Transcript Notes here - Adelphi University

TeachScheme, ReachJava
Adelphi University
Friday morning
July 16, 2010
Class composition
Define a class LogEntry to represent a runner's daily log. It
contains the Date of the run, the distance in miles, the time in
minutes, and a free-form comment.
Include
• a constructor
• several examples
• a toString method
• an avgSpeed method
• an addComment method (which takes in a String and returns a
LogEntry just like the old one but with the String added onto
whatever comments were already there).
Class composition
Define a class Circle to represent a circle on the screen. It contains a
center (of type Posn), a radius (double), and a color (String).
Include
• a constructor
• several examples
• a toString method
• an area method
• a contains method that takes in another Posn and returns a boolean
indicating whether that Posn is inside the circle
• a scale method that takes in a double scaling factor and returns a new
Circle like this one but with the radius multiplied by the scaling factor.
Class composition
Define a class Rectangle to represent a rectangle on the screen. It
contains a top-left corner (of type Posn), a width and height (both
double), and a color (String).
Include
• a constructor
• several examples
• a toString method
• an area method
• a contains method that takes in another Posn and returns a boolean
indicating whether that Posn is inside the rectangle
• a scale method that takes in a double scaling factor and returns a new
Circle like this one but with the width and height multiplied by the
scaling factor.
Definition by choices
Define a data type Shape which is either a
Circle or a Rectangle.
Since Circle and Rectangle both have
constructors, Shape doesn't need one.
Definition by choices
interface Shape
{
}
…
class Circle implements Shape
{
…
}
interface Shape { }
class Circle
implements Shape
{
Posn center;
double radius;
String color;
…
}
class Rectangle
implements Shape
{
Posn topLeft;
double width;
double height;
String color;
…
}
What can you do with this?
• A variable of type Shape can hold either a Circle
or a Rectangle:
Shape shape1 = new Circle
(new Posn(3,4),5,"blue");
Shape shape2 = new Rectangle
(new Posn (50,20), 30, 40, "orange");
What can't you do with this?
shape1.area() doesn't compile!
Why not?
In Java, every variable has two types: the static type from its
declaration, and the dynamic type from what it actually
contains.
shape1 was declared as a Shape, so that's its static type.
Static type is used to decide what's a legal call and what isn't.
To fix this…
interface Shape
{
public double area ();
public boolean contains (Posn other);
public Shape scale (double factor);
}
What can you do with this?
Now you can call the area, contains, and
scale methods on a Shape variable
shape1.area() // should return c. 78.54
shape2.area() // should return 1200
shape1.scale(2.0) // should return
// new Circle(new Posn(3,4), 10, "blue")
etc.
time check
Lists in Java
A StringList is either an EmptyStringList or a
NonEmptyStringList (ESL or NESL for
short).
An ESL has no parts.
A NESL has two parts: first (a String) and rest
(a StringList).
Lists in Java
Write classes ESL and NESL, and interface
StringList. For each class, provide
• a constructor
• examples
• a toString method
Lists in Java
Write the following methods on StringLists:
• countStrings : nothing -> int
• contains : String -> boolean
• countMatches : String -> int
Recall add-up function
(define (add-up nums)
(cond [(empty? nums) 0]
[(cons? nums)
(+ (first nums)
(add-up (rest nums)))]))
Trace add-up function
(add-up (list 3 5 2 1))
(+ 3 (add-up (list 5 2 1)))
(+ 3 (+ 5 (add-up (list 2 1))))
(+ 3 (+ 5 (+ 2 (add-up (list 1)))))
(+ 3 (+ 5 (+ 2 (+ 1 (add-up empty))))) ; lots of pending +'s
(+ 3 (+ 5 (+ 2 (+ 1 0))))
(+ 3 (+ 5 (+ 2 1))))
(+ 3 (+ 5 3))
(+ 3 8)
11
Another approach
(define (add-up-accum nums so-far)
(cond [(empty? nums) so-far]
[(cons? nums)
(add-up-accum (rest nums)
(+ (first nums) so-far))]))
(add-up-accum (list 3 5 2 1) 0)
(add-up-accum (list 5 2 1) (+ 3 0))
(add-up-accum (list 5 2 1) 3)
(add-up-accum (list 2 1) (+ 5 3))
(add-up-accum (list 2 1) 8)
(add-up-accum (list 1) (+ 2 8))
(add-up-accum (list 1) 10)
(add-up-accum empty (+ 1 10))
(add-up-accum empty 11)
11
; never more than 1 pending +
Another approach
Of course, add-up-accum is less convenient to use.
Easy fix:
(define (add-up nums)
(add-up-accum nums 0))
Another example
; multiply-positives : list-of-numbers -> number
(define (multiply-positives nums)
(mp-accum nums 1))
(define (mp-accum nums so-far)
(cond [(empty? nums) so-far]
[(cons? nums)
(mp-accum (rest nums)
(cond [(> (first nums) 0) (* (first nums) so-far)]
[else so-far]))]))
Generalize
Both functions look pretty similar. They
differ in
the answer to the "empty?" case, and
how to combine (first nums) with so-far
In Java…
See projects June26 v1 through June26 v5
v1: addUp written by structural recursion
v2: addUp written by accumulative recursion
v3: addUp becomes static, in a separate class
v4: addUp written using for-each loop
v5: addUp written using while-loop
Are we done yet?
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