L-2

Python Fundamentals:
Variables, Operators and Expressions

ENGG1330
Computer Programming I

$HKU School of Computing and Data Science$

We’re Starting at Different Levels

49% selected one of the two beginner options

Thought Python was about snakes

Complete beginner

40%

Written a few small programs

37%

Comfortable coding independently

11%

Expert

“I am Guido van Rossum”

How Did Learning to Code Feel?

“Excited” was the most common response

22%excited

5%interesting

4%exciting

4%nervous

3%fun

3%hard

2%anxious

2%curious

Would You Press the Button?

A guaranteed B · No work · Nothing learned

86% chose “No”

86%

I’m torn

Yes

L-1 Participation Snapshot

Approximate figures reported by Kevin

Completed survey + challenge

≈30%

Completed only the survey

≈50%

The survey is only one part: remember the coding challenges too

Terminology

ENGG1330
Computer Programming 1

$HKU School of Computing and Data Science$

Terminology

We combine literals and variables with operators
to form expressions

Variables?
Operators?
Literals?
Expressions?

Terminology

pi = 3.14159diameter = 3area = pi * ( ( diameter / 2) ** 2 )print(area)

Download: engg1330.dirk.hk/lec_02_circle_area.py

Terminology

pi = 3.14159diameter = 3area = pi * ( ( diameter / 2) ** 2 )print(area)

Download: engg1330.dirk.hk/lec_02_circle_area.py

Literals

Raw data in code

Terminology

pi = 3.14159diameter = 3area = pi * ( ( diameter / 2) ** 2 )print(area)

Download: engg1330.dirk.hk/lec_02_circle_area.py

Variables

Remember stuff by storing it in memory

Terminology

pi = 3.14159diameter = 3area = pi * ( ( diameter / 2) ** 2 )print(area)

Download: engg1330.dirk.hk/lec_02_circle_area.py

Operators

Special symbols that carry out operations

Terminology

pi = 3.14159diameter = 3area = pi * ( ( diameter / 2) ** 2 )print(area)

Download: engg1330.dirk.hk/lec_02_circle_area.py

Expressions

Combine literals, variables and operators

Terminology

●Literals

○Raw data in code

●Variables

○Store data in memory

●Operators

○Special symbols that carry out operations

●Expressions

○Combine literals and variables to create new values

Literals

ENGG1330
Computer Programming 1

$HKU School of Computing and Data Science$

Literals Have Different Data Types

pi = 3.14159diameter = 3area = pi * ( ( diameter / 2) ** 2 )print(area)

Download: engg1330.dirk.hk/lec_02_circle_area.py

3
Integer (int)

3.14159
Float (float)

'Bruce Lee'
String (str)

True
Boolean (bool)

Variables

ENGG1330
Computer Programming 1

$HKU School of Computing and Data Science$

Variables Are Used to Store Data in Memory

a = 3b = 3.14159c = 'Bruce Lee'd = True

Download: engg1330.dirk.hk/lec_02_example_17_02.py

Variables Have Unique Names (Identifiers)

a = 3b = 3.14159c = 'Bruce Lee'd = True

Download: engg1330.dirk.hk/lec_02_example_17_02.py

Variables Have a Value

a = 3b = 3.14159c = 'Bruce Lee'd = True

Download: engg1330.dirk.hk/lec_02_example_17_02.py

Variables Have a Type

a = 3b = 3.14159c = 'Bruce Lee'd = True

Download: engg1330.dirk.hk/lec_02_example_17_02.py

Use type(x) to Determine the Type of the Variable x

a = 3b = 3.14159c = 'Bruce Lee'd = Trueprint(type(a))print(type(b))print(type(c))print(type(d))

Download: engg1330.dirk.hk/lec_02_example_21_03.py

You Can Also Directly Write the Literal in type(x)

print(type(3))print(type(3.14159))print(type('Bruce Lee'))print(type(True))

Download: engg1330.dirk.hk/lec_02_example_22_04.py

One Name Can Refer to Values of Different Types

age = 18print(type(age))       # intage = "18"print(type(age))       # str

Download: engg1330.dirk.hk/lec_02_value_types.py

Type Hints

: int documents the intended type

int(...) converts a value

Python does not enforce type hints at runtime

age: int = 18print(type(age))

Download: engg1330.dirk.hk/lec_02_type_hints.py

A Variable Is Like a Blackboard

Image source

age = 18print('You are', age)age = age + 1print('You will be', age, 'soon')

Download: engg1330.dirk.hk/lec_02_example_23_05.py

Names Describe What a Variable Is Used For

d = 7

elapsedTimeInDays = 7

Choosing Variable Names

Choose meaningful names that describe the value

elapsed_days = 7student_count = 120total_cost = 85.50

Start with a letter or underscore
Then use letters, digits or underscores

Valid and Invalid Names

Invalid	Reason	Valid Alternative
`2nd_score`	Starts with a digit	`second_score`
`class`	Reserved keyword	`class_name`
`total-cost`	Contains a hyphen	`total_cost`

