Some Neat Things in Ruby

One thing I love about Ruby is how I'm always discovering more language features. I never had a "formal" tutorial on Ruby, so many of these things may be well known to you.

Some nice little things I've recently discovered:

• Backticks! You can use backticks to execute something on the command-line and put the result into a variable. Apparently this is from Perl. Check it out:

  output = `ls $HOME | grep someregex`

• The % operator for strings. I learned about this when trying some stuff in Python a while back, and though "wow, it would be cool if Ruby had this" and it turns out it does. Go like this:

  sql = "SELECT * FROM some_table WHERE id = %d " +
      "AND some_other_column = '%s'" %
      [params[:id].to_i, some_string]

  Of course this is a trivial example, but I'm sure you can think of some nastier SQL queries where this would come in handy. If you're like me and don't like the #{ .. Ruby code .. } stuff then the % operator is your friend.
• Regex variables - I learned this one a while back after reading some other programmer's Ruby code and I discovered that if you have a Regex with placeholders then you can use the $1, $2 variables to access those:

  puts $1 if html_code =~ /\<a href = "(.*?)"\>/i

  This code will check some HTML code for links (I used a simpler regex for links to illustrate the example, this is insufficient for scanning real HTML) and output the URL of the first link it finds. Very handy.
• Named parameters. I saw Rails do it, but never really understood how it worked. Now I do:

  def foo(p1, *params)
      ... some code ...
  end
  foo("asdf", :random_thing => 5, :test => "hello")

  When you call foo(), the first parameter you pass gets put into p1, and the rest of them get put into params, which is a hash of all the extra stuff you send. This is great, and I love it. Wish PHP had it, I hate having to go array(...) all the time for named parameters.

Some other stuff that I'm still wondering about:

• What the heck do $< and $> do? I'm pretty sure they're some kind of I/O things, but I'm not sure what. Unfortunately googling for "ruby $<" doesn't find anything about it since Google seems to ignore $<.
• What does & do when used as a unary operator? I'm still stuck in my C mindset where seeing & in front of something means "address of". From reading Raganwald, I think it calls the to_proc function of something, but I'm not sure. EDIT: Reg Braithwaite of Raganwald has answered this question for us.

If anyone has some answers to these things I don't know, or has anything neat they've discovered, feel free to comment.