08 - LISTS

Python Programming Study Guide: Lists as Mutable Sequence Types

Bert’s contempt for Lists knows no bounds!

List Syntax

Bert made a summary of the syntax for each of the Python list functions in the study guide:

  • append()
    • Adds an element to the end of the list.
    • Syntax: list.append(element)
  • index()
    • Returns the index of the first occurrence of the specified element.
    • Syntax: list.index(element, start, end)
    • The start and end parameters are optional and specify the search range.
  • insert()
    • Inserts an element at the specified position.
    • Syntax: list.insert(index, element)
  • pop()
    • Removes and returns the element at the specified position.
    • Syntax: list.pop(index)
    • If the index is not specified, pop() removes and returns the last element.
  • remove()
    • Removes the first occurrence of the specified element.
    • Syntax: list.remove(element)
  • reverse()
    • Reverses the elements of the list in place.
    • Syntax: list.reverse()
  • count()
    • Returns the number of times the specified element appears in the list.
    • Syntax: list.count(element)

Key Concepts

Lists as Mutable Sequence Types

  • Mutable: Lists in Python can be changed after they are created. This is in contrast to strings, which are immutable.
  • Sequence Type: A list is an ordered collection of items.

Indexing and Slicing

  • Indexing: Access an individual element of a list using its index, e.g., list[0] for the first item.
  • Slice Notation: Extract a subset of a list, e.g., list[start:stop] extracts elements from index start to stop-1.

List Functions and Operations

  • Add: append(item) adds an item to the end, insert(index, item) adds an item at a specified index.
  • Remove: remove(item) removes the first occurrence of an item.
  • Update: Directly assign a new value to a list element, e.g., list[index] = new_value.
  • Find: Use index(item) to find the index of the first occurrence of an item.

Common Patterns for List Management

  • Enumeration and Aggregates: Using loops to iterate over lists, and using functions like len(), max(), min() to aggregate data.
  • List Mutation: Changing, adding, or removing elements in a list.
  • Combining Lists: Use the + operator to concatenate two lists.

Multiple Choice Questions

  1. For x = [0,1,2,3,4,5], what is x[2:4]?
    • A. [2,3]
    • B. [1,2,3]
    • C. [1,2]
    • D. [2,3,4,5]
  2. How do you add a new element at the end of a list named myList?
    • A. myList.append(newElement)
    • B. myList.insert(len(myList), newElement)
    • C. myList.add(newElement)
    • D. myList.put(newElement)
  3. Which operation will combine two lists list1 and list2 into a new list list3?
    • A. list3 = list1 + list2
    • B. list3 = list1.append(list2)
    • C. list3 = list1.extend(list2)
    • D. list3 = combine(list1, list2)
  4. What does grades[:2] return if grades = ['A', 'B+','A','C+','B-']?
    • A. [‘A’, ‘B+’]
    • B. [‘B+’, ‘A’]
    • C. [‘A’, ‘B+’, ‘A’]
    • D. The first two characters of the first item in grades
  5. What is the result of the following operations on the list items = ['a','b','c','d']?
  • A. [‘a’,‘b’,‘c’,‘d’, ‘w’]
  • B. [‘a’,‘b’,‘d’, ‘w’]
  • C. [‘x’, ‘b’, ‘d’, ‘w’]
  • D. [‘i’,‘dont’,‘know’]

  1. What is the output of the following code?

    myList = [4, 2, 8, 6]
myList[2] = 5
print(myList)
    • A. [4, 2, 5, 6]
    • B. [4, 2, 8, 5]
    • C. [4, 5, 8, 6]
    • D. [5, 2, 8, 6]

  2. What is the output of the following code?

    x = ["apple", "banana", "cherry"]
x.remove("banana")
print(x)
    • A. ["apple", "banana"]
    • B. ["apple", "cherry"]
    • C. ["banana", "cherry"]
    • D. ["apple", "banana", "cherry"]

  3. What will be the length of the list after executing the following code?

    x = [1, 2, 3, 4, 5]
x.append(6)
x.extend([7, 8])
    • A. 5
    • B. 6
    • C. 8
    • D. 10

  4. What is the result of the following list slicing operation?

    myList = [10, 20, 30, 40, 50]
print(myList[1:4])
    • A. [10, 20, 30]
    • B. [20, 30, 40]
    • C. [30, 40, 50]
    • D. [20, 30, 40, 50]

  5. Which method is used to add an element at a specific index in a list?

    • A. append()
    • B. insert()
    • C. extend()
    • D. add()

Correct Answers and Explanations

  1. Answer: A. [2,3]
  • Explanation: The slice x[2:4] extracts elements from index 2 up to but not including index 4, resulting in [2, 3].
  1. Answer: A. myList.append(newElement)
  • Explanation: append() is the correct method to add an element to the end of a list. The other options are either incorrect methods or overly complicated.
  1. Answer: A. list3 = list1 + list2
  • Explanation: The + operator concatenates two lists. append() and extend() methods modify the list in place and do not return a new list.
  1. Answer: A. [‘A’, ‘B+’]
  • Explanation: The slice grades[:2] returns the first two elements of the list, which are [‘A’, ‘B+’].
  1. Answer: C. [‘x’, ‘b’, ‘d’, ‘w’]
  • Explanation: After each operation, the list changes as follows:
    • items.append('w') -> [‘a’,‘b’,‘c’,‘d’, ‘w’]
    • items.remove('c') -> [‘a’,‘b’,‘d’, ‘w’]
    • items.insert(0,'x') -> [‘x’, ‘a’, ‘b’, ‘d’, ‘w’]
    • items.pop(1) -> [‘x’, ‘b’, ‘d’, ‘w’]

  1. Answer: A. [4, 2, 5, 6]

    Explanation: The code changes the element at index 2 (third element) from 8 to 5.

  2. Answer: B. ["apple", "cherry"]

    Explanation: The remove() method removes the first occurrence of the element “banana” from the list.

  3. Answer: C. 8

    Explanation: The append() method adds one element, and extend() adds three more elements, making the total length 8.

  4. Answer: B. [20, 30, 40]

    Explanation: The slice [1:4] includes elements from index 1 to 3 (20, 30, 40).

  5. Answer: B. insert()

    Explanation: The insert() method is used to add an element at a specified index. append() adds to the end, extend() adds multiple elements, and add() is not a list method. ```

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