Names are case sensitive
age and Age are different names

Identifiers Are Case Sensitive

X = 'awesome'print('Python is',x)

Download: engg1330.dirk.hk/lec_02_example_32_07.py

x = 'awesome'print('Python is',x)

Download: engg1330.dirk.hk/lec_02_example_32_08.py

Operators: Arithmetic

$HKU School of Computing and Data Science$

Arithmetic Operators

Operator	Name	Description	Example	Result
`+`	Addition	Adds the operands	`1 + 1`	`2`
`-`	Subtraction	Subtracts the right operand	`5 - 4`	`1`
`*`	Multiplication	Multiplies the operands	`3 * 7`	`21`
`/`	Division	Divides the left by the right	`3 / 6`	`0.5`
`%`	Modulo	Returns the division remainder	`7 % 3`	`1`
`**`	Exponent	Raises the left to a power	`7 ** 3`	`343`
`//`	Floor Division	Rounds the quotient down	`7 // 3`	`2`

Arithmetic Operators: Addition

source

numCookiesToday = 3numCookiesYesterday = 4print('You had',numCookiesYesterday + numCookiesToday,'total!')

Download: engg1330.dirk.hk/lec_02_example_37_09.py

Arithmetic Operators: Division

a = 42b = 0print(a/b)

Download: engg1330.dirk.hk/lec_02_example_38_10.py

Modulus vs. Floor Division

Modulo: What Is Left Over?

7 objects, groups of 3

1 left over

print(7 % 3)

Download: engg1330.dirk.hk/lec_02_modulo_groups.py

Floor Division: How Many Complete Groups?

7 objects, groups of 3

2 complete groups

7Total=3Group size×2Complete groups+1Left over

print(7 // 3)

Download: engg1330.dirk.hk/lec_02_floor_groups.py

Arithmetic Operators: Modulus

a = 42b = 0print(a%b)

Download: engg1330.dirk.hk/lec_02_example_42_11.py

Modulo Checks Whether a Number Is Even or Odd

Numbers	Examples	Test
Even	2, 4, 6, 8, …	`x % 2 == 0`
Odd	1, 3, 5, 7, …	`x % 2 == 1`

Operators: Assignment

$HKU School of Computing and Data Science$

Assignment Operator

a = 5print(a)

Download: engg1330.dirk.hk/lec_02_example_46_12.py

Compound Assignment Operator

a = 5a += 5print(a)

Download: engg1330.dirk.hk/lec_02_example_49_14.py

L-2 Survey

Complete the full survey during the lecture

Operators: Boolean and Comparisons
(If / Else)

$HKU School of Computing and Data Science$

Comparisons Produce Boolean Values

x = 42xMod2 = x % 2even = xMod2 == 0print(x,'is even:',even)

Download: engg1330.dirk.hk/lec_02_example_52_16.py

x = 43xMod2 = x % 2even = xMod2 == 0print(x,'is even:',even)

Download: engg1330.dirk.hk/lec_02_example_52_17.py

if Runs Code When Its Condition Is True

x = 42xMod2 = x % 2even = xMod2 == 0if even:  print(x,'is even:',even)

Download: engg1330.dirk.hk/lec_02_example_53_18.py

x = 43xMod2 = x % 2even = xMod2 == 0if even:  print(x,'is even:',even)

Download: engg1330.dirk.hk/lec_02_example_53_19.py

Branching with if and else

x = Trueif (x):    print('yes')else:    print('no')

Download: engg1330.dirk.hk/lec_02_example_54_20.py

x = Falseif (x):    print('yes')else:    print('no')

Download: engg1330.dirk.hk/lec_02_example_54_21.py

Indentation Marks Blocks of Code

x = Trueif (x):    print('yes')else:    print('no')

Download: engg1330.dirk.hk/lec_02_example_54_20.py

← A block of code

← Another block of code

Comparison Operators with if and else

x = 42xMod2 = x % 2if (xMod2 == 0):    print(x,'is even')else:    print(x,'is odd')

Download: engg1330.dirk.hk/lec_02_example_56_22.py

x = 43xMod2 = x % 2if (xMod2 == 0):    print(x,'is even')else:    print(x,'is odd')

Download: engg1330.dirk.hk/lec_02_example_56_23.py

Greater Than or Equal To

age = 17if (age >= 18):    print('You are an adult')else:    print('You are under age')

Download: engg1330.dirk.hk/lec_02_example_57_24.py

age = 36if (age >= 18):    print('You are an adult')else:    print('You are under age')

Download: engg1330.dirk.hk/lec_02_example_57_25.py

A Flowchart Shows the Program’s Logic

age>=18

adult

under age

True

False

age = 17if (age >= 18):    print('You are an adult')else:    print('You are under age')

Download: engg1330.dirk.hk/lec_02_example_57_24.py

Advanced Example

age within
[18, 25]?

