Python Lecture Slides Variables and Assignments

Variables & Assignment
Lecture 2

Announcements for Today
If Not Done Already
• Install Python
§ Make sure right version
§ Make sure Kivy works
• Register your iClicker
• Sign into CMS
§ Fill out the Survey
§ Complete AI Quiz
Lab 1
• Labs are due at next class
§ So lab 1 is due now
§ By end of the lab section
§ Try to finish them before
• Makes T/W a little tight
§ Only 2 days (not 5)
§ Will keep them small
• Getting behind is bad!
8/25/22 Variables & Assignments 2

Helping You Succeed in this Class
• Consultants. Phillips 318 (after hours)
§ Daily office hours (see website) with consultants
§ Very useful when working on assignments
• AEW Workshops. Additional discussion course
§ Runs parallel to this class – completely optional
§ See website; talk to advisors in Olin 167.
• Ed Discussions. Forum to ask and answer questions
§ Go here first before sending question in e-mail
• Office Hours. Talk to the professor!
§ Couches in Statler Balcony between classes

Labs vs. Assignments
Labs
• Held twice a week
• Graded on completeness
§ Always S/U
§ Try again if not finished
• Indirect affect on grade
§ Can miss up to 3 labs
§ After that, grade reduced
• Similar to language drills
§ Simple, but take time
Assignments
• Every two weeks
§ First one due Sep. 16
• Graded on correctness
§ Assign points out of 100
• But first one is for mastery
§ Resubmit until perfect grade
• 40% of your final grade
• Can work with a partner!
§ Mixer coming soon

Academic Integrity
• Every semester we have cases of plagiarism
§ Claiming the work of others as your own
§ This is an Academic Integrity violation
• This course has a very specific policy
§ Do not listen to (non-staff) upperclassmen
§ Look at the course website for the new details
• Complete Academic Integrity Quiz on CMS
§ Must complete successfully to stay in class

iClickers
• Have you registered your iClicker?
• If not, visit (free service; no surcharge!)
§ https://cs1110.cs.cornell.edu/py/clicker
• See the course web page for more:
§ http://www.cs.cornell.edu/courses/cs1110/2022fa
§ Click “Materials/Textbook”
§ Look under “iClickers”

Warm-Up: Using Python
• How do you plan to use Python?
A. I want to work mainly in the Phillips lab
B. I want to use my own Windows computer
C. I want to use my own Macintosh computer
D. I want to use my own Linux computer
E. I will use whatever I can get my hands on

Type: Set of values and the operations on them
• Type int:
§ Values: integers
§ Ops: +, –, *, //, %, **
• Type float:
§ Values: real numbers
§ Ops: +, –, *, /, **
• Type bool:
§ Values: True and False
§ Ops: not, and, or
• Type str:
§ Values: string literals
• Double quotes: "abc"
• Single quotes: 'abc'
§ Ops: + (concatenation)
Will see more types
in a few weeks

Example: str
• Values: text, or sequence of characters
§ String literals must be in quotes
§ Double quotes: "Hello World!", " abcex3$g<&"
§ Single quotes: 'Hello World!', ' abcex3$g<&’
• Operation: + (catenation, or concatenation)
§ 'ab' + 'cd' evaluates to 'abcd’
§ concatenation can only apply to strings
§ 'ab' + 2 produces an error

Converting Values Between Types
• Basic form: type(expression)
§ This is an expression
§ Evaluates to value, converted to new type
§ This is sometimes called casting
• Examples:
§ float(2) evaluates to 2.0 (a float)
§ int(2.6) evaluates to 2 (an int)
§ Note information loss in 2nd example

Converting Values Between Types
• Conversion is measured narrow to wide
bool ⇒ int ⇒ float
• Widening: Convert to a wider type
§ Python does automatically
§ Example: 1/2.0 evaluates to 0.5
• Narrowing: Convert to a narrower type
§ Python never does automatically
§ Example: float(int(2.6)) evaluates to 2.0

Operator Precedence
• What is the difference between these two?
§ 2*(1+3)
§ 2*1 + 3

