🔬 Lab 1: 🐍 Python Tutorial#

Note

The only way to learn a new programming language is by writing programs in it. - Dennis Ritchie (inventor of C programming language)

Running Python#

Running Python in VS Code#

  • To open the ece487_wksp folder with VS Code, right-click on it and choose Open with Code from the menu.

  • To open the Terminal window, go to View and click on Terminal.

  • Type in python and press Enter

_images/Terminal.png

  • This will open the Python Shell. The Shell is where you can enter code and see the responses and output of code you have programmed into Python. This is a kind of sandbox, where you’re able to try out some simple code and processes. For example, in the Shell enter: 2+3. If you press enter, it will display the answer 5 on the next line.

  • Try the following code in the Shell.

2+3
"hello"+" world"
a = 2
print(a)

  • The Python Shell acts very much like a calculator since code is basically a series of mathematical interactions with the system.

  • Try the follownig code

2+2    # add two integers
2+2.0  # add an integer to a floating point number
2*3    # 2 times 3
2**2   # 2 squared
2**3   # 2 cubed
2**128 # Unlike C, integers in Python does not have a size limitation.    
2/3    # It returns 1 in C, but 0.6666666666666 in Pyton 
2/3.0  # Same as 2/3 in Python.
2//3   # modulo (remainder), same as 2%3 in C
4//3   # modulo (remainder), same as 4%3 in C

  • Now type in exit() and press Enter to leave the Python Shell. You can also use Ctrl+z to exti the Shell.

Running Python in Command Prompt (or Terminal for Mac)#

  • Open Command Prompt or PowerShell (or Terminal for Mac) and run python in Windows or python3 in Mac.

  • This will open the Python Shell.

Python Programming#

Variables#

  • Numeric types: int, float, long, complex

  • string

  • boolean: True / False

Python’s simple types are summarized in the following table:

Python Scalar Types

Type

Example

Description

str

x = 'abc'

String: characters or text

int

x = 1

integers (i.e., whole numbers)

float

x = 1.0

floating-point numbers (i.e., real numbers)

complex

x = 1 + 2j

Complex numbers (i.e., numbers with real and imaginary part)

bool

x = True

Boolean: True/False values

NoneType

x = None

Special object indicating nulls

Use the type() function to find the type for a value or variable

# String
c = 'hello'
print(type(c))

# Integer
a = 1
print(type(a))

# Float
b = 1.0
print(type(b))

# Boolean
d = True
print(type(d))

# None
e = None
print(type(e))

# Cast integer to string
print(type(str(a)))

Math Operators#

  • +, -, *, and /

  • Exponentiation **

  • Modulo % (Remainder)

Logic Operators#

x == y # x is equal to y
x != y # x is not equal to y
x > y # x is greater than y
x < y # x is less than y
x >= y # x is greater than or equal to y 
x <= y # x is less than or equal to y

Try the following code in Python Console.

2+2
2+2.0
2*3
2**3   # exponentiation 
2**128 
2/3
2/3.0
2//3   # integer division
4//3   
2%3    # modulo
4%3
bin(5|2) # convert a number to binary after bit operations
bin(5^1)
bin(5&1)
a = 6
b = 7
print(a==6)   # boolean expressions
print(a==7)
print(a==6 and b==7)
print(a==6 or b==6)
print(not a==6 and b==6)
print(not (a==6 and b==6))

Working with Variables#

In programming languages, variables are containers that store simple information. In Python, variables can store strings or numbers. A string is a block of text, such as “Stan Baek is the legendary pirate captain.” A number can be an integer or a decimal number. Try the the following code in Python Console.

# a string variable
name = "Stan Baek"

You can check the type in use by issuing the type() command

type(name)

It will return <class 'str'>

Add a new string variable

title = "the legendary pirate captain"

You can add (+) string variables to combine them into a new variable

character = name + ": " + title
print(character)  # It will print "Stan Baek: the legendary pirate captain"

# integer variable
age = 726

# Check the type using type()
type(name)

Let’s try to combine a string with an integer

# For string concatenation use + sign
print(character + age) # it will throw an error

Now let’s convert integer to string for printing

print(character + " is " + str(age) + " years old.")

It will print

Stan Baek: the legendary pirate captain is 726 years old.

You can ask for input from the user.

age = input("How old are you? ")
type(age)

# Since age is a string variable. We need to convert it to integer.
int(age)+10

# Numbers with a decimal point are floating point variables
x = 3.141
type(x)

print(x)  # It will print 3.141

print(int(x))  # It will print 3

A few things are going on here while creating variables. First, Python lets us define variables using the convention variable_name = value. As long as the variable name is unique (it is not the name of a function, module, or other variable), we can define our variables this way. For strings, single quotes or double quotes must be used around the text that needs to be in the variables; it doesn’t matter whether you choose single or double quotes, just stay consistent.

