1. Topic: Python’s building blocks -> Variables, Values, and Types
CMPT 120
Topic: Python’s building blocks > Variables, Values, and Types

2. Learning outcomes At the end of this course, a student is expected to:
Describe and apply fundamental concepts and terminology of Python:
Literals
Variables
Data types
Execution flow
Create (design) small size programs using Python:
Hand trace code (programs) and predict results of executing code (determining contents of variables and output)

3. Last Lecture Evolution of programming languages
Compiled versus interpreted programming languages
Introduced Python
Illustrated the Software Development Process using Python
Implement algorithm
into computer
program
State problem
Design algorithm
+ Identify data
Test it

4. Today’s Menu Start learning Python’s building blocks Variables Values
Types
and Literal values
Hand tracing
Execution Flow
i-clicker

5. In the real world, there are …
“Things” e.g.:
and
Actions e.g.:

6. In the computer world, there are …
Real World
“Things”
Actions
Computer World
“Things”
->
Actions
-> Scratch:
-> Python:
Data
Variables (objects)
Literal values
Statements (or blocks)
Operators
Functions + methods

7. Input-Process-Output Model
Most computer programs follow this model
Input
Process
Output
Data
Data
Data + Actions

8. Variable
We saw in Lecture 3 that computers are very good at manipulating lot’s of data
But in order for the computer to manipulate this data, we must tell the computer to remember the data
We do this by using variables in our programs

9. Variable Definition: A variable is a memory location
to which we give a name
to which we assign a value of a particular data type
In a computer program, variables are referred to by their name

10. Variable name Syntax rule: Can contain: Cannot contain:
Must start with

11. Variable name Convention 1: use camelCase (see wikipedia)
Example: finalLetterGrade
Convention 2: user underscore between words
Example: final_letter_grade
Python is case sensitive:
variable PRICE
variable Price
variable price
3 different variables

12. Value and their data type
A value can either be
an integral (whole) number
Example:
in Python, this data type is called: int (integer)
a floating point number
in Python, this data type is called: float
a sequence of character(s)
in Python, this data type is called: str (string)

13. Variable – Demo How to create a variable in Python
In doing so, we shall introduce the Assignment operator “=“ (assignment statement)
How to use a variable
In doing so, we shall introduce the output built-in function ”print” (output statement)
How to assign another value to an already existing variable
In doing so, we shall use the Assignment operator “=“ (assignment statement)
Using Python IDLE Interpreter Shell & Program Editor

14. Weakly Typed Python is a weakly typed high level programming language
This means that when we need to use a variable in our Python program, all we need to do is assign a value to our variable in order to create the variable (or the space in memory)

15. Strongly Typed Example: in C
This is not the case in strongly typed high level programming languages like C, C++ and Java, where a variable must first be declared, i.e., one must first state the type of value this variable is to reference, before the variable is used
Example: in C
float radius; <- variable declaration
float areaCircle; <- variable declaration
float pi; <- variable declaration
pi = 3.14; <- variable initialized
radius = 2.5; <- variable initialized
// area of circle = pi * radius2 <- comment
areaCircle = pi * pow(radius,2)<- variables used

16. Literal Value Examples: The values in red are literal values:
apple = 0.75
juice = 1.65
sandwich = 4.80
They are called “literal values” because they are literally typed into a program

17. GPS (Good Programming Style) – 1
Variable name must be descriptive, i.e., it must describe the purpose of the variable or the meaning of its value
For example:
Why?
Creates self-documented code

18. It follows that …
Reusing variable for different purposes is never a good idea!
For example:
Why?

19. GPS (Good Programming Style) – 2
We cannot use a Python keyword as a variable name
Python keywords: Section 2.2 (page 10) of our online textbook
For example:
Why?

20. GPS (Good Programming Style) – 3
Have a look at our Good Programming Style web page and see what it says about Literal Values

21. Summary Started learning Python’s building blocks Variables Values
Types
and Literal values
Hand tracing
Execution Flow
i-clicker

22. Next Lecture Continue learning Python’s building blocks
-> statements
Categories of Statements
Assignment statement
Input statement
Output statement
Operational statement