Backup your CiteULike library using MongoDB and Ruby

Well, that was easy.

#!/usr/bin/ruby
require "rubygems"
require "mongo"
require "json/pure"
require "open-uri"

db  = Mongo::Connection.new.db('citeulike')
col = db.collection('articles')
j   = JSON.parse(open("http://www.citeulike.org/json/user/neils").read)

j.each do |article|
  article[:_id] = article['article_id']
  col.save(article)
end

A brief introduction to “apply” in R

At any R Q&A site, you’ll frequently see an exchange like this one:

Q: How can I use a loop to [...insert task here...] ?
A: Don’t. Use one of the apply functions.

So, what are these wondrous apply functions and how do they work? I think the best way to figure out anything in R is to learn by experimentation, using embarrassingly trivial data and functions.