1. Winter 2018 CISC101
11/15/2018
CISC101 Reminders
Your group name in onQ is your grader’s name. Let me know if you are not in a group.
Prof’s office hours set to Thursdays 1:45 to 2:30 in GOO More times will be added, if needed.
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

2. Today From last week: Origins and History of Python.
How commands must be translated to binary for the CPU to be able to process them.
Upper level languages like Python are translated to Assembler which is then translated to machine code for execution.
Origins and History of Python.
The “Hello World” Ritual.
If we have time: Start Python Expressions.
Winter 2018
CISC101 - Prof. McLeod

3. Computer Languages: History
People became a bit frustrated with Assembler!
The next generation of computer languages went up one more level, getting closer to something readable - for example: Fortran, Cobol and Lisp.
These languages led to an explosion of over 200 languages being developed in the 60’s and 70’s, such as Basic, Pascal, C, Ada and Smalltalk.
Python is a relative newcomer, arriving on the scene in the early 90’s.
Winter 2018
CISC101 - Prof. McLeod

4. Aside – 99 Bottles of Beer…
See:
Programs in 1500 different programming languages to generate the lyrics to the “song”.
Winter 2018
CISC101 - Prof. McLeod

5. Aside - Malbolge
A programming language designed to be extremely hard to program in and impossible to read.
Named after the eighth circle of Hell in Dante’s “Inferno”.
Featured in an episode of Elementary (S01E10).
They supposedly had a program in Malbolge that de-crypted a safe’s combination.
What they really had was “Hello World” in Malbolge:
Winter 2018
CISC101 - Prof. McLeod

6. “Hello World” in Malbolge
Winter 2018
CISC101 - Prof. McLeod

7. Yikes! Python is much, much friendlier!! “Hello World” in Python:
print("Hello World")
Winter 2018
CISC101 - Prof. McLeod

8. CISC101
History of Python
The language was created by Guido van Rossum at Stichting Mathematisch Centrum in the Netherlands in the early 90’s.
He is still very involved with the language and retains the title “BDFL”, which stands for “Benevolent Dictator for Life”.
Python is named after “Monty Python”, not the snake!!
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

9. History of Python, Cont. He wanted to make the language:
an easy and intuitive language, but just as powerful as major competitors.
open source, so anyone can contribute to its development.
use code that is as understandable as plain English.
to be suitable for everyday tasks, allowing for short development times.
First released in 1994, the language was inspired by Modula-3, Lisp, SETL, Haskell, Icon and Java.
A compilation of all the “Best-Of’s” from many other languages!
Winter 2018
CISC101 - Prof. McLeod

10. Features of Python High Level Object Oriented Scalable Extensible
Most notable are the built-in data structures.
Object Oriented
OOP helps you to build code in a modular way. But, Python allows you to write code without knowing anything about OOP!
Scalable
Packaging of code allows even very large programming projects to be manageable.
Extensible
You can easily use external code modules written in Python, C, C++, C#, Java or Visual Basic.
Winter 2018
CISC101 - Prof. McLeod

11. Features of Python, Cont.
Portable
Runs on any platform/OS combination that can run C.
Easy to Learn (!)
Relatively few keywords, simple language structure and clear syntax. OOP can be avoided while learning.
Easy to Read
Much less punctuation than other languages. Forces you to use good indentation.
Easy to Maintain
Results from the two above features!
Winter 2018
CISC101 - Prof. McLeod

12. Features of Python, Cont.
Robust
Exception handlers provide safe, sane and informative crashes.
Good for Rapid Prototyping
Often used with other languages to create prototypes and provide a testing platform.
Built-In Memory Management
Like Java. Avoids a major problem in C/C++.
Interpreted
Not compiled – each command is executed as it is read from the program. (Speed can be increased a bit using byte-compiled files (like Java does).)
Winter 2018
CISC101 - Prof. McLeod

13. “Easter Egg” Execute the command import this at the Python prompt…
For more detail on the history of Python, see:
Winter 2018
CISC101 - Prof. McLeod

14. Hello World Ritual Print “Hello World” at the >>> prompt.
Do the same thing from a script, or “program”.
Write a program that prompts the user for his first name and then displays “Hello name” to the console. Put the code in a main function.
Just for fun, disassemble the program when it is done.
Winter 2018
CISC101 - Prof. McLeod

