Ruby debugging gems

A list of debugger add-ons for ruby:

• debug (old, built-in)
• ruby -rdebug foo.rb
• # works
• # but can't view code around current line automatically like Perl's  {{v
• pry (no step-through)
• require 'pry' # at the top
• binding.pry # in code to define a breakpoint
• ruby foo.rb
• pry with step through:
• pry-debugger (2014. failed to install)
• pry-debug (2014. installed. works)
• pry_debug foo.rb
• ruby-debug_pry (2011)
• pry-byebug (requires ruby >= 2.2.0)

Ruby iterator cheat sheet

A list of many useful iterators in Ruby for hashes and arrays:

• each - Do block for each item. Works on hash or array. Can use 'break'. Return nothing.
• each_index - Same as each but pass in the index to the block
• collect (aka map) - Do block for each item. Return array. Same as Perl map.
• collect! / map! - Same as map, but modify array in place
• select (aka find_all) - Do block for each item. Return array if block is true. Same as Perl grep.
• select! (aka keep_if) - Same as select but modify array in place
• reject - Opposite of select. Return if block is false.
• reject! (aka delete_if) - Same as reject but modify array in place
• inject(xyzzy) (aka reduce) - Do block for each pair of items, starting with xyzzy & first item.
• compact - Return array with nil items removed
• compact! - Same as collect but modify array in place
• delete(foo) - Remove all items that are equal to foo
• clear - Remove all items
• empty? - Return true if array has no items
• count - Return number of items
• length (aka size) - Return number of items
• flatten! - Modify array to be 1-dimensional
• shuffle! - Shuffle in place
• sort! - Sort in place
• bsearch - Find item using binary search
• combination(3) - Return all combainations of length 3
• permutation(4) - Return all permutations of length 4
• sort_by! - Sort in place using keys in block
• transpose - swap rows and columns
• zip - step through two arrays at the same time

(source, source, source)

Detailed Ruby notes

A list of somewhat advanced Ruby topics:

• Scope of constants, and namespace: link, link.
• Keyword arguments: link.

How to install ruby, rake, rubygems

https://www.ruby-lang.org/en/documentation/installation/

For ruby, tried:

• https://rvm.io/ - wouldn't work on my Suse setup
• https://github.com/sstephenson/rbenv#readme - worked on Suse. Promising, I like.
• https://github.com/sstephenson/ruby-build#readme - doesn't work

For rubygems, tried:

• https://rubygems.org/pages/download - worked

For rake:

• It comes with ruby

Now learn Ruby:
https://github.com/neo/ruby_koans

Ruby basics

• Install Ruby
• Setup gems
• gem install nokogiri
• gem install builder
• Database: SQLite (docs)
• Database driver:
• ! gem install dbi (docs don't work, but these docs do)
• ! gem install dbd-sqlite3

• ? gem install sqlite-ruby (docs)

• Write script offline
• Host on Heroku

Save URL to a file

require 'open-uri'
open('image.png', 'wb') do |file|
    file << open('http://example.com/image.png').read
end

is this the same as:

require 'open-uri'
file = open('image.png', 'wb')
file << open('http://example.com/image.png').read

Fetch URL through a proxy

print Net::HTTP::Proxy(proxy_addr, proxy_port, proxy_user, proxy_pass).get URI.parse('http://www.compufer.com/')

Hello world

puts "Hello, what's your name?"
STDOUT.flush
name = gets.chomp
puts 'Hello, ' + name + '.'

Blocks

def try  

  if block_given?  

    yield  

  else  

    puts "no block"  

  end  

end  

try # => "no block"  

try { puts "hello" } # => "hello"  

try do puts "hello" end # => "hello" 

Notes

• Whichever variables(s) you pass into the block, the method will fill them with something, depending on what its function is. e.g. "open" puts a file handle in there.
• List of file modes
• Hash, Hashes
• ---------------
• Builder tut1, tut2, tut3

• Read docs
• open-uri docs
• open returns an IO like object
• File
• Ruby blocks (closures)

Install ruby gems

sudo yum -y install gcc ruby-devel rubygems
sudo gem install nokogiri

Parse HTML with Ruby and Nokogiri

doc = Nokogiri::HTML(open(filename, 'r'))
puts doc.xpath('//title')[0].inner_html
puts doc.xpath('//h2')

Ruby: Read+write to database

require 'sqlite3'

db = SQLite3::Database.new( "db1" )
db.execute('INSERT INTO table_name (id, text) VALUES (1, "hello")')
db.execute( "SELECT* FROM table_name" ) do |row|
    puts row.inspect
end

Web scraping with Ruby

In order of goodness:

• Nokogiri
• Hpricot
• Scrapi

Ruby Net::HTTP cheat sheet

http://august.lilleaas.net/blog/ruby-net-http-cheat-sheet

Misc Ruby stuff

• Net::HTTP - fetch a webpage

• Write to a file:
File.open(local_filename, 'w') {|f| f.write(doc) }

• Read from a file:
File.open(local_filename, 'w') {|f| f.write(doc) }

• stringify: .inspect