Python Programming : Intermediate (Leaving Cert Computer Science): Revision Notes

Python Programming : Intermediate

Strings

We will begin by revisiting strings and examining them more closely.

A string is an object, it has methods that can be invoked and fields we can access.

Assuming str is our string, common string methods include :

• str.lower() : Converts all characters in the string to lowercase.
• str.upper() : Converts all characters in the string to uppercase.
• str.capitalise() : Capitalises the first character of the string
• str.title() : Capitalises the first character of each word in the string

sentence = "this is my string"
print(sentence.upper()) # THIS IS MY STRING
print(sentence.title()) # This Is My String

• str.strip() : Removes leading and trailing whitespace.
• str.lstrip() : Removes leading whitespace.
• str.rstrip() : Removes trailing whitespace.

sentence = "    this is my string    "
print(sentence.strip())  #  "this is my string"
print(sentence.title())  #  "this is my string    "

Strings are indexed, meaning each character has a position (index) starting from 0. You can access individual characters or slices of a string using square brackets [] .

Consider the string "Hello John", the index of each character is as follows :

greeting = "Hello John"

first_letter = greeting[0] # "H"
sixth_letter = greeting[5] # " " (whitespace character)
last_letter = greeting[len(greeting) - 1] # "n"

Indexing also allows us to slice strings, which let's us take substrings.

• [start : end] slices a string from the start index up until but not including the end index.
• [start : ] slices a string from the start index up until the end of the string.
• [ : end] slices a string from the start of the string up until but not including the end index of the string.

greeting = "Hello John"

first_word = greeting[0 : 5]  # "Hello"
substring = greeting[2 : 7]   # "llo J"
second_word = greeting[6 : ] # "John"

• str.find(sub) : Returns the lowest index in the string where substring sub is found, or 1 if not found.
• str.rfind(sub) : Returns the highest index in the string where substring sub is found, or 1 if not found.
• str.replace(old, new): Returns a new string where all occurrences of substring old are replaced by new.

text = "Hello, World!"

print(text.find('World'))      # 7
print(text.rfind('o'))         # 8
print(text.replace('World', 'Python'))  # 'Hello, Python!'

• str.startswith(prefix) : Returns True if the string starts with the specified prefix.
• str.endswith(suffix) : Returns True if the string ends with the specified suffix.
• str.isalpha() : Returns True if all characters in the string are alphabetic.
• str.isdigit() : Returns True if all characters in the string are digits.
• str.isalnum() : Returns True if all characters in the string are alphanumeric.

text = "Python3"
print(text.startswith('Py'))   # True
print(text.endswith('3'))      # True
print(text.isalpha())          # False
print(text.isdigit())          # False
print(text.isalnum())          # True

Escape Characters

Escape characters are used to include special characters in a string. They are prefixed with a backslash - [].

• \n : Newline
• \t : Tab
• \ : Backslash
• ' : Single quote
• " : Double quote

print("Hello\nWorld")     # Newline
print("Hello\tWorld")     # Tab
print("Backslash: \\")    # Backslash
print("Single quote: \'") # Single quote
print("Double quote: \"") # Double quote

Boolean Expressions

Boolean expressions in Python are expressions that evaluate to either True or False.

• and : Returns True if both operands are true
• or : Returns True if at least one operand is true
• not : Returns True if the operand is false

x = True
y = False

print(x and y)  # False
print(x or y)   # True
print(not x)    # False
print(not y)    # True

Conditionals

Conditionals allow you to execute different code blocks based on certain conditions. In Python, conditional statements are used to make decisions in the code by evaluating Boolean expressions.

The if statement is used to test a condition. If the condition evaluates to True, the code block under the if statement is executed. If the condition is False, the code block is skipped.

age = 18

if age >= 18:
    print("You are an adult.")

The else statement follows an if statement and is executed if the if condition is False. It provides an alternative set of instructions when the if condition is not met.

age = 16

if age >= 18:
    print("You are an adult.")
else:
    print("You are a minor.")

The elif (short for "else if") statement allows you to check multiple conditions.

It must follow an if statement or another elif statement. When an elif condition is True, its associated code block is executed, and the remaining elif and else blocks are skipped.

age = 20

if age < 13:
    print("You are a child.")
elif age < 18:
    print("You are a teenager.")
else:
    print("You are an adult.")

