How do I create an average for a temperature data set? - ruby-on-rails

Question

I am writing a program which takes a set of data and then averages it. This data set is the average temperature at Laguardia Airport for every month which corresponds with a temperature.
Here is an example one data point:
2009-07,23.6
Which is year-month, temp.
I have 163 data points. All of the data points are in a .txt file, each on a new line.
I made the data into an array and split it. For some reason, my program says my average is 0 every time I try to run it
Here is my code:
data = File.open("avg_temp.txt", "r+")
contents = data.read
contents = contents.split("\r\n")
contents.collect! do |x|
x.split(',')
end
sum = 0
data.each do |x|
sum = sum + x[1].to_f
end
avg = sum / contents.length
puts avg

Answer 1

assign avg = sum/contents.length outside the loop after the end. Variables defined within a loop have a scope limited to that loop. Also you are looping over data, should be content
contents.each do |x|
puts x[1] # to check if the right value is being evaluated
sum = sum + x[1].to_f
end
avg = sum/contents.length

Answer 2

Your avg is not the top level local variable. Thus you can't access it from the top level. avg is scoped to the block only. You can do below :-
avg = 0
data.each do |x|
sum = sum + temp[1].to_f
avg = sum / contents.length
end
puts avg

Answer 3

sum = 0
data.each do |x|
sum = sum + temp[1].to_f
avg = sum / contents.length
end
puts avg
The avg variable is no longer in scope, as it's outside of the each block. To print the avg, simply move it inside of the block like so
sum = 0
data.each do |x|
sum = sum + temp[1].to_f
avg = sum / contents.length
puts avg
end
Or move the avg variable outside of the loop (to perform the action once, after your loop has exited)
sum = 0
data.each do |x|
sum = sum + temp[1].to_f
end
avg = sum / contents.length
puts avg
More info at http://rubyflare.com/2009/09/30/variables-scope-and-iterators/

Answer 4

I'd write this more simply:
lines = 0
sum = 0
File.foreach("avg_temp.txt") do |data|
sum += data.split(',').last.to_f
lines = $.
end
puts 'Average: %f over %d datapoints' % [sum / lines.to_i, lines]
# >> Average: 23.375000 over 4 datapoints
Which outputs:
Average: 23.375000 over 4 datapoints
Starting with "avg_temp.txt" looking like:
2009-07,23.6
2009-08,23.7
2009-09,23.6
2009-10,22.6
To 'splain it:
$. is the current line number. Each time through the loop it reassigns the last read line-number to lines so we know how many to divide by.
foreach reads line-by-line, so there's no need to split the file. That also explains why I use $. to keep track of the number of elements. It also means this could read a file containing millions or billions of values without a problem, where read would go to its knees.