What does the syntax ?. mean in this Ruby example?

Question

I am learning Ruby and found this code sample in some documentation:

require 'find'

  total_size = 0

  Find.find(ENV["HOME"]) do |path|
    if FileTest.directory?(path)
      if File.basename(path)[0] == ?.
        Find.prune       # Don't look any further into this directory.
      else
        next
      end
    else
      total_size += FileTest.size(path)
    end
  end

The purpose is to sum up the file sizes of all files in a tree, excluding directories that start with a dot. The line "if File.basename(path)[0] == ?." is obviously performing the directory name test. I would have written it like this:

if File.basename(path)[0] == "."

What does "?." do? (Could be a typo, I suppose.) I have not seen this syntax described elsewhere.

Answer 1

It means to return the character code of the next character. In Ruby, saying string[0] gives you a character code, so it's like saying is the first character a ".".

Interestingly, it means that expressions like ???!?!?? are perfectly legal in Ruby.

Take a look here.

Answer 2

It is a shorthand for the ASCII code point of the "." character. See the documentation on numeric literals in the Ruby syntax.

>> ?.
=> 46
>> ?a
=> 97

Answer 3

?. returns the ASCII value of the dot. You can put pretty much any char after the question mark to get its ASCII value, like ?a or ?3 or ?\\, etc. The reason they are not comparing it to the string "." is that when you index into a string, you get the ASCII value of the char at that index rather than the char itself. To get the char at a certain index you can use [0, 1] as the index. So the options are:

if File.basename(path)[0] == ?.

Or:

if File.basename(path)[0, 1] == "."

Or even:

if File.basename(path)[0].chr == "."