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1. 3/25/2014
1
More Ruby programming
Iterators, duck typing, inheritance,
and mixins
Iterators
• 3.times { puts "hi" } # output “hi” 3 times
• [4,6,8].each { puts "hi" } # can "ignore" argument
• y = 7
[4,6,8].each { |x|
y = y + x
puts y
}
What is the value of y?
Iterators
• arr = [4,6,8,10]
arr2 = arr.map { |x| x + 1 }
puts arr2 # output another array,
# each element is the result of the block
• sum = arr.inject { |acc,elt| acc + elt }
puts sum # inject is like fold function
# acc is the initial value, elt is the array element
# result of the block is used as initial value
# to apply the block to the next array element
Iterators
• puts (arr.any? { |elt| elt < 0 })
# prints true if one element is negative
• def foo
eight = yield 4
twelve = yield 6
eight + twelve
end
• puts (foo { |x| x + x } ) # what is the output?
Closure
• cl = lambda {|z| z * y}
q = cl.call(9)
puts q
• def foo2 f
eight = f.call 4
twelve = bar f
eight + twelve
end
def bar f
f.call 6
end
puts (foo2 (lambda { |x| x + x })
Duck typing
• Use an instance that “behaves enough like”
the expected ones.
def double x
x + x
end
– double applies to any class of objects that has a +
method that takes self as argument

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Inheritance
• If a class C extends class D, then every
instance of C is also an instance of D
– C inherits the methods of D
– C can add new methods
– C can override inherited methods
– Unlike Java, Ruby fields are not part of a class
definition and therefore, not inherited
Point class
class Point
attr_reader :x, :y
attr_writer :x, :y
def initialize(x,y)
@x = x
@y = y
end
def distFromOrigin
Math.sqrt(x * x + y * y) # uses getter methods
end
end
ColorPoint extends Point
class ColorPoint < Point
attr_reader :color
attr_writer :color
# (say p.color = "green" rather than needing
# p.myColorSetter("green") which does @color="green" in its body)
def initialize(x,y,c="clear") # or could skip this and color starts unset
super(x,y)
@color = c
end
end
3D point extends point
class ThreeDPoint < Point
attr_reader :z
attr_writer :z
def initialize(x,y,z)
super(x,y)
@z = z
end
def distFromOrigin
d = super
Math.sqrt(d * d + z * z)
end
end
Polar point extends point
class PolarPoint < Point
def initialize (r, theta)
@r = r
@theta = theta
end
def x
@r * Math.cos(@theta)
end
def y
@r * Math.sin(@theta)
end
…
# distFromOrigin already works!!!
end
Polar point extends point
class PolarPoint < Point
…
def x= a
b = y # avoids multiple calls to y method
@theta = Math.atan (b / a)
@r = Math.sqrt(a*a + b*b)
self
end
def y= b
a = x # avoid multiple calls to x method
@theta = Math.atan (b / a)
@r = Math.sqrt (a*a + b*b)
self
end
# distFromOrigin already works!!!
end

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Dynamic dispatch
def distFromOrigin
Math.sqrt(x * x + y * y) # uses getter methods
end
Math.sqrt(self.x() * self.x() + self.y() * self.y())
This method inherited from Point class still works
because the method “x” and “y” are overridden
in the subclass PolarPoint
Multiple inheritance, interfaces, and
mixins
• Languages (C++) with multiple inheritance let one class
extend multiple other classes
– Most powerful option
– Has semantic problems
– Java and Ruby do not use it
• Interfaces: Java has single inheritance but a Java class can
implement multiple interfaces
– An interface defines method signatures but not implementation
• Mixins: Ruby allows a class to have only one super class but
can include any number of mixins
– Mixin is “just a pile of methods”
– Mixin provides behavior while interface only provides types,
which is not an issue for dynamic languages such as Ruby
Multiple Inheritance
• In some languages (such as C++) a class can
have more than one base class
• Seems simple at first: just inherit fields and
methods from all the base classes
• For example: a multifunction printer
MultiFunction
Printer Copier Scanner Fax
Collision Problem
• The different base classes are unrelated,
and may not have been designed to be
combined
• Scanner and Fax might both have a
method named transmit
• When MultiFunction.transmit is
called, what should happen?
MultiFunction
Printer Copier Scanner Fax
Diamond Problem
• A class may inherit from the same base
class through more than one path
• If A defines a field x, then B has one and so
does C
• Does D get two of them?
D
B C
A
Solvable, But…
• A language that supports multiple inheritance
must have mechanisms for handling these
problems
• Not all that tricky
• The question is, is the additional power worth
the additional language complexity?
• Java’s designers did not think so

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Living Without Multiple Inheritance
• One benefit of multiple inheritance is that a
class can have several unrelated types (like
Copier and Fax)
• This can be done in Java by using interfaces:
a class can implement any number of
interfaces
• Another benefit is inheriting
implementation from multiple base classes
• This is harder to accomplish with Java
public class MultiFunction {
private Printer myPrinter;
private Copier myCopier;
private Scanner myScanner;
private Fax myFax;
public void copy() {
myCopier.copy();
}
public void transmitScanned() {
myScanner.transmit();
}
public void sendFax() {
myFax.transmit();
}
…
}
Forwarding
Interfaces
• A method prototype just gives the method
name and type—no method body
• An interface in Java is a collection of
method prototypes
public interface Drawable {
void show(int xPos, int yPos);
void hide();
}
Implementing Interfaces
• A class can declare that it implements a
particular interface
• Then it must provide public method
definitions that match those in the interface
Examples
public class Icon implements Drawable {
public void show(int x, int y) {
… method body …
}
public void hide() {
… method body …
}
…more methods and fields…
}
public class Square implements Drawable, Scalable {
… all required methods of all interfaces implemented …
}
Why Use Interfaces?
• An interface can be implemented by many
classes:
• Interface name can be used as a reference
type:
public class Window implements Drawable …
public class MousePointer implements Drawable …
public class Oval implements Drawable …
Drawable d;
d = new Icon("i1.gif");
d.show(0,0);
d = new Oval(20,30);
d.show(0,0);

