Group by key in array and get max and average - ruby-on-rails

Question

I have an array that is structure as such:
{"status": "ok", "data": [{"temp": 22, "wind": 351.0, "datetime": "20160815-0330"}, {"temp": 21, "wind": 321.0, "datetime": "20160815-0345"}]}
I'm looking to group by the datetime key (ignoring the time), find the max temp and the average wind.
I've tried something as follows, but unsure of how to do max_by and average in the same map:
#data['data'].group_by { |d| d.values_at("datetime") }.map { |_, v| v.max_by { |h| h["temp"] } }

Answer 1

So, when you do "data": { ... }, the data actually becomes a symbol, not a string so you would need to do something like:
#data[:data].group_by { |data| data[:datetime].split('-')[0] }
in order to group by the :datetime key, ignoring the time portion (I assume, the time portion is just everything after the -). Then you end up with a hash looking like:
{"20160815"=>[{:temp=>22, :wind=>351.0, :datetime=>"20160815-0330"}, {:temp=>21, :wind=>321.0, :datetime=>"20160815-0345"}]}
and to find the max :temp and average of the :wind you can do:
results = #data[:data].group_by { |data| data[:datetime].split('-')[0] }.map do |date, values|
[date, {
maximum_temp: values.max_by { |value| value[:temp] }[:temp],
average_wind: values.sum { |value| value[:wind] }.to_f / values.length
}]
end.to_h
# => {"20160815"=>{:maximum_temp=>22, :average_wind=>336.0}}

Answer 2

The above method work very well, the code seems bit complicated by making use of max_by and the access the value [:temp] and then sum and explicit to_h. So, if you consider for performance and good readability wise you could use the basic each like below,
data = {"20160815"=>[{:temp=>22, :wind=>351.0, :datetime=>"20160815-0330"}, {:temp=>21, :wind=>321.0, :datetime=>"20160815-0345"}]}
data.map do |k, v|
winds = []
temps = []
v.each do |item|
winds << item[:wind]
temps << item[:temp]
end
{k => {max_temp: temps.max, avg_wind: winds.inject(:+).to_f/winds.length}}
end
And the output is below,
# => {"20160815"=>{:max_temp=>22, :avg_wind=>336.0}}
Below is the small benchmark between making use of each and max_by,
data = {"20160815"=>[{:temp=>22, :wind=>351.0, :datetime=>"20160815-0330"}, {:temp=>21, :wind=>321.0, :datetime=>"20160815-0345"}]}
def by_each(data)
data.map do |k, v|
winds = []
temps = []
v.each do |item|
winds << item[:wind]
temps << item[:temp]
end
{k => {max_temp: temps.max, avg_wind: winds.inject(:+).to_f/winds.length}}
end
end
def by_max(data)
data.map do |date, values|
[date, {
maximum_temp: values.max_by { |value| value[:temp] }[:temp],
average_wind: values.sum { |value| value[:wind] }.to_f / values.length
}]
end.to_h
end
Benchmark.ips do |x|
x.config(times: 10)
x.report 'BY_EACH' do
by_each(data)
end
x.report 'BY_MAX' do
by_max(data)
end
x.compare!
end
And the benchmark o/p is like below,
Warming up --------------------------------------
BY_EACH 18.894k i/100ms
BY_MAX 13.793k i/100ms
Calculating -------------------------------------
BY_EACH 226.160k (± 5.3%) i/s - 1.134M in 5.025488s
BY_MAX 154.745k (± 5.8%) i/s - 772.408k in 5.006365s
Comparison:
BY_EACH: 226159.5 i/s
BY_MAX: 154744.8 i/s - 1.46x slower
Hence, you can see BY_MAX is 1.46 times slower than BY_EACH. But, of course you can make use of any approach that suits for your understanding and usability.