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Thurs 2016.1.21
Python
Chapter 9.1 through 9.5
same
Buffalo Nickel Moment
Show the random selector program first.
A. Scope of names
Scope – where things are visible (things = names)
Also talk about global code, i.e.
code inside a file but not inside a
function
Global vrs. Local
Class vrs. Instance
Global variables and global functions exist outside of any other functions or classes.
Local variables are only visible inside functions.
Parameters are always local.
Inside a fucntion…
If you use a name, Python assumes it is global and looks for it outside the function.
If you assign to a name, Python assumes it is a local and creates a new local version.
If you want to assign to a global variable, you must first declare it as a global.
names = ["Mark", "Sally", "Kathy"]
def silly(s):
if s in names:
print ("We know",s)
names = ["Mark", "Sally", "Kathy"]
def silly(s):
names = ["weird", "stuff", s]
names = ["Mark", "Sally", "Kathy"]
def silly(s):
global names
names = ["weird", "stuff", s]
names = ["Mark", "Sally", "Kathy"]
def silly(s):
names[0] = s
Since names is used in silly but not assigned
to, Python looks for names outside, and it uses
the global variable.
Since you assign to names inside the function,
Python thinks names is a local. So the names
inside is NOT the same as names outside, and
no change occurs.
Here you are assigning but you have to tell
Python that the names you are assigning to
inside the function is the same as the one
outside.
Here you are not assigning to names itself, but
are actually changing the existing value in the
variable, so global is not needed. Python
assumes you mean the global names.
B. Object Oriented programming Part 1: evolution of ideas
Arrays were the first composite objects. Parallel arrays were the first form of quasi-database.
Objects are homogeneous in arrays (all the same) at least the same size. Nobody talked about
"objects" in 1957. They talked about values and arrays.
COBOL (1960) introduced the record. C programmers know it as "struct". The record is
heterogeneous. You talked about fields, not positions, within the collection. And these fields
were named.
struct employee {
char name[200];
int yearBorn;
double salary;
int married;
// Boolean
char race;
};
struct employee
fulltimers[1000];
// array of structs
fulltimers[0].yearBorn = 1957;
fulltimers[1].salary = 100573.44;
Of course to make the mini-database, you'll need an array of these records or structs.
Python's dictionary is a generalization of a struct. In C, you associate a name (which is a string)
to a value inside a struct (see above.) In Python, you can do the same (although the syntax is
different, don't use the dot notation.) But in Python, the "name" can be any kind of object,
including numbers, strings, sets, tuples, etc!
Simula-67 (created by Ole-Johan Dahl and Kristen Nygaard in the 1960s in Oslo, Norway) was
used for simulation. Introduced so many concepts that are in all our languages today. Amazing!
It was the first object oriented programming lang.
Think of a class as a struct with methods added to it. These methods often work on the fields of
the struct:
struct employee {
char name[200];
int yearBorn;
double salary;
int married;
// Boolean
char race;
void getRaise(double howMuchPct) {
this.salary = this.salary + this.salary * howMuchPct;
}
};
Bjarne Stroupstrup created a "C with classes" in 1979 that did exactly this! He later renamed it
to C++ because ++ means "add 1" or "go to the next level."
Later C# was so named because it was C++++ or put two of the pluses underneath the others.
C++ was wildly successful. Most modern OS are written in it! But it is complex and hard to
learn.
Smalltalk was another OOP system dating from 1980 at Xerox PARC where the mouse and
WIMP interface (the GUI) was invented.
Apple stole some of the ideas from Smalltalk and pasted them onto C and called it Objective C.
That was what a lot of ios apps are written in. It has been replaced with Swift (2014).
C. Object Oriented programming Part 2: nuts and bolts
Read my Very quick tutorial (on D2L) starting with program 18
Modules and classes: what is the difference?
a module is essentially just a file containing Python code. It may contain global
variables, functions, classes. Modules cannot be nested. You do not make instances of
modules. You import modules.
Functions versus methods
How to define a class
How to make an instance of a class (i.e. an object)
Ways that classes relate to each other:
Inheritance (isa)
Composition (hasa)
Interaction (toucha)
What is a pointer (a reference)?
Functions and their parameters:
call by value (for integers and strings and floats)
call by reference (objects)
Some examples:
Point class: what are its members and methods? (pretty minimal, could convert to polar)
Line: what are its members? what are its methods? (slope, distance between points)
is the line finite or infinite? Defined by two points, or by a point and a slope?
2-way dictionary (1-to-1, not 1-to-many)
Several different ways of implementing this (2 dicts, or 1 dict plus searching)
RWOs = Real World Objects
Examples: book, library, employee, department
Clocks and calendars
clocks—military time (24-hour)
12-hour clock
Inheritance:
In Python:
class X(Y):
In Java:
class X extends Y;
Class X is the subclass. Class Y is the superclass. Two ways of relating:
1. The subclass "decorates" or augments the superclass. It extends it, adds stuff to it.
2. The subclass is a special version of the superclass. It restricts or constricts it. The
superclass is a more general version of it.
Taxonomy of living things:
http://en.wikipedia.org/wiki/Zoology_mnemonic
Kingdom  Phylum  Class  Order  Family  Genus  Species
All domestic cats are Felis catus. Felidae, all ar carnivora, mammalia, chordata, animalia
It is more common to use the augmentation version, because in business you want to say for
example that you have employees. Some are workers, some are managers, etc. Some part-time,
some semi-retired, etc.
Point3D (x,y,z)
Point2D (x,y)
is a special version of Point3D where z=0 always
or You could let Point be the superclass, and then Point2D and Point3D come off that.
box-shapes
quadrilateral
parallelogram
rectangle
square
Containers:
list, set, tuple, bag, limited bag, ordered set
List: ordered, heterogeneous, not unique
Set: unordered, hetero-, unique
Bag: unordered, hetero-, not unique
Limited Bag: like a Bag but there are limits on the number of instances (applies to all or a
different limit for each value?)
Ordered Set: ordered, hetero-, unique
In some strongly typed prog. langs like Java or C, arrays are homogeneous.
ArrayList is a kind of generic type. ArrayList<String> is a subclass of ArrayList with a
limitation on it.
We thought about a OneWayDict()
G2E = OneWayDict()
# German to English (and back)
API
get
set
del
in
lookup a word
assign a new word pair or change existing word pair
remove a word pair
2 functions
a. __contains__ (if x in mylist: )
b. iterator (for x in mylist:)
How can we implement this?
Implement a class called Oracle.