Lists – Class 11 Computer Science notes, important questions and MCQs

Chapter overview

A list is an ordered, mutable sequence of elements enclosed in square brackets, and its elements can be of different data types. Lists support indexing, slicing, concatenation, repetition and membership, and they can be changed in place. Python provides many list methods and built-in functions, and lists can be nested, copied and passed to functions.

List operations

• Creating: L = [10, 20, 30, 40]; indexing L[0] is 10 and L[-1] is 40; slicing L[1:3] is [20, 30].
• Lists are mutable: L[1] = 25 changes the second element; concatenation [1, 2] + [3] gives [1, 2, 3]; repetition [0] * 3 gives [0, 0, 0]; membership 20 in L tests presence.
• Traversal uses for x in L or for i in range(len(L)); nested lists such as M = [[1, 2], [3, 4]] are accessed as M[1][0], which is 3.

Methods, copying and functions

• Methods: append(x) adds one element at the end; extend(list) adds many; insert(i, x) inserts at position i; remove(x) deletes the first occurrence; pop(i) removes and returns the element at i (last by default); count(), index(), reverse(), sort(); built-ins len(), sorted(), min(), max(), sum() and list().
• Assignment L2 = L1 makes both names refer to the same list, so changes appear in both; a true copy is made with L1.copy(), list(L1) or L1[:].
• When a list is passed to a function, changes made to it inside the function are reflected outside because lists are mutable; sort() changes the list in place and returns None, while sorted() returns a new list.

Key terms

List

an ordered, mutable sequence in square brackets

append()

a method that adds one element at the end of a list

extend()

a method that adds all elements of another sequence

Nested list

a list that contains other lists as elements

Aliasing

two names referring to the same list object

Important questions with answers

Quick revision

• Creating, indexing and slicing lists.
• Mutability; concatenation, repetition, membership.
• List methods and built-in functions.
• Copying lists; lists as function arguments.

Exam tips

• append() adds one element; extend() adds each element of a sequence.
• Use L.copy() to avoid aliasing.

Practice MCQs

1. Lists in Python are enclosed in

   1. parentheses ()
   2. braces {}
   3. square brackets []
   4. quotes
   Show answer

   Answer: (c) square brackets []. Example: [1, 2, 3].

2. The method that adds one element at the end of a list is

   1. insert()
   2. add()
   3. extend()
   4. append()
   Show answer

   Answer: (d) append(). L.append(5).

3. Lists in Python are

   1. mutable
   2. immutable
   3. fixed in size
   4. only numeric
   Show answer

   Answer: (a) mutable. Elements can be changed.

4. If L = [1, 2, 3] and L.extend([4, 5]) is run, L becomes

   1. [1, 2, 3, [4, 5]]
   2. [4, 5, 1, 2, 3]
   3. [1, 2, 3, 4, 5]
   4. [1, 2, 3, 9]
   Show answer

   Answer: (c) [1, 2, 3, 4, 5]. extend adds each element.

5. If L = [10, 20, 30, 40], L.pop() returns

   1. 10
   2. 30
   3. 40
   4. None
   Show answer

   Answer: (c) 40. Without an index, pop removes the last element.

6. For M = [[1, 2], [3, 4]], M[1][0] is

   1. 2
   2. 1
   3. 4
   4. 3
   Show answer

   Answer: (d) 3. Row 1, column 0.

7. After A = [1, 2]; B = A; B.append(3), the value of A is

   1. [1, 2, 3]
   2. [1, 2]
   3. [3]
   4. an error
   Show answer

   Answer: (a) [1, 2, 3]. A and B refer to the same list.

8. The statement x = L.sort() assigns to x

   1. the sorted list
   2. None
   3. the original list
   4. the length of the list
   Show answer

   Answer: (b) None. sort() works in place and returns None.