How do I screen out NaNs when comparing floats in Ruby? - ruby

Question

I'm using Rails 5 wiht Ruby 2.4. How do I determine if a variable is a number? I thought is_a?(Numeric) was the way to go, but apparently not with me. I have this code
puts "guesses arr: #{guesses_arr}"
guesses_arr.map!{|str| str.is_a?(Numeric) && str == guesses_arr.max ? str : 0}
which dies with
guesses arr: [NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN, NaN]
ArgumentError: comparison of Float with NaN failed
from /Users/davea/Documents/workspace/myproject/app/services/text_table_to_race_time_converter_service.rb:2121:in `max'
from /Users/davea/Documents/workspace/myproject/app/services/text_table_to_race_time_converter_service.rb:2121:in `block (2 levels) in guess_headers_from_data_cols'
from /Users/davea/Documents/workspace/myproject/app/services/text_table_to_race_time_converter_service.rb:2121:in `map!'
I'm unable to reproduce this in a console because I can't seem to produce an array with all those "NaN" values. The point is, how do I make sure those don't get compared in my loop?

Answer 1

You can produce an array of NaN values in the console like so:
[ 0.0 / 0.0 ] * 3
# => [NaN, NaN, NaN]
It's not entirely clear what your code is doing, but it might be worth noting that it's the max which is throwing the exception, and not the equality test. Incidentally, if you are trying to compare each array value against the maximum value - you might want to max outside of the map, to avoid having to evaluate it multiple times.
Building on top the other answer here on rejecting NaNs, you can put it all together to get something like:
# set up dummy data - 2 values and 3 NaNs
guesses_arr = [ 0.0 / 0.0 ] * 3 + [ 2.0, 3.0 ]
puts "guesses arr: #{guesses_arr}"
# => [NaN, NaN, NaN, 2.0, 3.0]
# remove NaNs in place
guesses_arr.reject!(&:nan?)
maximum = guesses_arr.max
guesses_arr.map!{|str| str.is_a?(Numeric) && str == maximum ? str : 0}
# => [ 0, 3.0 ]
Edit: if you have integers mixed up in the array, try converting them into floats first:
# set up dummy data - including floats and integers
guesses_arr = [ 0.0 / 0.0 ] * 3 + [ 2.0, 3.0, 0 ]
guesses_arr.map!(&:to_f).reject!(&:nan?)
# => [ 2.0, 3.0, 0.0 ]

Answer 2

You can get NaN by calling Float::NAN:
[7] pry(main)> Float::NAN
=> NaN
[8] pry(main)> Float::NAN.nan?
=> true
If you just want to remove NaN:
guesses_arr.reject! { |x| x.nan? }
For example:
[4] pry(main)> [1.0, Float::NAN].reject(&:nan?)
=> [1.0]

Answer 3

There are a couple of things that we can do. To answer the specific question.
arr = [1.0, Float::NAN, "1.0", "1"]
arr.map(&:to_f).reject(&:nan?)
would be a concise example around removing NaN
With a guess that you are trying to get a max value from inconsistent data, first we need to clean it up with a method like the following, which will convert a string or a integer to a float, or otherwise return nil.
def to_number(maybe_a_number)
Float(maybe_a_number)
rescue ArgumentError
nil
end
Assuming we have the above method
guesses_arr.map(&:to_number).max # or .min
That being said, the reason it's been hard to debug is the fact that you had a .map! that causes guesses_arr to be changing as you move over it. (Generally we avoid use of the standard libraries bang methods as they object errors can be hard to debug.