Functions and Modules#

The next step is to tackle built-in functions.

name = "Stan Baek"
len(name)  # return the lenght of string
type(name) # return variable type

Python has tens of built-in functions. To view the list of the functions, navigate to https://docs.python.org/3.8/library/functions.html

In Python you can use help to display the description of a function.

help(len)

In Python, we can create functions to perform a specific task. Python functions start with the keyword def followed by the function name, an open parenthesis, any inputs to pass to the function, a close parenthesis, and a colon. When we start writing what the function will do, we need to start on a new line and “tab-in” so that Python knows we are writing within the scope of the function. The function finishes when it gets to the last line of code or it reaches a return keyword. Let’s write a function that adds two numbers

def add(a, b):
    return a+b
    
print(add(3, 4))

Let’s look at the print function more carefully. There are a few different methods to print a string with variables.

title = "The legendary pirate captain"
age = 726.25

# method 1: (formatted-string)%(values)
print("%s is %f years old."%(title, age))     # print a string and a floating point
print("%s is %.4f years old."%(title, age))   # 4 decimal places
print("%s is %10.4f years old."%(title, age)) # allocate 10 spaces
print("%s is %d years old."%(title, age))     # print it as integer
print("%s is %10d years old."%(title, age))   # allocate 10 spaces

# method 2: (formatted-string).format(values)
print("{} is {} years old.".format(title, age)) 
print("{} is {:0.4f} years old.".format(title, age))
print("{} is {:10.4f} years old.".format(title, age))

# method 3: f(formatted-string with values)
print(f"{title} is {age} years old.")
print(f"{title} is {age:.4f} years old.")
print(f"{title} is {age:10.4f} years old.")

The legendary pirate captain is 726.250000 years old.
The legendary pirate captain is 726.2500 years old.
The legendary pirate captain is   726.2500 years old.
The legendary pirate captain is 726 years old.
The legendary pirate captain is        726 years old.
The legendary pirate captain is 726.25 years old.
The legendary pirate captain is 726.2500 years old.
The legendary pirate captain is   726.2500 years old.
The legendary pirate captain is 726.25 years old.
The legendary pirate captain is 726.2500 years old.
The legendary pirate captain is   726.2500 years old.

Python Modules#

Additional functions can be added to python through modules. Modules are packages of additional functionality built by others. Typically, a module is comprised of compiled code that serves a specific function. To use advanced mathematical functions, enter import math

import math
math.sqrt(16)
math.sin(math.pi/2)
math.cos(0)

With Python code, the convention is to import all required modules at the top of the python script. By importing all modules at the beginning of the script, we ensure that all modules have been loaded by the time we need them later in the script. Also, it ensures anyone reading our code can very quickly see all the modules (dependencies) needed to run the code.

We can also import individual functions from a module

from math import sqrt, sin, pi
print(sqrt(16))
print(sin(pi/2))
cos(0)   # error because cos was not imported

import math as m
m.sqrt(16)
m.sin(m.pi/2)
m.cos(0)

Conditions#

The if, else, and elif statements control conditional code execution. The general format of a conditional state is as follows:

if expression:
    statements
elif expression:
    statements
elif expression:
    statements
else:
    statements

If no action is to be taken, you can moit both the else and elif clauses of a conditional. Use the pass statement if no statements exists for a particular clause:

if expression:
    pass       # Do nothing.
else:
    statements

Here is an example

word = input("Enter a four-letter word: ")
if len(word) == 4:
    print(word, "is a four-letter word. Well done.")
elif len(word) == 3:
    print(word, "is a three-letter word.")
else:
    print(word, "is not a four-letter word.")

Let’s take a look at the following if-elif-else statement.

hand = input("What would you like to play? ")
if hand == "Rock":
    print("It is a valid play.")
elif hand == "Paper":
    print("It is a valid play.")
elif hand == "Sciccors":
    print("It is a valid play.")
else:    
    print("It is an invalid play.")

