Introduction to Python and Programming

Winter 2025

1

Adrian Salguero

Announcements

• Homework 0 is available - due this Friday at 11:59pm on Gradescope
• Check-in 0 is available - due this Friday at 11:59pm on Gradescope
• Coding Practice 1 is available - due next Friday (Jan 17) at 11:59pm on Gradescope
• Appointment Calendar available for 1-1 meetings with instructor (~15 minutes)

2

3

1. Python is a calculator

​

2. A variable is a container

​

3. Different types cannot be compared

​

4. A program is a recipe

​

1. Python is a calculator

4

You type expressions.�Python computes their values.

• 5
• 3 + 4
• 44 / 2
• 2 ** 3
• 3 * 4 + 5 * 6
• If precedence is unclear, use parentheses: 3 * (4 + 5) * 6
• (72 – 32) / 9 * 5

5

Try typing some of these expressions into a python interpreter

An expression is evaluated�from the inside out

• How many expressions are in this Python code?

6

(72 – 32) / 9.0 * 5

an expression

values

(72 – 32) / 9.0 * 5

(40) / 9.0 * 5

40 / 9.0 * 5

4.44 * 5

22.2

Another evaluation example

7

(72 – 32) / (9.0 * 5)

(40) / (9.0 * 5)

40 / (9.0 * 5)

40 / (45.0)

40 / 45.0

.888

Order of Operations

​

​

​

8

2. A variable is a container

9

Variables hold values

• Recall variables from algebra:
• Let x = 2 …
• Let y = x …
• In Python: “varname = expression”

pi = 3.14

pi

avogadro = 6 * 10 ** 23

avogadro

22 = x # Error!

• Not all variable names are permitted

10

Nothing printed from an assignment statement

An expression that can be typed into a python interpreter to be evaluated. Not a statement to put into a python program.

Try typing into a python interpreter

Changing existing variables�(“re-binding” or “re-assigning”)

x = 2

x

y = 2

y

x = 5

x

y

• “=” in an assignment is not a promise of eternal equality
• This is different than the mathematical meaning of “=”
• Evaluating an expression gives a new (copy of a) number, rather than changing an existing one

​

11

An expression that can be typed into a python interpreter to be evaluated. Not a statement to put into a python program.

Nothing printed from an assignment statement

Try typing into a python interpreter

How an assignment is executed

1. Evaluate the right-hand side to a value
2. Store that value in the variable

​

x = 2

print(x)

y = x

print(y)

z = x + 1

print(z)

x = 5

print(x)

print(y)

print(z)

​

12

State of the computer:

Printed output:

​

​

​

​

​

​

To visualize a program’s execution: http://pythontutor.com A custom link to this program is here

This is a python program that could be typed into a text editor.

How an assignment is executed

1. Evaluate the right-hand side to a value
2. Store that value in the variable

x = 2

print(x)

y = x

print(y)

z = x + 1

print(z)

x = 5

print(x)

print(y)

print(z)

​

13

State of the computer:

Printed output:

x: 2

y: 2

z: 3

x: 5

To visualize a program’s execution: http://pythontutor.com A custom link to this program is here

This is a python program that could be typed into a text editor.

2

2

3

5

2

3

Boolean Expressions�(value is True or False)

22 > 4

22 < 4

22 == 4

x = 100 # Assignment, not conditional!

22 = 4 # Error!

x >= 5

x >= 100

x >= 200

not True

not (x >= 200)

3 < 4 and 5 < 6

4 < 3 or 5 < 6

temp = 72

water_is_liquid = temp > 32 and temp < 212

14

Order of Precedence:

Numeric operators: +, *, **

Mixed operators: <, >=, ==

Boolean operators: not, and, or

Also: see a program printing these expressions in python tutor

Try typing these expressions into a python interpreter

What do you think?

What is printed out by the following Python code:�

1. print(2 < 7 or 3 > 12)�
2. print(not ((2 < 3) and (4 > 100)))�

temp = 72

is_liquid = temp > 32 and temp < 212

print(is_liquid)

temp = 300

print(is_liquid)

15

See this program in python tutor

More expressions: strings

A string represents text

'Python'

this_class = "CSE 160"

""

Empty string is not the same as an unbound variable

​

Operations on strings:

• Length:

len(this_class)

• Concatenation:

"CSE" + "160" + 'is' + "fun!"

• Containment/searching:

'0' in this_class

"O" in this_class

16

Try typing these expressions into a python interpreter

3. Different types cannot be compared

17

Types of values

• Integers (int): -22, 0, 44
• Arithmetic is exact

​

• Real numbers (float): 2.718, 3.1415
• float, for “floating point”
• Arithmetic is approximate

​

• Strings (str): "I love Python", ""

​

• Truth values (bool): True, False
• bool, for “Boolean”

18

Operations behave differently�on different types

3.0 + 4.0

3 + 4

3 + 4.0

"3" + "4"

3 + "4" # Error

3 + True # Don’t do this.

​

Moral: Python sometimes tells you when you do something that does not make sense.

19

Also: see a program printing these expressions in python tutor

Try typing these expressions into a python interpreter

Operations behave differently�on different types

15.0 + 4.0

15 + 4

15.0 + 4

15 + 4.0

​

Type conversion:

float(15)

int(15.0)

int(15.5)

int("15")

str(15.5)

float(15) + 4

20

See a program printing these expressions in python tutor

Try typing these expressions into a python interpreter

4. A program is a recipe

21

What is a program?

• A program is a sequence of instructions
• The computer executes one after the other
• Saving your work as a program is better than re-typing from scratch

​

x = 1

y = 2

x + y

print(x + y)

print("The sum of", x, "and", y, "is", x + y)

22

See this program in python tutor

Interlude: The print statement

• The print statement always prints one line
• The next print statement prints below that one
• Write 0 or more expressions inside the parentheses, separated by commas
• In the output, the values are separated by spaces
• Examples:

print(3.1415)

print(2.718, 1.618)

print()

print(20 + 2, 7 * 3, 4 * 5)

print("The sum of", x, "and", y, "is", x + y)

23

See this program in python tutor

Expressions, statements, and programs

• An expression evaluates to a value
• 3 + 4
• pi * r ** 2
• A statement causes an effect
• pi = 3.14159
• print(pi)
• Expressions appear within other expressions and statements
• (fahr - 32) * (5.0 / 9)
• print(pi * r ** 2)
• A statement may not appear within an expression
• 3 + print(pi) # Error!!
• A program is made up of statements
• A program should do something or communicate information
• Just evaluating an expression does not accomplish either goal

24

25

1. Python is a calculator

​

2. A variable is a container

​

3. Different types cannot be compared

​

4. A program is a recipe

​

Todos for next lecture

• Start Homework 0, Check-in 0, and Coding Practice 1
• Set up your Python environment (Anaconda or VS Code)
• Attend Section this Thursday
• Come to OHs or post on Ed if you have any questions

26

Using Gradescope for CSE160

Ungraded

You will see some, but not all of the autograder tests

Can resubmit as many times until the due date

Graded

Automatically Graded

Autograder Score: Points given from all autograder tests

Manually Graded

Autograder Correction: TA’s giving/taking away points in case of autograder error

Internal Correctness: Points given for following directions, good style, efficiency, etc.

Autograder + Autograder Correction + Internal Correctness = Total Score

Comments

Two tabs - results and code. Click on ‘Code’ to see list of files you submitted. Each file will have the number of comments that were made on it.

​

Comments Example

Resubmissions

Two steps:

1. Fill out the google form! (Due 11 pm)
2. Resubmit on gradescope

(Also, please read comments before resubmitting!)