Chapter 2: Variables, expressions, and statements

Contents

This notebook is based on "Think Python, 2Ed" by Allen B. Downey
https://greenteapress.com/wp/think-python-2e/

Values and types



In [1]:

    
# Integer data
type( 17 )









    Out[1]:





int



In [2]:

    
# Floating-point data
type( 17.0 )









    Out[2]:





float



In [3]:

    
# A number inside a string
type( '17' )









    Out[3]:





str



In [4]:

    
count = 55
size = 42.0



In [5]:

    
print( count )



In [6]:

    
type( count )









    Out[6]:





int

Variables names need to be meaningful
Don't use x, y, and z as names unless they have real meaning in the problem (like 3-d points)
Variable names can be arbitrarily long and can contain:
- letters
- numbers
- underscores
They have to begin with a letter
You can use uppercase letters, but start variables with a lowercase letter
Python is case sensitive so max and Max are different variables
An illegal variable name is a syntax error
Python has 30 keywords which are words that mean something specific in the language

Operators are special symbols that represent computations like addition and multiplication
The value(s) to which the operator is applied is called an operand



In [7]:

    
# Operator: + (addition)
# Operands: 3 and 4
3 + 4









    Out[7]:





7



In [8]:

    
# Operator: - (subtraction)
# Operands: 3 and 4
3 - 4









    Out[8]:





-1



In [9]:

    
# Operator: *tiplication (mul)
# Operands: 3 and 4
3 * 4









    Out[9]:





12



In [10]:

    
# Operator: / (division)
# Operands: 3 and 4
3 / 4









    Out[10]:





0.75



In [11]:

    
10 / 3









    Out[11]:





3.3333333333333335



In [12]:

    
10 // 3









    Out[12]:





3



In [13]:

    
5 * 2









    Out[13]:





10



In [14]:

    
5 * 2.0









    Out[14]:





10.0



In [15]:

    
5.0 * 2









    Out[15]:





10.0



In [16]:

    
5.0 * 2.0









    Out[16]:





10.0

An expression is a combination of values, variables and operators
A value or a variable is itself an expression
The interpreter evaluates expressions and uses the result for other expressions or simply displays it (if it is executed in the console)
A statement is comprised of one or more expressions

The order in which operators are evaluated in an expression depends on the rules of precedence
Python follows the standard mathematical convention
You may know it as PEMDAS
1. Parenthesis
2. Exponent
3. Multiplication
4. Division
5. Addition
6. Subtraction



In [17]:

    
1.0 / ( 2 * 3.14159 )









    Out[17]:





0.15915507752443828

In general, you can't use mathematical operators with string values or variables
Even if the strings look like numbers



In [18]:

    
# Can't divide a string by an integer
# Uncomment to demonstrate
# '13' / 42



In [19]:

    
'fizz' + 'buzz'









    Out[19]:





'fizzbuzz'



In [20]:

    
'la' * 3









    Out[20]:





'lalala'

Comments are a way for you to add notes to your program
They improve understanding of your code by describing the why of the code
Code already provides the how, so don't have your comments repeat that
This assumes that your code is understandable an you have chosen appropriate variable names



In [21]:

    
# Poor example
# Calculate it
y = 2
x = 2 * 3.14 * y



In [22]:

    
# Good example
# Calculate circumference of a circle
pi = 3.14
radius = 2
circumference = 2 * pi * radius

There are multiple ways to denote comments in your code
The easiest is to use the # character
Everything from the # to the end of the line is ignored
Although the book shows comments to the right of the code, I prefer that you put the comment above the code it describes

Complete Exercise 2.2 from page 15 of the textbook "Think Python, 2Ed". It has three (3) parts. Write Python code that computes the answer of all three.
1. The volume of a sphere with radius $r$ is $\frac{4}{3} \pi r^3$. What is the volume of a sphere with radius 5?
2. Suppose the cover price of a book is \$24.95, but bookstores get at 40% discount. Sipping costs \\$3 for the first copy and 75 cents for each additional copy. What is the total wholesale cost for 60 copies?
3. If I leave my house at 6:52am and run 1 mile at an easy pace (8:15 per mile), then 3 miles at tempo (7:12 per mile) and 1 mile at easy pace again, what time do I get home for breakfast?
Write Python code that converts three (3) different Fahrenheit temperatures to Celsius temperatures.
Write Python code that converts three (3) different distances given in miles to kilometers.