courses by semester and then transposing would do the trick, but only if there were exactly the same number of courses taken each semester. If even one semester differs, Ruby will blow up. What we really want is for Ruby to just ignore the fact that each grouping may have differently sized arrays and transpose anyway, filling in the empty spaces with nils.

Here’s how to do just that:

class Array
  def safe_transpose
    result = []
    max_size = self.max { |a,b| a.size <=> b.size }.size
    max_size.times do |i|
      result[i] = Array.new(self.first.size)
      self.each_with_index { |r,j| result[i][j] = r[i] }
    end
    result
  end
end

Now we call safe_transpose on our matrix of courses and Ruby does the right thing. It calculates the length of the longest row and uses that as the baseline to perform the transposition. So our original example becomes:

>> a = [[1,2], [3,4], [5]]
>> puts a.transpose.inspect
[[1, 3, 5], [2, 4, nil]]

Nice and neat. Caveats: the code above hasn’t been refactored or tested. Your mileage may vary. If you see a better way to do this, let me know and I’ll post an update.