Ruby notes

Ruby

--
Files

Get a file          f = File.new("testfile")
Get line number     f.lineno # note: you can't seek with lineno
Get a line          f.gets  # equal to the var $_
Read each line      f.each { |line| puts "#{f.lineno}: #{line}" }
                    # note: after using .each on a file once, you must .rewind the file
Into an array       arr = f.readlines
Get a line quickly  arr[3]
                    # like .each, you must .rewind after running .readlines once, unless place it in a variable

--
Arrays/Hashes

Array               []; [1, 'a', /^A/]
A quick array       Array.new(3){ |index| index ** 2 } # => [0, 1, 4]
Access an array     arr[0]
Sort an array       arr = arr.sort; arr.sort!
Sort in reverse     sort!.reverse; sort! { |x,y| y <=> x }  # The second is more process efficient
Hash                {}; { "a" => 100, "b" => 200 }; { a: 100, b: 200 }
A quick hash        h = Hash.new { |hash, key| hash[key] = "Go Fish: #{key}" }
                    h["c"]  #=> "Go Fish: c"
                    h["d"]  #=> "Go Fish: d"
                    h.keys  #=> ["c", "d"]

Note: arrays (and any object you modify) can sometimes be confusing when you change them. Consider:

                    a = []; b = a; b << 1; print a
                    #=> [1]

This is because ruby pointed a and b to the same array object: a.object_id == b.object_id... You really wanted:

                    a = []; b = a.dup

--
Loops/Iterators

While               while a < b { a += 1 }
Until               until a > b { a += 1 }
For                 for num in 1..10
                    for num in 1...11
                    for item in arr
Loop                i = 0; loop { i += 1; puts i; break if i == 10 }
Each                arr.each { |v| puts v }
                    hash.each { |k,v| puts k, v }
Times               10.times


--
Control flow

If                  if hungry
                      puts "I hunger"
                    elsif tired
                      puts "I tire"
                    else
                      puts "I am neither hungry nor tired"
                    end
Unless              unless hungry
                      puts "I am not hungry"
                    end
Shorthand           puts "I'm hungry" if hungry
                    puts "I'm no hungry" unless hungry

RubyDB

There are several ways one can connect to a DB (in this case SQL) in Ruby --

ActiveRecord - http://www.rubydoc.info/gems/activerecord/4.2.0/frames
             - http://blog.aizatto.com/2007/05/21/activerecord-without-rails/
DataMapper   - http://datamapper.org/why.html
Sequel       - http://sequel.jeremyevans.net/
MySQL2       - https://github.com/brianmario/mysql2
Arel         - https://github.com/rails/arel