1. Variables, Lists, and Objects
Professor Hugh C. Lauer CS-1004 — Introduction to Programming for Non-Majors
(Slides include materials from Python Programming: An Introduction to Computer Science, 2nd edition, by John Zelle and copyright notes by Prof. George Heineman of Worcester Polytechnic Institute)
CS-1004, A-Term 2014
Variables, Lists, and Objects

2. Variables, Lists, and Objects
Before we start
Chapter 3 is about Computing with Numbers
Talked about numeric data types and conversion
Table 3.2 is a useful list of math library functions
§3.3 illustrates the factorial function
How it is constructed
Useful example of counting a for-loop from highest index to lowest
Be sure to read thru this chapter so that …
… you know where to look when you need to know this stuff during programming
CS-1004, A-Term 2014
Variables, Lists, and Objects

3. Variables, Lists, and Objects
“A variable is used to give a name to a value so that we can refer to it at other points in a program.”
p. 16
E.g., ints, floats, etc.
An assignment is the act of associating a value with that name
The association of name and value may change whenever the assignment statement is executed
CS-1004, A-Term 2014
Variables, Lists, and Objects

4. Variables (continued)
Variable may refer to other kinds of things beyond simple values
Lists
Objects
Functions
Every variable in Python “knows” its type
i.e., the kind of thing it currently refers to
Unlike many other programming languages
Type of a variable is fixed at time program is written!
Cannot be changed
Only things of right type can be assigned to variable
Java, C, C++, etc.
CS-1004, A-Term 2014
Variables, Lists, and Objects

5. Variables, Lists, and Objects
Questions?
CS-1004, A-Term 2014
Variables, Lists, and Objects

6. Variables, Lists, and Objects
A collection of values (and other things) enclosed in square brackets, separated by commas
[1, 2, 3, 5, 7, 11, 13, 17, 19, 23]
May be all of the same type …
Examples
… or of different types
May be assigned to a variable
May be arguments and results of functions!
CS-1004, A-Term 2014
Variables, Lists, and Objects

7. Variables, Lists, and Objects
Accessing Lists
L = [1, 2, 3, 4]
Individual list items referred to as
List_Name[item_#]
Examples
List items indexed from zero
Error if indexing off end of list
Individual list entries may be assigned to
List_Name[item_#] = expression
Lists may change in size
Add elements to beginning, middle, or end
Delete elements
CS-1004, A-Term 2014
Variables, Lists, and Objects

8. Variables, Lists, and Objects
Lists (continued)
May be used as control of for-loop
for i in [1, 2, 3, 5, 7, 11, 13, 17, 19]:
for j in L:
The empty list
E = []
Many operations on lists
What operation needed for HW #2?
A list in Python is a little bit like an array in C or other programming languages …
… more powerful conceptually
… less efficient computationally
Where a list had been previously
assigned to L
Your time is much more valuable than your computer’s time!
CS-1004, A-Term 2014
Variables, Lists, and Objects

9. Variables, Lists, and Objects
Thinking about Lists
A list is (essentially) a collection of variables represented sequentially under one name
Each element of a list knows the type of the thing it refers to
Just like a variable
Each element of a list can be changed independently of other elements
List and each element retains value until changed (or forgotten)
Not like a variable!
What do we mean by “forgotten”?
CS-1004, A-Term 2014
Variables, Lists, and Objects

10. Variables, Lists, and Objects
Questions?
CS-1004, A-Term 2014
Variables, Lists, and Objects

11. Variables, Lists, and Objects
Reading assignment:– Chapter 4
Needed for HW #3
And everything else we do in this course!
Object: computational abstraction that includes
Data
Methods (a. k. a. Functions)
P. 81:–
Objects “know” stuff
Objects “do” stuff
Objects can refer to other objects
Objects can interact with other objects
An object may be assigned to a variable
Or, equivalently, to an element of a list
CS-1004, A-Term 2014
Variables, Lists, and Objects

12. Variables, Lists, and Objects
Objects (continued)
Objects are organized into Classes
All objects in a class have the same kind of information
All objects in a class have the same set of methods (a.k.a. functions)
Classes are discussed in Chapter 10
Including how to define new classes
How to construct objects of a class
How to define methods of a class
How to define data elements of a class object
Probably will not get to this chapter in this course
CS-1004, A-Term 2014
Variables, Lists, and Objects

13. Variables, Lists, and Objects
Object Notation
If X is an object of class C, …
… and if class C has a method/function named f, …
… then the notation
X.f()
means “Apply the function f (and its arguments) to the object X”
I.e., f may cause the object X to DO something
Or f may cause the object X to change what it knows
CS-1004, A-Term 2014
Variables, Lists, and Objects

14. Object Notation (continued)
Already encountered similar notation
E.g., appending to a list
L = [] for x in range(10): L.append(sin(x))
E.g., access members and member functions of a package
from matplotlib import pyplot pyplot.plot( <arguments> )
… that is associated with this object, list, class, or package
Call the function with this name …
CS-1004, A-Term 2014
Variables, Lists, and Objects

15. Variables, Lists, and Objects
Creating Objects
If C is a class, then the constructor function for that class is also named C
V = C(arg1, arg2, …) creates a new object of class C and assigns it to variable V
All of the methods associated with class C are now available for object V
Examples
CS-1004, A-Term 2014
Variables, Lists, and Objects

16. Variables, Lists, and Objects
Assigning Objects
V = SomeClass(<arguments> )
W = V causes W and V to now refer to the very same object
Not to copies of each other, but exactly the same object
Incidentally,
The same is true for lists.
If L is a list, M = L causes M to refer to the same list, not a copy of the list
Changes to one are reflected in the other!
CS-1004, A-Term 2014
Variables, Lists, and Objects

17. Variables, Lists, and Objects
Questions?
CS-1004, A-Term 2014
Variables, Lists, and Objects

18. Why introduce Objects so early in the course?
Because graphics.py is an “object-oriented” package
Simple enough to introduce now
A chance to do some cool stuff in your first course in programming!
CS-1004, A-Term 2014
Variables, Lists, and Objects