Ruby

I'm currently learning Ruby to improve my Jekyll blog. At the moment, I can't really say if I like Ruby.

Just like PHP and Python, Ruby is implicitly typed. This means you can do the following:

# A list
a = [1,2,3,4,5]
# An integer
b = 42
# A floating point number
c = 3.141

Ruby is also dynamically typed:

a = 42
a = "now a is a string!"

One thing that is pretty annoying is the usage of begin ... end. I thought LaTeX was the only "language" in use that had this syntax.

Syntax examples

If, else if, else

a = 1

if a == 1 || a % 2 == 0
    puts "a equals 1 or is even"
elsif a == 3
    puts "a equals 3"
else
    puts "a does not equal 1 or 3 and is not even"
end

String manipulation

String manipulation is weird in Ruby. Take a look at the last example. What would you expect?

s = "0123456789"
puts s[2..4]
puts s[2..s.length()]
puts s[2..-2]

Output is:

234
23456789
2345678

Now what would you expect to happen with "0123456789"[2..4]? I would expect Ruby to give "234". But this one actually fails.

Dictionaries

Dictionaries, which are sometimes also called 'maps' or 'associative arrays' are a neat data structure. Ruby uses this syntax:

dictionary = {"a"=> 7, "b"=> "x"}

if dictionary.has_key?("a")
    puts "key 'a' is in dictionary! It is " + dictionary["a"].to_s
    dictionary["a"] = 42
    puts "Now it is " + dictionary["a"].to_s
    puts dictionary.size()
    puts dictionary.length()
    puts dictionary.count()
end

map

myList = [1,2,3,4,5]
x = myList.map{|element| [element,element+1]}
x.each do |a, b|
    puts "a: " + a.to_s + ", b: " + b.to_s
end

will print

a: 1, b: 2
a: 2, b: 3
a: 3, b: 4
a: 4, b: 5
a: 5, b: 6

.count(), .length(), .size()

At a first glance, those three seem to be aliases. But .count() is not quite the same as .length() and .size() (source).

Blocks

A very interesting idea in Ruby is that of blocks. It seems to be similar to Pythons decorators:

def time_method(method=nil, *args)
  beginning_time = Time.now
  if block_given?
    yield
  else
    self.send(method, args)
  end
  end_time = Time.now
  puts "Time elapsed #{(end_time - beginning_time)*1000} milliseconds"
end

time_method do
  (1..10000).each { |i| i }
end

Logging

Logging in Ruby is very convenient. See Logger Documentation

Notable libraries and projects

Nokogiri: Parsing HTML

The following code will parse yourHTMLcode and select all a-tags (also known as links):

require 'nokogiri'

doc = Nokogiri::HTML.parse(yourHTMLcode)
links = doc.css('a').map { |link| [link['href'],link.text] }
links.each do |link, linktext|
    puts "Link '"+link+"' was used with '"+linktext+"'"
 end

Sequel

This way you can select elements from a SQLite-Database:

require 'sequel'

db = Sequel.sqlite("search.db")
rows = db.fetch( "SELECT rowid FROM pages WHERE url='/imprint';" ).all
#puts rows
#puts rows.count()
puts rows[0][:rowid]