This code can be simplified by

hand = input("What would you like to play? ")
if hand == "Rock" or hand == "Paper" or hand == "Sciccors":
    print("It is a valid play")
else:    
    print("It is an invalid play")

It can be even further simplified by

hand = input("What would you like to play? ")
if hand in  ("Rock", "Paper", "Scissors"):  # This is Pythonian
    print("It is a valid play")
else:    
    print("It is an invalid play")

The ternary operator is a way of writing conditionals in Python.

y = 1

if y == 1:
    x = 1
else: 
    x = 0
print(x)

The code above can be simplified by

x = 1 if y == 1 else 0  # This is Pythonian
print(x)

Loops#

Let’s start with a simple while statement.

x = 1
while x < 5:
    print(x)
    x = x + 1
    
while True:
    print(x)
    x -= 1
    if x == 0:
        break

Let’s try for loops

for loop is probably the most popular loop construct in Python:

for target in sequence:
    do_statements

for i in [0, 1, 2, 3, 4]:
    print(i)

0
1
2
3
4

for x in range(0,5):  # 0 <= x < 5
    print(x)

0
1
2
3
4

for i in range(5):
    print(i)

0
1
2
3
4

for i in range(2,5):
    print(i)

2
3
4

names = ['Peter Parker', 'Clark Kent', 'Wade Wilson', 'Bruce Wayne', 'Dr. Baek']
for i in range(0,len(names)):
    print(names[i])

It will print

Peter Parker
Clark Kent
Wade Wilson
Bruce Wayne
Dr. Baek

You can accomplish the same with the following code, called Pythonic,

for name in names:
    print(name)

It will print

Peter Parker
Clark Kent
Wade Wilson
Bruce Wayne
Dr. Baek

# indexing using for loops
index = 0
for name in names:
    print(index, name)
    index += 1

It will print

0 Peter Parker
1 Clark Kent
2 Wade Wilson
3 Bruce Wayne
4 Dr. Baek

# Pythonic with enumerate
for index, name in enumerate(names):
    print(index, name)

It will print

0 Peter Parker
1 Clark Kent
2 Wade Wilson
3 Bruce Wayne
4 Dr. Baek

# the index can start with 1 
for index, name in enumerate(names, start=1):
    print(index, name)

It will print

1 Peter Parker
2 Clark Kent
3 Wade Wilson
4 Bruce Wayne
5 Dr. Baek

heroes = ['Spiderman', 'Superman', 'Deadpool', 'Batman', 'Pirate Captain']

for index, name in enumerate(names):
    hero = heroes[index]
    print(f'{name} is actually {hero}')

It will print

Peter Parker is actually Spiderman
Clark Kent is actually Superman
Wade Wilson is actually Deadpool
Bruce Wayne is actually Batman
Dr. Baek is actually Pirate Captain

# Pythonic with zip
for name, hero in zip(names, heroes):
    print(f'{name} is actually {hero}')

It will print

Peter Parker is actually Spiderman
Clark Kent is actually Superman
Wade Wilson is actually Deadpool
Bruce Wayne is actually Batman
Dr. Baek is actually Pirate Captain

universes = ['Marvel', 'DC', 'Marvel', 'DC', 'USAFA']
for name, hero, universe in zip(names, heroes, universes):
    print(f'{name} is actually {hero} from {universe}')

It will print

Peter Parker is actually Spiderman from Marvel
Clark Kent is actually Superman from DC
Wade Wilson is actually Deadpool from Marvel
Bruce Wayne is actually Batman from DC
Dr. Baek is actually Pirate Captain from USAFA

example_string = "Hello World"
for c in example_string:
    print(c)

It will print

H
e
l
l
o
 
W
o
r
l
d

animals = list()
# appending to lists in for loop
for value in ['Horse','Mouse','Aardvark']:
    animals.append(value)
    
print(animals)

It will print

['Horse', 'Mouse', 'Aardvark']

# manipulating lists using for loops
for value in ['a','n','i','m','a','l','s']:
    print(value.upper())

It will print

A
N
I
M
A
L
S

Deliverables#

Deliverable 1#

Write a Python script that computes square roots using the Newton’s method of successive approximation, which says that whenever we have a guess \(y\) for the value of the square root of a number \(x\), we can perform a simple manipulation to get a better guess by averaging \(y\) with \(x/y\). For example we can compute the square root of 2 as shown below

Guess

Quotient

Average

\(1\)

\(2/1=2\)

\((2+1)/2=1.5\)

\(1.5\)

\(2/1.5=1.333\)

\((1.333+1.5)/2=1.4176\)

\(1.4176\)

\(2/1.4176=1.4188\)

\((1.4188+1.4176)/2=1.4142\)

\(1.4142\)

\(\cdots\)

\(\vdots\)

You need to iterate until \(|y^2 - x| < \epsilon\), where \(\epsilon = 10^{-9}\) is the tolerance. You are not allowed to use any functions in the math module except math.fabs for the absolute values.

Deliverable 2#

Write the num_vowels function inside vowels.py that takes a string argument and returns the number of lowercase vowels in the string.