Operator Precedence
§ 2*(1+3)
§ 2*1 + 3
• Operations are performed in a set order
§ Parentheses make the order explicit
§ What happens when no parentheses?
add, then multiply
multiply, then add

Operator Precedence
§ 2*(1+3)
§ 2*1 + 3
• Operations are performed in a set order
§ Parentheses make the order explicit
§ What happens when no parentheses?
add, then multiply
multiply, then add
Operator Precedence:
The fixed order Python processes
operators in absence of parentheses

Precedence of Python Operators
• Exponentiation: **
• Unary operators: + –
• Binary arithmetic: * / %
• Binary arithmetic: + –
• Comparisons: < > <= >=
• Equality relations: == !=
• Logical not
• Logical and
• Logical or
• Precedence goes downwards
§ Parentheses highest
§ Logical ops lowest
• Same line = same precedence
§ Read “ties” left to right
§ Example: 1/2*3 is (1/2)*3
• There is a video about this
• See website for more info
• Was major portion of Lab 1

Expressions vs Statements
Expression
• Represents something
§ Python evaluates it
§ End result is a value
• Examples:
§ 2.3
§ (3+5)/4
Statement
• Does something
§ Python executes it
§ Need not result in a value
• Examples:
§ print('Hello')
§ import sys
Will see later this is not a clear cut separation
Literal
Complex

Variables
• A variable
§ is a box (memory location)
§ with a name
§ and a value in the box
• Examples:
5
x Variable x, with value 5 (of type int)
20.1
area Variable area, w/ value 20.1 (of type float)

Using Variables
• Variables can be used in expressions
§ Evaluate to the value that is in the box
§ Example: 1 + x evaluates to 6
• Variables can change values
§ Example: 1 + x evaluates to 2.5
§ Can even change the type of their value
§ Different from other languages (e.g. Java)
5
x
5
x 1.5
x

Naming Variables
• Python has strict rules of how to assign names
§ Names must only contain letters, numbers, _
§ They cannot start with a number
• Examples
§ e1 is a valid name
§ 1e2 is not valid (it is a float)
§ a_b is a valid name
§ a+b is not valid (it is + on two variables)

Variables and Assignment Statements
• Variables are created by assignment statements
x = 5
• This is a statement, not an expression
§ Expression: Something Python turns into a value
§ Statement: Command for Python to do something
§ Difference is that has no value itself
• Example:
>>> x = 5
(NOTHING)
x
the value
the variable
5
But can now use x
as an expression

Variables Do Not Exist Until Made
• Example:
>>> y
Error!
>>> y = 3
>>> y
3
• Changes our model of Python
§ Before we just typed in one line at a time
§ Now program is a sequence of lines

Assignments May Contain Expressions
• Example: x = 1 + 2
§ Left of equals must always be variable: 1 + 2 = x
§ Read assignment statements right-to-left!
§ Evaluate the expression on the right
§ Store the result in the variable on the left
• We can include variables in this expression
§ Example: x = y+2
§ Example: x = x+2
This is not circular!
Read right-to-left.
x 5
y 2

Execute the Statement: x = x + 2
• Draw variable x on piece of paper:
5
x

• Step 1: evaluate the expression x + 2
§ For x, use the value in variable x
§ Write the expression somewhere on your paper
5
x

• Step 2: Store the value of the expression in x
§ Cross off the old value in the box
§ Write the new value in the box for x
5
x

• Step 2: Store the value of the expression in x
§ Cross off the old value in the box
§ Write the new value in the box for x
• Check to see whether you did the same thing as your
neighbor, discuss it if you did something different.
5
x

Which One is Closest to Your Answer?
A: B:
C: D:
¯_(ツ)_/¯
5
x 7
x 5
x
5
x x
7
x
7
x

A: B:
C:
5
x 7
x 5
x
5
x x
7
x
7
x
✓
x = x + 2

Execute the Statement: x = 3.0 * x + 1.0
• You have this:
5
x 7
x

• You have this:
• Execute this command:
§ Step 1: Evaluate the expression 3.0 * x + 1.0
§ Step 2: Store its value in x
5
x 7
x