True

No, Bye bye

height >= 180cm?

True

False

Yes, I do

Expressions

$HKU School of Computing and Data Science$

Parentheses Bind Operators and Operands Together

x = 42xMod2 = x % 2if (xMod2 == 0):    print(x,'is even')else:    print(x,'is odd')

Download: engg1330.dirk.hk/lec_02_example_56_22.py

x = 42if ((x % 2) == 0):    print(x,'is even')else:    print(x,'is odd')

Download: engg1330.dirk.hk/lec_02_example_61_26.py

Expressions - Example

My Mr. Right must be 18 or older and no older than 25 years old and his height must be at least 180cm

Let’s use a flow chart to screen potential boyfriends

Expressions - Example

age within
[18, 25]?

No, Bye bye

height >= 180cm?

True

False

True

False

Yes, I do

Combining Conditions in an Expression

age = 25height = 179if (((age >= 18) and (age <= 25)) and height >= 180):    print('Yes, I do')else:    print('No, Bye bye')

Download: engg1330.dirk.hk/lec_02_example_64_27.py

Which Operation Happens First?

print(2 + 3 * 4)print((2 + 3) * 4)

Download: engg1330.dirk.hk/lec_02_precedence_demo.py

Highest to Lowest

**
* / // %
+ -
Comparisons
not
and
or

Parentheses make the intended grouping explicit

Casting and
Square Bracket Operator

$HKU School of Computing and Data Science$

input() Returns a String

age = input('age: ')if (age >= 18):    print('You are an adult')else:    print('You are under age')

Download: engg1330.dirk.hk/lec_02_example_67_28.py

A String and an Integer Cannot Be Ordered

age = input('age: ')if (age >= 18):    print('You are an adult')else:    print('You are under age')

Download: engg1330.dirk.hk/lec_02_example_67_28.py

Converting Values to a Different Type

Conversion	Resulting Type
`int(x)`	Integer
`str(x)`	String
`float(x)`	Float

int() Converts the Input to an Integer

age = input('age: ')print(type(age))age = int(age)print(type(age))if (age >= 18):    print('You are an adult')else:    print('You are under age')

Download: engg1330.dirk.hk/lec_02_example_70_29.py

Accessing String Characters by Index

Python uses zero-based indexing

Expression	Character
`s[0]`	First
`s[1]`	Second
`s[-1]`	Last
`s[-2]`	Second last

Keep the index within the string’s bounds

Accessing Characters by Index

name = input('name: ')index = int(input('index: '))print(name[index])

Download: engg1330.dirk.hk/lec_02_example_72_30.py

Coding Challenge

To Earn Your Participation Mark

Complete the full survey during the lecture
Pass all tests in at least 2 of the 3 challenges
by 23:59 HKT today

Both required · 1 mark · No partial credit

Python Fundamentals:Variables, Operators and Expressions

We’re Starting at Different Levels

How Did Learning to Code Feel?

Would You Press the Button?

L-1 Participation Snapshot

Terminology

Literals

Variables

Operators

Expressions

Literals

Literals Have Different Data Types

Variables

Use type(x) to Determine the Type of the Variable x

You Can Also Directly Write the Literal in type(x)

One Name Can Refer to Values of Different Types

Type Hints

A Variable Is Like a Blackboard

Names Describe What a Variable Is Used For

Choosing Variable Names

Valid and Invalid Names

Identifiers Are Case Sensitive

Operators: Arithmetic

Arithmetic Operators

Arithmetic Operators: Addition

Arithmetic Operators: Division

Modulus vs. Floor Division

Modulo: What Is Left Over?

Floor Division: How Many Complete Groups?

Arithmetic Operators: Modulus

Modulo Checks Whether a Number Is Even or Odd

Operators: Assignment

Assignment Operator

Compound Assignment Operator

Operators: Boolean and Comparisons(If / Else)

Comparisons Produce Boolean Values

if Runs Code When Its Condition Is True

Branching with if and else

Indentation Marks Blocks of Code

Comparison Operators with if and else

Greater Than or Equal To

A Flowchart Shows the Program’s Logic

Expressions

Parentheses Bind Operators and Operands Together

Combining Conditions in an Expression

Which Operation Happens First?

Casting andSquare Bracket Operator

input() Returns a String

A String and an Integer Cannot Be Ordered

Converting Values to a Different Type

int() Converts the Input to an Integer

Accessing String Characters by Index

Accessing Characters by Index

Python Fundamentals:
Variables, Operators and Expressions

Operators: Boolean and Comparisons
(If / Else)

Casting and
Square Bracket Operator