Whether to use String or Integer in BigDecimal in Ruby

Question

Referring to the documentation of BigDecimal class,

n,m = a.precs
prec returns number of significant digits (n) and maximum number of significant digits (m) of a.

I am puzzled by the following output related to BigDecimal.

require 'bigdecimal'    
BigDecimal.new('1').precs # => [9, 18] 
BigDecimal.new(1).precs   # => [9, 27]

I cannot figure out why when a String is passed, the maximum number of significant digits is less compared to when a Fixnum is passed.

Also will it result in any precision issues?

Answer 1

If you can read C code, you can start at https://github.com/ruby/ruby/blob/trunk/ext/bigdecimal/bigdecimal.c#L2509 - that's the initializer for any BigDecimal object. If you follow that code to the next method, which is BigDecimal_new you'll notice that when passed an integer argument there are a few more steps to go through before allocating and creating an internal big decimal object as opposed to when passing a string argument.

In any case, you shouldn't worry about loss of precision - the significant digits attributes are more like hints than absolute values. Even the documentation mentions it: The actual number of significant digits used in computation is usually larger than the specified number.