9.1 Modifying and iterating lists

Lists in Python are versatile data structures that allow for easy modification and iteration. They're mutable, meaning you can change their contents after creation, and offer various methods for adding, removing, and accessing elements.

Iterating through lists is a fundamental skill in Python programming. Whether using for loops, list comprehensions, or built-in functions, mastering list iteration techniques enables efficient data manipulation and analysis in your code.

List Modification and Iteration

List modification operations

• append() adds an element to the end of a list
  • Syntax: list_name.append(element)
  • Appends the specified element to the end of the list (numbers.append(10) adds 10 to the end of the numbers list)
• remove() removes the first occurrence of a specified element from a list
  • Syntax: list_name.remove(element)
  • Removes the first occurrence of the specified element from the list (fruits.remove("apple") removes the first occurrence of "apple" from the fruits list)
  • Raises a ValueError if the element is not found in the list
• pop() removes an element at a specified index and returns its value
  • Syntax: list_name.pop(index)
  • Removes the element at the specified index from the list and returns its value (numbers.pop(2) removes the element at index 2 from the numbers list and returns its value)
  • If no index is provided, pop() removes and returns the last element in the list
• Lists are mutable, allowing for in-place modification of their elements

Iteration through lists

• for loops iterate through each element in a list
  • Syntax: for element in list_name:
  • Iterates through each element in the specified list (for fruit in fruits: iterates through each element in the fruits list)
• Access list elements using their indexes
  • List indexes start at 0 for the first element and increment by 1 for each subsequent element
  • Syntax: list_name[index]
  • Accesses the element at the specified index in the list (fruits[0] accesses the first element in the fruits list)
• range() function with len() iterates through list indexes
  • Syntax: for i in range(len(list_name)):
  • Iterates through the indexes of the specified list (for i in range(len(numbers)): iterates through the indexes of the numbers list)

List comprehensions for creation

• List comprehensions create new lists based on existing lists concisely
• Syntax: new_list = [expression for element in list_name if condition]
  • expression is the operation performed on each element
  • element is the variable representing each element in the original list
  • list_name is the name of the original list
  • if condition is an optional filter to include elements based on a condition
• Create a new list with each element from the original list squared (squared_numbers = [x ** 2 for x in numbers] creates a new list with each element from numbers squared)
• Create a new list with only even numbers from the original list (even_numbers = [x for x in numbers if x % 2 == 0] creates a new list with only even numbers from numbers)

Advanced List Operations

• Slicing allows for extracting a portion of a list using a start index, end index, and optional step
• The in operator can be used to check for the presence of an element in a list during iteration
• Iteration can be performed using various methods, including for loops, list comprehensions, and built-in functions like map() and filter()