Lists

Lists are one of the most versatile and commonly used data structures in Python. They are used to store multiple items in a single variable.

Lists are mutable, meaning their elements can be changed after the list has been created.

Lists are defined by enclosing items in square brackets ([]), separated by commas.

numbers = [1, 2, 3, 4, 5]
fruits = ["apple", "banana", "cherry"]
mixed_list = [1, "apple", 3.14, True]

Like strings, lists are also indexed.

fruits = ["apple", "banana", "cherry"]

print(fruits[0])  # apple
print(fruits[1])  # banana
print(fruits[2])  # cherry

You can access a range of elements in a list using slicing. The syntax is list[start : end : step].

numbers = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

print(numbers[2:5])   # Output: [2, 3, 4]
print(numbers[:4])    # Output: [0, 1, 2, 3]
print(numbers[5:])    # Output: [5, 6, 7, 8, 9]
print(numbers[::2])   # Output: [0, 2, 4, 6, 8]
print(numbers[::-1])  # Output: [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]

Lists are also objects, and support their own methods.

• append() : Adds an element to the end of the list.
• insert() : Inserts an element at a specified position.
• extend() : Adds all elements of a list to the end of the current list

fruits = ["apple", "banana", "cherry"]
fruits.append("orange")
print(fruits)  # ['apple', 'banana', 'cherry', 'orange']

fruits.insert(1, "blueberry")
print(fruits)  # ['apple', 'blueberry', 'banana', 'cherry', 'orange']

more_fruits = ["mango", "pineapple"]
fruits.extend(more_fruits)
print(fruits)  # ['apple', 'blueberry', 'banana', 'cherry', 'orange', 'mango', 'pineapple']

• remove() : Removes the first occurrence of a specified value.
• pop() : Removes and returns the element at a specified position. If no index is specified, pop() removes and returns the last item.
• clear() : Removes all elements from the list.

fruits = ["apple", "banana", "cherry", "banana"]
fruits.remove("banana")
print(fruits)  # ['apple', 'cherry', 'banana']

fruits.pop(1)
print(fruits)  # ['apple', 'banana']

fruits.pop()
print(fruits)  # ['apple']

fruits.clear()
print(fruits)  # []

Iteration

Iteration is a fundamental concept in programming that refers to the process of repeatedly executing a block of code. In Python, we perform iteration using loops.

The two primary types of looping are while loops and for loops.

A while loop repeatedly executes a block of code as long as a given condition is True.

count = 0
while count < 5:
    print(count)
    count = count + 1
    
"""
0
1
2
3
4
"""

The count = count + 1 portion of the code is a crucial component of the loop. This is called incrementation.

If we didn't increment, count will never get changed, and the loop will go on forever. This is a problem which may cause the programme to crash.

Incrementation is so widely used that Python supports shorthand notations for different types of variables reassignments.

count = 0

count += 1  # equivalent to a = a + 1
count -= 2  # equivalent to a = a - 2
count *= 2  # equivalent to a = a * 2
count /= 2  # equivalent to a = a / 2 

By declaring a counter before entering a loop, we can iterate over lists and string.

Here is an example that replaces all negative integers in a list to 0.

index = 0
list = [2, 4, 5, -1, -3, 5, 1, 0, -4]

while (index < len(list)) :
    if list[index] < 0 :
        list[index] = 0
    index += 1    
    
print(list)  # [2, 4, 5, 0, 0, 5, 1, 0, 0]

Here is an example that counts the number of vowels in a string.

index = 0
counter = 0
vowels = ['a', 'i', 'e', 'o', 'u']
sentence = "here is my string"

while (index < len(sentence)) :
    if sentence[index] in vowels :
        counter += 1
    index += 1    
    
print(counter) # 4

A for loop in Python is used to iterate over anything that is iterable which is any object that can be looped over such as lists or strings.

The general syntax of a for loop in Python is :

for variable in sequence :
	# do something

fruits = ["apple", "banana", "cherry", "banana"]

for fruit in fruits:
	print(fruit)

The range function generates a sequence of numbers. It is useful for iterating a specific number of times.

# Iterating from 0 to 4
for i in range(5):
    print(i)

# Iterating from 1 to 5
for i in range(1, 6):
    print(i)

# Iterating with a step
for i in range(0, 10, 2):
    print(i)