CS111 – Fundamentals of CS�Lecture 10�Python III�

​

Mohammad El-Ramly, PhD

Fall 2021-2022

​

http://www.acadox.com/class/63592 6EQYX4

https://www.geeksforgeeks.org/python-programming-language/

Reading for this & last lectures

​

1. Python Crash Course (Chapters 1 to 8 and 10)
2. Share any useful resources you find
3. https://www.freecodecamp.org/
4. https://www.w3schools.com/python/default.asp

​

2

Lecture 10 Outline

1. Dictionaries
2. Files
3. Recursion
4. Mutable and immutable objects
5. Tuples

3

Remember

• Any fool can write code that a computer can understand. Good programmers write code that humans can understand. 
• Martin Fowler, 2008.

4

Python 3 Keywords

5

6

1. Dictionaries

• Dictionaries are a very important data structure in Python
• A dictionary stores (key, value) pairs in an optimized way.
• Each key is hashed to an index in the table.
• It is an instance of a map or hashtable.

1. Dictionaries

• What dictionaries are
• The main features
• How to iterate through a dictionary

1. Dictionaries Are Hastables

0

1

2

3

Key 1

Hash Function

Key 2

Key 0

Python Dictionaries Are Ordered

Key 1

Hash Function

Key 2

Key 0

1. Dictionaries

• Dictionaries are indexed by key.
• Keys must be hashable.
• The hash for a key is unique.
• Values can be of any type

Building a Dictionary

marks = {'Ali': 30, 'Sami': 35}

print (marks)

​

marks ['Mai'] = 39

print (marks)

marks ['Ali'] = 32

print (marks['Ali'])

del marks ['Ali']

​

Building a Dictionary

marks = {

'Ali': 30,

'Sami': 35,

'Mai': 28

}

​

marks

Building a Dictionary

marks = {

'Ali': 30,

'Sami': 35,

'Mai': 28,

'Ali': 20 # Change Ali’s age

}

​

marks

All Functions of a Dictionary

>> dir (int) # All functions of int

>> dir ({}) # Dictionary func

['__class__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'items', 'keys', 'pop', 'popitem', 'setdefault', 'update', 'values']

​

List vs. Dictionary

• List
• Items are ordered
• Access by index O(1)
• Insert or delete O(n)
• No access by key (Loop and search for key)
• Allows duplication
• Dictionaries
• Items are ordered in Python implementation
• No access by index
• Access by key, insert or delete O(1)
• No duplication

​

Iterating over Dictionary #1

marks = {

'Ali': 30,

'Sami': 35,

'Amina ': 28,

'Aliaa': 20

}

​

print (marks)

for key, value in marks.items():

if (key[0].upper() == 'A'):

print (key, ' ', value)

17

Iterating over Dictionary #2

marks = {

'Ali': 30,

'Sami': 35,

'Amina ': 28,

'Aliaa': 20

}

​

print (marks)

for key in marks.keys():

print(key, ' ', marks[key])

18

Iterating over Dictionary #3

marks = {

'Ali': 30,

'Sami': 35,

'Amina ': 28,

'Aliaa': 20

}

​

print (marks)

for key in marks:

print(key, '->', marks[key])

19

What is really happening?

marks = {

'Ali': 30,

'Sami': 35,

'Amina ': 28,

'Aliaa': 20

}

​

itr = marks.__iter__()

while itr.__length_hint__() > 0:

key = itr.__next__()

print(key, '->', marks[key])

20

Iterating over Dictionary #4

marks = {

'Ali': 30,

'Sami': 35,

'Amina ': 28,

'Aliaa': 20

}

​

print (marks)

sum = 0

for value in marks.values():

sum += value

print(sum / len(marks))

21

Deleting Dictionary Items

marks = {

'Ali': 30,

'Sami': 35,

'Amina ': 28,

'Aliaa': 20

}

​

print (marks)

del marks['Sami']

print (marks)

22

Accessing Dictionary Items

marks = {

'Ali': 30,

'Sami': 35,

'Amina ': 28,

'Aliaa': 20

}

​

print (marks['Ali'])

print (marks['Amir']) #Error

print (marks.get('Ali'))

print (marks.get('Amir')) #None

marks.get('Amir', 'No Such Key')

23

24

2. Files

• We cannot input data every time
• Files are the permanent way to store data for future use.
• Actual file management on disk is handled by operating system.
• Main file operations
1. Open a file
2. Read or write (perform operation)
3. Close the file

Reading from Files

# Open existing file for reading

file = open("test.txt")

f = open("C:/Python38/readme.txt")

​

26

27

Opening Files

# equivalent to 'r' or 'rt'

f = open("test.txt")

​

# write in text mode

f = open("test.txt",'w')

​

# read and write in binary mode

f = open("img.bmp",'r+b')

28

Reading from Files

• Default encoding is platform dependent. In windows, it is cp1252 (like ASCII) but utf-8 in Linux.
• If not ASCII, specify which one it is:
• f = open("test.txt", encoding='utf-8') f = open("test.txt", encoding='utf-16')

​

• If not, program will behave different on different systems.

​

29

Reading from Files

• f = open("test.txt", encoding = 'utf-8')
• f.read(4) # read the first 4 data
• f.read() # read in the rest of file
• f.read() # returns empty string
• f.seek(0) # returns cursor to start
• f.tell() # returns cursor position

30

Reading from Files

• f = open("test.txt", encoding = 'utf-8')
• for line in f:

print(line, end = '')

​

• f.seek(0)
• for line in f:
• print (line.split(), end = '')

​

• f.readline()

31

Writing to File

• ??
• ??
• ??
• Search about it

​

32

Closing a File

• When finished, close the file
• This frees the memory resources used for it

​

• file = open('file_path', 'w')
• file.write('hello world !')
• file.close()

​

• # Auto close
• with open('file_path', 'w') as file:
•     file.write('hello world !')

​

33

Exercise #1

​

• Modify hangman game to read words from a file.

Homework …..

• Read chapters 1 to 8 and 10
• Work on assignment 3
• Prepare for Quiz 2

35

Readings

• Readings are essential for this course. You must read related chapters from

36