Lecture 1, Tue 01/07

Introduction, Python Review

Recorded Lecture: 1_7_25

• Course webpage: https://ucsb-cs9.github.io/w25/
  • Please read and understand the syllabus: https://ucsb-cs9.github.io/w25/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

• Data types can have methods (functions that can be called on an instance of a type (or object))

>>> x = [1,1,2,2,2]
>>> x.count(2)  # counts the number of times 2 appears in the list
3
>>> x.count(3)
0
>>> x = [1,'2',3,'4']
>>> '1' in x  # Returns True if '1' is in the list, False otherwise
False
>>> 1 in x
True
>>> x.insert(2, 2.5)  # inserts 2.5 in index 2 of the list, shifts right elements over
>>> x
[1, '2', 2.5, 3, '4']
>>> x.pop()  # removes and returns last element of list
'4'
>>> x
[1, '2', 2.5, 3]
>>> x.pop(1)  # removes element at index 1
'2'
>>> x
[1, 2.5, 3]
>>> del x[1]  # Notice that there isn’t any output, but still removes element
>>> x
[1, 3]

• Python methods and functions may or may not have a return value
• A special value (None) is used to represent a non-returned value
• It may be useful to store a return value to a variable, because once a value returns and is not stored, then it’s gone!