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Polymorphism With Interfaces
• Class of object referred to by d is not
known at compile time
• It is some class that implements
Drawable, so it has show and hide
methods that can be called
static void flashoff(Drawable d, int k) {
for (int i = 0; i < k; i++) {
d.show(0,0);
d.hide();
}
}
A More Complete Example
• A Worklist interface for a collection of
String objects
• Can be added to, removed from, and tested
for emptiness
public interface Worklist {
/**
* Add one String to the worklist.
* @param item the String to add
*/
void add(String item);
/**
* Test whether there are more elements in the
* worklist: that is, test whether more elements
* have been added than have been removed.
* @return true iff there are more elements
*/
boolean hasMore();
/**
* Remove one String from the worklist and return
* it. There must be at least one element in the
* worklist.
* @return the String item removed
*/
String remove();
}
Interface Documentation
• Comments are especially important in an
interface, since there is no code to help the
reader understand what each method is
supposed to do
• Worklist interface does not specify
ordering: could be a stack, a queue, or
something else
• We will do an implementation as a stack,
implemented using linked lists
/**
* A Node is an object that holds a String and a link
* to the next Node. It can be used to build linked
* lists of Strings.
*/
public class Node {
private String data; // Each node has a String...
private Node link; // and a link to the next Node
/**
* Node constructor.
* @param theData the String to store in this Node
* @param theLink a link to the next Node
*/
public Node(String theData, Node theLink) {
data = theData;
link = theLink;
}

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/**
* Accessor for the String data stored in this Node.
* @return our String item
*/
public String getData() {
return data;
}
/**
* Accessor for the link to the next Node.
* @return the next Node
*/
public Node getLink() {
return link;
}
}
/**
* A Stack is an object that holds a collection of
* Strings.
*/
public class Stack implements Worklist {
private Node top = null; // top Node in the stack
/**
* Push a String on top of this stack.
* @param data the String to add
*/
public void add(String data) {
top = new Node(data,top);
}
/**
* Test whether this stack has more elements.
* @return true if this stack is not empty
*/
public boolean hasMore() {
return (top!=null);
}
/**
* Pop the top String from this stack and return it.
* This should be called only if the stack is
* not empty.
* @return the popped String
*/
public String remove() {
Node n = top;
top = n.getLink();
return n.getData();
}
}
A Test
• Output: The cut worm forgives the plow.
• Other implementations of Worklist are
possible: Queue, PriorityQueue, etc.
Worklist w;
w = new Stack();
w.add("the plow.");
w.add("forgives ");
w.add("The cut worm ");
System.out.print(w.remove());
System.out.print(w.remove());
System.out.println(w.remove());
Mixins
• Ruby mixins are somewhere between multiple
inheritance and interfaces
– They provide actual code for classes to include them but
they are not classes themselves
– Do not have constructors or a separate notion of fields
module Color
attr_accessor :color
def darken
self.color = "dark " + self.color
end
end
Include a mixin in class definition
module Color
attr_accessor :color
def darken
self.color = "dark " + self.color
end
end
class ColorPoint < Point
include Color
end

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Method look up
• obj.m # obj is an instance of the class C
– Look in C for method m first
– Then look in the mixins included in C
• Later ones shadow earlier ones
– Look in C’s superclass
– Look in C’s superclass’ mixins
– Look in C’s super-superclass
– Continue until m is found or reach the Object class
Mixin may call hook methods
module Doubler
def double
self + self # uses self’s + method, not defined in Doubler
end
end
class AnotherPoint
attr_accessor :x, :y
include Doubler
def + other # add two points
ans = AnotherPoint.new
ans.x = self.x + other.x
ans.y = self.y + other.y
ans
end
end
class String
include Doubler
end
Convenient Ruby mixins
• Both Enumerable and Comparable are mixins in
Ruby
• Comparable provides =, !=, >, >=, <, and <=
– Assume the classes that include comparable has the
method <=>
• a <=> b < 0 if a < b
• a <=> b > 0 if a > b
• a <=> b = 0 if a == b
– A class like Integer only needs to define <=> method
and then include Comparable to have a bunch of
methods for comparison
An example use of Comparable
class Name
attr_accessor :first, :middle, :last
include Comparable
def initialize(first,last,middle="")
@first = first
@last = last
@middle = middle
end
def <=> other
l = @last <=> other.last # <=> defined on strings
return l if l != 0
f = @first <=> other.first
return f if f != 0
@middle <=> other.middle
end
end
Define Comparable methods
def > other
(self <=> other) > 0
end
Enumerable module
• Enumerable defines methods such as any?,
map, and inject
– Assume the enumerable classes have “each”
method
– Array class defines “each” method and includes
Enumerable mixin

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Enumerable example
class MyRange
include Enumerable
def initialize(low,high)
@low = low
@high = high
end
def each
i=@low
while i <= @high
yield i
i=i+1
end
end
end
Enumerable example
class MyRange
include Enumerable
def initialize(low,high)
@low = low
@high = high
end
def each
i=@low
while i <= @high
yield i
i=i+1
end
end
end
MyRange.new(4,8).inject{|x,y| x+y}
Enumerable example
class MyRange
include Enumerable
def initialize(low,high)
@low = low
@high = high
end
def each
i=@low
while i <= @high
yield i
i=i+1
end
end
end
MyRange.new(4,8).inject{|x,y| x+y}
def map
arr = []
each {|x| arr.push x }
arr
end