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?

Orelia

The constructor does take a second argument if you want consistency: BigDecimal.new('1', 27).precs for example. And the documentation does say that the "number of significant digits is determined from the initial value" if you don't specify it yourself. – Saccule

@muistooshort your right that we can specify that. But my question was why the precs are different in the first place? – Orelia

Here are more data points. The following observations refer to strings containing the character 1 only (e.g., "111111") and numbers whose digits are all 1's (e.g., 111111). For fewer than 10 characters, [9,18] is returned. For fewer than 10 digits, [9,27] is returned. Between10 and 18 characters or digits, [18,27] is returned (i.e., the same tuple is returned for both strings and numbers), between 19 and some greater number of characters or digits, [27,36] is returned. Aside: I suggest you add require 'bigdecimal' to your code. – Caveman

@CarySwoveland, thanks for your findings. – Orelia

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.

Pelagic