Question

We get our data from a sensor which records and stores data like hashes. At any time it measures a few stuff like that:

{:temperature => 30, :pression => 100, :recorded_at => 14:34:23}
{:temperature => 30, :pression => 101, :recorded_at => 14:34:53}
{:temperature => 31, :pression => 102, :recorded_at => 14:34:24}
{:temperature => 30, :pression => 101, :recorded_at => 14:34:55}
{:temperature => 30, :pression => 102, :recorded_at => 14:34:25}
{:temperature => 31, :pression => 101, :recorded_at => 14:34:56}

We need to be able to export that data on a JSON format, but we have way too much data (the sensor records about every 30 seconds) and we need to remove some of the data. Ideally we d want to export 1 measure per hour in the last 24 hours so we have something like

{0 => {:temperature => 30, :pression => 100}, 1 => {:temperature => 30, :pression => 100}, 2 => {:temperature => 30, :pression => 100}, 3 => {:temperature => 30, :pression => 100}, 4 => {:temperature => 30, :pression => 100}}

For each hour, the temperature is the average of all temperatures measured within that hour. Also, if for any reason some data is missing for 1hour, I d like to to extrapolate it by being the mean between the previous and next hour. Anybody can help?

Answer 1

More functional version (with simple interpolation of missing values)

probs = [{:temperature => .. }] # array of measurings

def average(list, key)
  list.reduce(0){|acc,el| acc+el[key]} / list.length unless list.empty
end

prob_groups = probs.group_by{|prob| prob[:recorded_at][0,2].to_i}
average_groups = prob_groups.map do |hour,prob_group|
  { hour => {
      :temperature => average(prob_group, :temperature),
      :pression    => average(prob_group, :pression)
  }}
end.reduce{|acc,el| acc.merge(el)}

def interpolate(p, n, key)
  (p[key] + n[key])/2 unless p.nil? || n.nil? || p[key].nil? || n[key].nil?
end

resuls = (1..24).map do |hour|
  if average_groups[hour]
    { hour => average_groups[hour] }
  else
    { hour => {
      :temperature => interpolate(average_groups[hour-1], average_groups[hour+1], :temperature),
      :pression => interpolate(average_groups[hour-1], average_groups[hour+1], :pression)
    }}
  end
end.reduce{|acc,el| acc.merge(el)}

Hope it works

Answer 2

something like this

t = [....] - array of measurings
result = {}

(1..24).each do|hour| 
    #  measurings of given hour
    measurings = t.select{|measuring| measuring[:recorded_at][0, 2].to_i == hour}

    #  average temperature of hour
    sum = measurings.inject(0){|sum, measuring| sum + measuring[:temperature].to_i} 
    average_temperature = (measurings.length == 0)? nil: sum/measurings.length.to_f

    result[hour] = average_temperature
end

Answer 3

If you are not interested on the history but only on an approximation of actual value(s), consider to use a "moving metric" (http://en.wikipedia.org/wiki/Moving_average).

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