Question

I have an if/else condition, and the if and else sections are identical, save for the operator used. In one case <, and in the other >. Is there a way to conditionally set that operator, to DRY out the code?

if count_year < end_year
    while count_year <= end_year
        if count_year % 4 == 0
            if count_year % 100 != 0
                all_years << count_year unless (count_year % 400 == 0)
            end
        end
        count_year += 1
    end
    puts all_years
elsif count_year > end_year
    while count_year >= end_year
        if count_year % 4 == 0
            if count_year % 100 != 0
                all_years << count_year unless (count_year % 400 == 0)
            end
        end
        count_year -= 1
    end
    puts all_years.reverse
end

This is part of a program for printing out leap years between two given years. I feel like there must be a way to not have to repeat the loop twice. Something like: count_year < end_year ? operator = "<" : operator = ">" – Then using that variable to substitute the operator into a code block or something? Any ideas?

Answer 1

For one small improvement, you can extract really identical parts into a method. Then duplication stops being so massive.

# I'm too lazy to come up with a proper name for it.
def foo count_year, all_years
  if count_year % 4 == 0
    if count_year % 100 != 0
      all_years << count_year unless (count_year % 400 == 0)
    end
  end
end


# later...

if count_year < end_year
  while count_year <= end_year
    foo count_year, all_years
    count_year += 1
  end
  puts all_years
elsif count_year > end_year
  while count_year >= end_year
    foo count_year, all_years
    count_year -= 1
  end
  puts all_years.reverse
end

But, the operator substitution…

Yes, there is a way to dynamically choose an operator for evaluation. You see, operators in ruby are just method calls, nothing more. These two lines are equivalent:

7 > 5
7.>(5)

And here’s a snippet that chooses random operator for comparison. I leave it up to you to adapt it for your problem (if you want, that is. I advise you against this).

def is_7_greater_than_5
  operator = [:<, :>].sample # pick random operator
  7.send(operator, 5)
end

is_7_greater_than_5 # => false
is_7_greater_than_5 # => false
is_7_greater_than_5 # => true
is_7_greater_than_5 # => true
is_7_greater_than_5 # => true