How do I round a float to a specified number of significant digits in Ruby?

Question

It would be nice to have an equivalent of R\'s signif function in Ruby.

For example:

>> (11.11).signif(1)
10
>> (22.22).signif(2)
22
>         


        

Answer 1

You are probably looking for Ruby's Decimal.

You could then write:

require 'decimal/shortcut'
num = 1.23541764
D.context.precision = 2
num_with_2_significant_digits = +D(num.to_s) #  => Decimal('1.2')
num_with_2_significant_digits.to_f #  => 1.2000000000000002

Or if you prefer to use the same syntax add this as a function to class Float like this:

class Float
  def signif num_digits
    require 'decimal/shortcut'
    D.context.precision = num_digits
    (+D(self.to_s)).to_f
  end
end

Usage would then be the same, i.e.

 (1.23333).signif 3
 # => 1.23

To use it, install the gem

gem install ruby-decimal