• You have this:
• Execute this command:
5
x 7
x

A: B:
C: D:
¯_(ツ)_/¯
5
x 7
x 5
x
5
x x
22.0
x
22.0
x
22.0
x 7
x
7
x

A: B:
C:
5
x 7
x 5
x
5
x x
22.0
x
22.0
x
22.0
x 7
x
7
x
✓
x = 3.0 * x + 1.0

• You now have this:
• The command:
• This is how you execute an assignment statement
§ Performing it is called executing the command
§ Command requires both evaluate AND store to be correct
§ Important mental model for understanding Python
5
x 7 22.0
x x

Exercise: Understanding Assignment
• Add another variable, interestRate, to get this:
• Execute this assignment:
interestRate = x / interestRate
4
interestRate
5
x 7 22.0
x x

A: B:
C: D:
4
interestRate
5
x 7 22.0
x x 5.5
x
5.5
x 4
interestRate
5
x 7 22.0
x x
x
4
interestRate
5
x 7 22.0
x x
5.5
x 4
interestRate
5
x 7 22.0
x x
5
x
5.5
interestRate

A: B:
C: D:
4
interestRate
5
x 7 22.0
x x 5.5
x
5.5
x 4
interestRate
5
x 7 22.0
x x
x
4
interestRate
5
x 7 22.0
x x
5.5
x 4
interestRate
5
x 7 22.0
x x
5
x
5.5
interestRate
E:
¯_(ツ)_/¯

B:
C: D:
4
interestRate
5
x 7 22.0
x x
x
4
interestRate
5
x 7 22.0
x x
5.5
x 4
interestRate
5
x 7 22.0
x x
5
x
5.5
interestRate
✓
interestRate = x/interestRate

Exercise: Understanding Assignment
• You now have this:
• Execute this assignment:
intrestRate = x + interestRate
4
interestRate 5.5
5
x 7 22.0
x x x

A: B:
C: D:
4
interestRate
5
x 7 22.0
x x
5.5
x 4
interestRate
5
x 7 22.0
x x
x
interestRate
5
x 7 22.0
x x
interestRate
5
x 7 22.0
x x
27.5
intrestRate
5.5
4
x 5.5
27.5
intrestRate
x
27.5
x
4 5.5
x
27.5
x

A: B:
C: D:
4
interestRate
5
x 7 22.0
x x
5.5
x 4
interestRate
5
x 7 22.0
x x
x
interestRate
5
x 7 22.0
x x
interestRate
5
x 7 22.0
x x
27.5
intrestRate
5.5
4
x 5.5
27.5
intrestRate
x
27.5
x
4 5.5
x
27.5
x
E:
¯_(ツ)_/¯

A: B:
4
interestRate
5
x 7 22.0
x x
5.5
x 4
interestRate
5
x 7 22.0
x x
x
27.5
intrestRate
5.5
27.5
x
✓
^
e

A: B:
4
interestRate
5
x 7 22.0
x x
5.5
x 4
interestRate
5
x 7 22.0
x x
x
27.5
intrestRate
5.5
27.5
x
✓
^
e
Spelling mistakes in
Python are bad!!

Dynamic Typing
• Python is a dynamically typed language
§ Variables can hold values of any type
§ Variables can hold different types at different times
• The following is acceptable in Python:
>>> x = 1
>>> x = x / 2.0
• Alternative is a statically typed language
§ Each variable restricted to values of just one type
§ This is true in Java , C, C++, etc.
ç x contains an int value
ç x now contains a float value

Dynamic Typing
• Often want to track the type in a variable
§ What is the result of evaluating x / y?
§ Depends on whether x, y are int or float values
• Use expression type(<expression>) to get type
§ type(2) evaluates to <type 'int'>
§ type(x) evaluates to type of contents of x
• Can use in a boolean expression to test type
§ type('abc') == str evaluates to True

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