Pickaxe Book

This chapter is about inheritance, modules and mixins.

To make a subclass:

class Child < Parent
end

You can invoke parent class methods with “super”, just like in Java. You will usually want to call “super” in the initialize method if you are inheriting.

On page 95, it would be nice if the “private” keyword was tabbed in a few spaces.

Modules provide namespaces and allow some reusability. But unlike packages in Java it seems like they are similar to Ruby classes with two differences:
1. They have the word “module” in front of the name instead of the word “class”
2. Methods defined in the module have the module name in front of them. So if a Trigonometry class had a method “sin(x)” tyou would see this:

class Trigonometry
    # This is a constant
    PI = 22/7
    def sin(x)
        # implementation
    end
    # other stuff
end

A module would do it like this:

module Trigonometry
    def Trigonometry.sin(x)
        # implementation
    end
    # other stuff
end

If you use a module constant, you need to specify the module name and use the double-colon operator:

y = Trigonometry.sin(Trigonometry::PI/2)

Modules cannot have instances, so I guess they are like static classes in Java.

Is there inheritance for modules? I may find out

A mixin is when you include a module in a class. Then you can call the module’s method from a class instance as if they were parent methods.

The book talks quite a bit about the Enumerable mixin. I guess all you have to do is provide an “each” method, and you get all the iteration methods in collections (Array and Hash).

At the end of the chapter, they recommend designing programs with composition (has-a) and delegation (uses-a) instead of inheritance (is-a). They seem to say that mixins are more of a composition technique than an inheritance technique.

Image from Wikimedia, assumed allowed under Fair Use. Image from the Ambrosian Iliad, a 5th century manuscript of the Iliad.

This chapter starts out with arrays (square brackets) and hashes (curly braces), which are important to know. It says that blocks when combined with collections are good iterators. The elements in arrays and hashes can be any type. Using different methods, arrays can be different data structures: sets, queues, dequeues, FIFO queues, instead of using different classes. In Ruby hashes, it keeps the objects in the order they were entered. Keep the names of the block parameters unique. You could use a colon, but I think it is better to keep them unique.

A block can be empty:

def three_times
    puts "in method three_times"
    yield
    yield
    yield
    puts "leaving three_times"
end

three_times { } 
# { puts "Hello" }

But you must put in a block if a method has “yield” in it, even if yield has a parameter

To keep track of how many times you have gone through the loop, instead of each, use each_with_index. You will need a second variable for the block: index to track the count.

f = File.open("para.txt")
f.each_with_index do |line,index|
    puts "Line #{index} is: #{line}"
f.close

I did not quite get the “inject” method, and I skipped over some of the stuff about enumerators since it said I could.
Class methods start with the word “self” in front of them

At the end it gets into using blocks for closures and lambdas. Do we get into that with Rails? I think I get this stuff, but sometimes I have to look at it for a few minutes to get it. I don’t think I really know too well. Plus it appears there is another syntax for Ruby 1.9, which makes it more confusing.

There is an encouraging note at the end that if you don’t get it now, it’s okay. Thanks, Dave!

Image from Wikimedia, assumed allowed under Fair Use. Image from the Vatican Virgil, a 5th century manuscript of poems by Virgil.

This chapter was pretty easy. Mostly about objects and variables.

So what is an attribute? It seems like it is another name for an instance variable. They start with an “at” symbol. I am still not too clear what a symbol is.

I did not pay too much attention to the part on protected methods. I read somewhere that Bjarne Stroustrup, the creator of C++, at one point said the “protected” keyword was a mistake. Ever since then I have not paid much attention to protected.

I have finished the first chapter of the Pickaxe book. The first chapter was no big deal for me. It was about getting and installing Ruby and a few basic commands.

I have used rvm to install Ruby on my Ubuntu laptop, and Rails Installer to install Ruby and Rails on my Windows laptop.