Can I implement round half to even?

问题

I need to do round half to even on floats, i.e.,

if the value is half-way between two integers (tie-breaking, fraction part of y is exactly 0.5) round to the nearest even integer,
else, standard round (which is Round to nearest Integer in Ruby).

These are some results, for example:

0.5=>0
1.49=>1
1.5=>2
2.5=>2
2.51=>3
3.5=>4

回答1:

I would reopen the Float class to create a round_half_to_even function :

class Float
  def round_half_to_even(precision)
    if self % 1 == 0.5
      floor = self.to_i.to_f
      return floor % 2 == 0 ? floor : floor + 1
    else
      return self.round(precision)
    end
  end
end

回答2:

The BigDecimal class has the rounding mode half to even already implemented. You just have to set the ROUND_MODE to :half_even with BigDecimal.mode method:

require 'bigdecimal'

def round_half_to_even(float)
  BigDecimal.mode(BigDecimal::ROUND_MODE, :half_even)
  BigDecimal.new(float, 0).round
end

Or by using the round with some arguments:

require 'bigdecimal'

def round_half_to_even(float)
  BigDecimal.new(float, 0).round(0, :half_even).to_i
end

Please note that BigDecimal#round returns an Integer when used without arguments, but a BigDecimal when used with arguments. Therefore the need to call to_i in the second example but not in the first.

回答3:

def round_half_to_even f
  q, r = f.divmod(2.0)
  q * 2 +
  case
  when r <= 0.5 then 0
  when r >= 1.5 then 2
  else 1
  end
end

round_half_to_even(0.5) # => 0
round_half_to_even(1.49) # => 1
round_half_to_even(1.5) # => 2
round_half_to_even(2.5) # => 2
round_half_to_even(2.51) # => 3
round_half_to_even(3.5) # => 4

回答4:

You could do the following:

def round_half_to_even(f)
  floor = f.to_i
  return f.round unless f==floor+0.5
  floor.odd? ? f.ceil : floor
end

round_half_to_even(2.4) #=> 2
round_half_to_even(2.6) #=> 3
round_half_to_even(3.6) #=> 4
round_half_to_even(2.5) #=> 2
round_half_to_even(3.5) #=> 4

来源：https://stackoverflow.com/questions/35359699/can-i-implement-round-half-to-even

标签

ruby

rounding