Lecture 1, Thu 09/25

Introduction, Python Review

Recorded Lecture: 9_25_25

• Course webpage: https://ucsb-cs9.github.io/f25/
  • Please read and understand the syllabus: https://ucsb-cs9.github.io/f25/info/syllabus/

Python Basics

• Python is an example of an interpreted language (unlike C/C++)
• Each line is interpreted one at a time
• Does have some flexibility, especially when simply running a program from top-to-bottom
  • But is also dangerous since it doesn’t check for type errors and may fail in the middle of execution
• Python interactive shell (IDLE) can execute lines of code by typing it in
• Stores state of variables that can be reused, BUT…
  • Once you exit the interactive shell, memory is cleared and your work is lost
  • So python programs are usually NOT written in the interactive shell and in separate .py files

Python Buit-in Atomic Types

• Python has some basic types of data that come straight out-of-the-box
• Examples are integers (int) and floats (decimals)
  • May affect the output type you’re getting, even if it’s numerically the same

Example

>>> 2/2
1.0 # float
>>> 2 + 2
4 # int
>>> 2 + 2.0
4.0 # float

• And there are certain functionality that may work with certain types, but not others

>>> x = 10.0
>>> int(x)
10
>>> float(x)
10.0
>>> x = "10.0" # string type
>>> type(x)
<class 'str'>
>>> x = float(x)
>>> type(x)
<class 'float'>
>>> x = int(x)
>>> type(x)
<class 'int'>
>>> x = "10.0"
>>> x = int(x)
Traceback (most recent call last):
  File "<pyshell#105>", line 1, in <module>
    x = int(x)
ValueError: invalid literal for int() with base 10: '10.0'
>>> len(x)
4
>>> len(float(x))
Traceback (most recent call last):
  File "<pyshell#107>", line 1, in <module>
    len(float(x))
TypeError: object of type 'float' has no len()

Relational and Logical Operators

• Output of these operators result in a Boolean value
  • True , False
• Boolean values are important for control structures (while loops, if statements)
• Basically, allows you to fine-tune your program and define what / when instructions should be executed.

Example

>>> 5 == 5
True
>>> 5 != 5
False
>>> 5 < 5
False
>>> 5 <= 5
True
>>> 5 > 4
True
>>> 4 >= 5
False
>>> True or False
True
>>> not False or False
True
>>> False and True
False

Python Lists

• A dynamic collection of data (heterogeneous types) that can be indexed (index starting at 0).

Example

>>> x = []
>>> len(x)  # returns number of elements in list
>>> x.append(1)	 # adds to the end of the list
>>> x
[1]
>>> len(x)
1
>>> x = [1,2,2,3,3,3]
>>> len(x)
6
>>> x[3]  # extracts an element at an index (index starts at 0)
3
>>> x[3] = "3"  # assigns a value at a certain index
>>> x
[1, 2, 2, '3', 3, 3]