15. Questions Do you have to put code in a main function?
Why is it called “main()”?
What’s with the “()”? They are empty!
Why is there a call to main() at the end of the program?
How can you tell what code is inside main() and what is outside?
Winter 2018
CISC101 - Prof. McLeod

16. Questions, Cont. What does the input() BIF return?
How can you get a number, like an int, from input()?
What does the print() BIF return?
What does “+” do with strings?
I can add numbers with “+”, as well. Can I add a number to a string?
Winter 2018
CISC101 - Prof. McLeod

17. Python Expressions A program is a series of expressions, one per line.
An expression is built from one or many of the following pieces:
Literal values
Variables
Keywords
Function and method calls
Punctuation
“Syntax” supplies the rules about how these pieces go together so the interpreter can understand our commands.
Winter 2018
CISC101 - Prof. McLeod

18. Numeric Types in Python
CISC101
Numeric Types in Python
In Exercise 1 you (have or will?) discovered that literal values can be of different types.
What numeric types did you find?
int
float
complex
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

19. Numeric Types, Cont.
Each type’s literal value is characterized by the way it is typed into a program.
Do you remember the characteristics of each type? How does the interpreter tell them apart?
Winter 2018
CISC101 - Prof. McLeod

20. Numeric Types, Cont.
The int type is an integer (no decimal or exponent) and there is no limit to its size.
The float type is characterized by a decimal place and/or an exponent. It is limited to about 17 digits.
(We won’t use the complex type much!)
Winter 2018
CISC101 - Prof. McLeod

21. float Type For example to code the real number:
You would write:
2.43E-4 or 2.43e-4
exponent
decimal
Winter 2018
CISC101 - Prof. McLeod

22. Other Bases
Normally we view numbers in base 10, or in a “radix” of 10. That’s the default in Python.
How can you create literal numbers in base 2, 8 or 16?
Use the prefixes: 0b, 0o or 0x on literals.
Use the BIFs: bin(), oct() and hex() to display a value in another base.
Winter 2018
CISC101 - Prof. McLeod

23. Other Types What other types did you find? bool str
How about the collections?
bool
str
list
tuple
dict
set
We’ll talk more about these later…
Winter 2018
CISC101 - Prof. McLeod

24. Python Types, Cont.
Python determines the type of a literal value by examination.
When the value is assigned to a variable, the variable is typed to match the type of the value.
Python is a dynamically typed language. Which means that a variable can change types.
Winter 2018
CISC101 - Prof. McLeod

25. From Exercise 1 – Literal Types
What are the types of the following literals?
45.237
3.4e-7
45e10
123
0b100101
0x45cde
'Hello!'
"Again"
"""line1
Line2"""
True
[4, 6, 7.9, "Dingdong"]
(4, 5, True)
{'first':'Alan', 'last':'McLeod'}
{3, 4, 7, 10}
"H"
3.4E10
Winter 2018
CISC101 - Prof. McLeod

26. From Exercise 1 – Types, Cont.
How can you discover the type of a variable?
How can you change a literal of one type to another?
Winter 2018
CISC101 - Prof. McLeod

27. CISC101
Variables
A variable is a name used in a program to refer to a specific location of memory.
In Python, variables are created by an assignment statement (or in function parameter lists).
A variable takes the type of the value being assigned to it when the program runs. A variable’s value can change any time, as can its type.
Winter 2018
CISC101 - Prof. McLeod
Prof. Alan McLeod

28. Assigning/Creating a Variable
In code:
myVal = 20
Now myVal refers to some location in RAM that stores the int type value 20.
We don’t have to worry about what the actual memory address is.
Winter 2018
CISC101 - Prof. McLeod

29. Variable Naming Syntax Rules
Variable names are case sensitive.
You can’t use a Python keyword for a variable name.
No spaces!
Start with a letter (use lower case, by convention), or an underscore _.
The rest of the name can contain numbers, letters or the underscore.
Why no spaces, anyways?
Winter 2018
CISC101 - Prof. McLeod

30. Variable Naming Style Rules
Use descriptive names.
Capitalize successive words in a name using camelCase.
No limit to the length of a variable name, but don’t write an essay!!
Don’t use single letter variable names, except if you need a loop counter that has no intrinsic meaning, then you can use i, j or k.
Winter 2018
CISC101 - Prof. McLeod

31. Aside – The Worst Variable Name!
My favourite bad variable name: l1
Winter 2018
CISC101 - Prof. McLeod