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>>> numbers = [72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33] >>> filter(lambda n: n % 2 == 0, numbers) [72, 108, 108, 44, 32, 114, 108, 100] The lambda expression simply checks whether the remainder of a given number when divided by 2 is zero (which is another way of saying that the number is even). Now, map and filter can be very useful, but they were added to the language before list comprehension came along. If you think about it, anything that map and filter can accomplish can also be done with list comprehensions: >>> [chr(n) for n in numbers] # characters corresponding to numbers ['H', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd', '!'] >>> [ord(c) for c in 'Hello, world!'] # numbers corresponding to characters [72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33] >>> [n for n in numbers if n % 2 == 0] # filters out the odd numbers [72, 108, 108, 44, 32, 114, 108, 100] In my opinion, list comprehensions are, in most cases, more readable than using map and filter. I won t go so far as to say that you always should use list comprehensions: it s largely a matter of taste, and the demands of each specific programming task.

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Once the mysql gem is installed, connecting to and using a MySQL server from Ruby is almost as simple as using a SQLite database:

require 'rubygems' require 'mysql' # Connect to a MySQL database 'test' on the local machine # using username of 'root' with no password. db = Mysql.connect('localhost', 'root', '', 'test') # Perform an arbitrary SQL query db.query("INSERT INTO people (name, age) VALUES('Chris', 25)") # Perform a query that returns data begin query = db.query('SELECT * FROM people') puts "There were #{query.num_rows} rows returned" query.each_hash do |h| puts h.inspect end rescue puts db.errno puts db.error end # Close the connection cleanly db.close

One of the most common verbal distractions is the use of ller words like um, uh, I mean, and you know. Most people aren t even aware that w they use these words even some of the most experienced speakers. To reduce distracting verbal llers, record yourself when you speak so that you can listen to yourself and count the ums. Or ask someone you know to count the number of ller word occurrences while you speak. The surest cure for this distracting habit is becoming aware that you do it in the rst place.

This code demonstrates a basic, arbitrary SQL query, as well as a query that results in data being returned (in a row-by-row hash format). It also features basic error reporting by catching exceptions with a rescue block and using the error-retrieval methods provided by the MySQL library.

Note If it is speed you are after, you may want to go with map and filter after all. When used with builtin functions, they are faster than list comprehensions.

Note You can also access MySQL databases using the database-agnostic DBI library, covered later in this

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Like MySQL, PostgreSQL (pronounced post-gres-Q-L) is a free relational database server that s available under an open source license, allowing you to download and use it for free. Although PostgreSQL offers many of the same features as MySQL, it s quite different. PostgreSQL users claim it s faster and more stable and offers more features. It is also often claimed that PostgreSQL follows SQL standards more correctly, whereas MySQL is more pragmatic (in the sense that it s more willing to break away from established standards) and extends the SQL language for its own benefits. As with MySQL, PostgreSQL access is achieved in Ruby with a library, also available as a RubyGem. To install, use this code:

On some setups it might be necessary to specify where PostgreSQL is installed, like so: