Precedence

Today when practised Ruby, I encountered an issue about precedence.

Common sense

Let’s start with simple exercise, the old fashion if statement is as below

if a > b
  1
else
  2
end 

And we can also shorten to a > b ? 1 : 2. Easy right? Please look at the next example.

Dictionary Class

class Dictionary

  def initialize
    @entries = {}
  end

  def entries
    @entries
  end

  def add(entry)
    if entry.is_a? Hash
      @entries.merge!(entry)
    else
      @entries[entry] = nil
    end
  end

  def include?(key)
    @entries.key?(key)
  end

end

If we do

d = Dictionary.new
d.include?'nothing' ? d.entries['nothing'] : nil

I was expecting the result is nil. But it returns false and throws warning string literal in condition. What is it??

Investigation

In Ruby, if on a non-boolean input will check existence, the check for existence with ?? considered true; you receive a warning (warning: string literal in condition)

> "1" ? 1 : 2
(irb): warning: string literal in condition
 => 1

So, d.include? is executed without ‘nothing’. Then the codes become: 'nothing' ? d.entries['nothing'] : nil

Solution

Add a bracket for ‘nothing’,

d = Dictionary.new
d.include?('nothing') ? d.entries['nothing'] : nil

 => nil 

Yay! So, please do remember add the bracket if using ? : expression.