Ruby Expression at First Glance

I started to learn Ruby language over the weekend.

If you have a C-like syntax language background like me, planned to learn Ruby for years, but never get started (with a valid reason, I know :-) Something quick to share with you today: a short highlights of Ruby expression.

Every statement returns a value

• Every statement is an expression. You can code something like this:

  a = b = 1  # => 1
  
  bar = "spam"
  foo = if bar == "spam"
          "great!"
        else
          "keep going"
        end
  p foo  # => "great!"
  
  defined? var                 # => nil
  var = var || "default value" # => "default value"
  

Using defined?

• If var is not defined, defined? var returns nil. Otherwise, it returns the var description.
• It's a handy tool when learning the language in irb. But just curious how it is used in production code..

Parallel assignment

• Swapping values of two variables is easy, like Python
  

  a = 1
  b = 2
  a, b = b, a   # => a=2, b=1
  

• Or you can perform neat stunts using splat (asterisk prefix)

  list = [1,2,3,4]
  
  first, *, sec_last, last = list
    # => first=1, sec_last=3, last=4
  
  first, *middle, last = list     
    # => first=1, middle=[2,3], last=4
  
  # more stunts..
  a, b, c, d = *(1..2), *[3, 4]  # => a=1, b=2, c=3, d=4
  

The Ruby Truth

• Everything is true, except nil, false.
• That means 0 and '' is evaluated to true! Beeee careful.

Conditions

• You can use normal if..elsif..else..end or unless..end statements
• or append if/unless to a statement, like Perl

  process(order) if order.valid?
  

No breaks in case statement

• Great to see there is no breaks :)
• Note that, when statement uses === operator (case equality) to perform matching.

  case command 
  when "debug"
    dump_debug_info
    dump_symbols 
  when /p\s+(\w+)/
    dump_variable($1) 
  when "quit", "exit"
    exit 
  else
    print "Illegal command: #{command}" 
  end

Looping

• Ruby provides while..end and until..end, or you can append while/until to a statement:

  puts a += 1 until a == 100

• We may use for..in..end and loop do..end to iterate through a list, but I believe many Rubyists prefer list.each{}.

Variable scoping

• Block introduces new scope. Block is a chunk of codes enclosed within {..} or do..end. A few ground rules:
  1. New variable defined in a block is not visible outside the block.
  2. Variables exist before a block, can be accessed within the block (think closure).
  3. A block's parameter will shadow a variable outside the block with same name.
  Here's an example,

  defined? local  # => nil
  x = y = -1
  [1,2,3].each do |x|
    local = y = x
  end
  defined? local  # => nil
  p x             # => -1
  p y             # => 3
  

• Built-in expressions such as if, unless, while, until, for do not use block. So they do not introduce new scope. As a result, new variables defined in a while loop, will continue to exist after the loop.

Learn Ruby?
My rubyist friend, Yasith recommended to start with this book: Programming Ruby. The latest edition Programming Ruby 1.9 is available on bookshelf.