“My favorite language for maintainability is Python. It has simple, clean syntax, object encapsulation, good library support, and optional named parameters.” - Bram Cohen

Introduction to Python (continued)

Set

A set is an unordered collection data type that is unchangeable and does not allow duplicate values. Python’s set class represents the mathematical notion of a set. The major advantage of using a set, as opposed to a list, is that it has a highly optimized method for checking whether a specific element is contained in the set.

# Create a set
Pollutants = {"PM10", "PM2.5", "O3", "CO", "SO2", "NO2"}
print(Pollutants) 

# A set is a collection that is both unordered and unindexed
Pollutants[0]

# Once a set is created, you cannot change its items
Pollutants[0] = "VOCs"

Here are some functions to handle a set:

# Update a set
New_pollutants = {"VOCs", "PAH"}
Pollutants.update(New_pollutants)
print(Pollutants)

# Remove an item using remove
Pollutants.remove("PAH")
print(Pollutants)

# Remove an item using discard
Pollutants.discard("VOCs")
print(Pollutants)

For more functions, please refer to Built-in Types - Set Types.

Dictionary

Finally, a dictionary is a collection which is ordered, changeable and does not allow duplicates. Dictionaries are used to store data values in key: value pairs. Let’s define a dictionary for the 6 air pollutants as follow:

# Create and print a dictionary:
Air_pollutants = {
  "PM10": "Inhalable particles",
  "PM2.5": "Fine particulate matter",
  "O3": "Ozone",
  "CO": "Carbon monoxide",
  "SO2": "Sulfur dioxide",
  "NO2": "Nitrogen dioxide",
}
print(Air_pollutants)

Dictionary items can be referred to by using the key name:

print(Air_pollutants["CO"])

The key and value are not necessarily at string type. Numbers also work.

# Key and value can be other types
Air_pollutants = {
  "PM10": "Inhalable particles",
  "PM2.5": "Fine particulate matter",
  "O3": "Ozone",
  "CO": "Carbon monoxide",
  "SO2": "Sulfur dioxide",
  "NO2": "Nitrogen dioxide",
  100:   "The number 100",
  "Density": 1.69
}

Air_pollutants[100]
Air_pollutants["Density"]

# Show all keys
Air_pollutants.keys()

# Show all values
Air_pollutants.values()

# Get the value
Air_pollutants.get("SO2")
Air_pollutants.get("PO2")

# If key is not found, return default value
Air_pollutants.get("PO2", "Not a valid key")

Some functions to handle a dictionary:

# Change the value
Air_pollutants["Density"] = 1.99
Air_pollutants

# Add a new item
Air_pollutants[200] = "The number 200"
Air_pollutants

# Remove an item with pop()
Air_pollutants.pop(200)
Air_pollutants

# Remove an item with del
del Air_pollutants["Density"]
Air_pollutants

For more functions, please refer to Built-in Types - Dictionaries.

Summary of types

So far, we have covered all 4 built-in data types in Python to store collections of data: list, tuple, set, and dictionary. Here is a quick summary:

Type Ordered Changeable Allow duplicate members How to define
List Y Y Y [ ]
Tuple Y N Y ( )
Set N N N { }
Dictionary Y (as of v3.7) Y N { } with key

while loop

With the while loop, we can execute a set of statements as long as a condition is true.

# Initialize the ID
ID = 0

# while loop, print the first 5 numbers
while ID < 5 :
    print(ID)
    ID = ID + 1
    # Another way to write the increment 
    # ID += 1

The code can be translated as: As long as ID is less than 5, python will print ID and then increase it by 1. Now can you figure out what is ID after the while loop?

You can also use the else statement to run a block of code once when the condition is no longer true. For example:

# Initialize the ID
ID = 0

# while loop, print the first 5 numbers
while ID < 5 :
    print(ID)
    ID = ID + 1
# if the condition is no longer true, do following
else:
    print("ID is beyond 4, ID = " + str(ID))

Even more, you can use break and continue combined with if statement to control the flow.

# Use break and continue to control the flow
  
# Initialize the ID
ID = 0

# while loop, print the first 5 numbers
while ID < 5 :
    print(ID)
    ID = ID + 1
    # use continue
    if ID == 2 :
        print("ID becomes 2 now")
        continue
    # use break
    if ID == 3 :
        print("Breaks with ID = 3")
        break
# if the condition is no longer true, do following
else:
    print("ID is beyond 4, ID = " + str(ID))

for loop

A for loop is used for iterating over a sequence (i.e. list, tuple, dictionary, set, or string).

Let’s use for loop to print each element in a collection:

# Loop element in a collection (list, tuple, dict, and set)
for x in Air_pollutants:
    print(x)

This also works for a string:

# for loop also works for a string
for letter in "ESE5023":
    print(letter)

Similar to the while loop, one can use for combined with break, continue, and if to control the flow:

# for loop with if, break, and continue
for letter in "ESE5023":
    if letter == "E":
        print(letter.lower())
        continue
    else:
        print(letter)
    if letter == "5":
        break

To loop through a set of code a specified number of times, we can use the range() function, which returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.

# for loop with range, starting from 0
for i in range(5):
    print(i)        

# for loop with range, starting from 1
for i in range(1,5):
    print(i)    

# for loop with range, starting from 1, increment 2
for i in range(1,5,2):
    print(i)

Finally, you can run nested for loop:

# nested for loop
for letter in "ESE5023":
    for i in range(3):
        print(letter)

Modules

A module allows you to logically organize your Python code. Grouping related code into a module makes the code easier to understand and use. A module is a Python object with arbitrarily named attributes that you can bind and reference. Simply, a module is a file consisting of Python code. A module can define functions, classes, and variables.

For example, copy the following lines and save the file as my_module.py to your working directory.

# Create a function to print say hi
def say_hi():
    name = input("Please type you name: ")
    print("Hello " + name)

# Also define some variables
age = 28
gender = "Male"

To import my_module.py:

import my_module

We can then use functions and variables defined in the above module:

# Does this work?
say_hi()

# Call a function from the imported module
my_module.say_hi()

# Call a variable from the imported module
my_module.age

Python provides a range of official modules to do various tasks, see Python Module for the comprehensive list. For example, we previously imported ramdom module to generate random values:

# import python official modules
import random
random.random()

As we are using Python from Anaconda, official module files are located at: C:\Users\xxx\anaconda3\Lib, where xxx is your user name. Or C:\ProgramData\anaconda3\Lib depending on your system.

Python also has a huge collection of third-party modules available for you to install. To do so, we use a package manager called pip (Pip Installs Packages).

  • Open Anaconda Powershell Prompt

  • Type pip install yyy, where yyy is the package name (for example python-docx)

  • The new package will be installed at C:\Users\xxx\anaconda3\Lib\site-packages\, where xxx is your user name. Or C:\ProgramData\anaconda3\Lib\site-packages\ depending on your system.

  • To uninstall the package yyy, type pip uninstall yyy.

In-class exercises

Exercise #1 Fibonacci series

Use for loop to display Fibonacci series up to 100 terms.

Exercise #2 Building a number translator

Build a translator that first asks a user to input a string, then converts every number in the string to *, and finally prints the result.

[Hint: you may find the keyword in useful]

Exercise #3 Building a guessing game (v2)

  • Set a secret integer from 0 to 9, name the integer answer. Make sure your deskmate does not know the secret number.

  • Write a function that does the following: if your deskmate is able to guess your secret number correctly within 3 tries, print You Win!, otherwise print You Lose! I Win!

  • Let your deskmate play the game.

  • Now it’s your turn to play your deskmate’s game.

[Hint: consider creating a module and later import it]