Tuples and Dictionaries – Class 11 Computer Science notes, important questions and MCQs

Chapter overview

A tuple is an ordered, immutable sequence of elements enclosed in parentheses. A dictionary is a mutable collection of key-value pairs enclosed in curly braces, in which keys are unique and immutable. Tuples are useful for fixed data and for returning multiple values, while dictionaries allow fast lookup of values by key. Both support useful operations, methods and built-in functions.

Tuples

• Creating: T = (10, 20, 30); a single-element tuple needs a comma, such as (5,), because (5) is just an integer; tuples support indexing, slicing, concatenation, repetition and membership.
• Tuples are immutable, so T[0] = 1 raises a TypeError; however, a mutable element inside a tuple, such as a list, can be changed.
• Tuple methods and functions: count(), index(), len(), min(), max(), sum(), sorted() and tuple(); tuple assignment allows swapping, as in a, b = b, a.

Dictionaries

• Creating: D = {'roll': 1, 'name': 'Asha'}; D['name'] gives 'Asha'; D['age'] = 16 adds a new pair, and assigning to an existing key updates its value.
• Keys must be unique and immutable (numbers, strings, tuples); values can be of any type; membership with in checks keys, not values.
• Methods: keys(), values(), items(), get() (returns None or a default value if the key is absent), update(), pop(), popitem(), clear(), copy(), fromkeys(), setdefault(); del D[key] deletes a pair; dictionaries are used for counting frequencies, as in counting characters in a string.

Key terms

Tuple

an ordered, immutable sequence in parentheses

Dictionary

a mutable collection of unique keys mapped to values

Key

the unique, immutable identifier in a dictionary pair

get()

a dictionary method that returns a value without raising an error for a missing key

Tuple assignment

assigning several values at once, as in a, b = 1, 2

Important questions with answers

Quick revision

• Creating and accessing tuples; immutability.
• Tuple operations, methods and assignment.
• Creating, accessing and modifying dictionaries.
• Dictionary methods; counting frequencies.

Exam tips

• Remember: (5) is an int, (5,) is a tuple.
• Use D.get(key) to avoid KeyError.

Practice MCQs

1. Tuples in Python are enclosed in

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

   Answer: (d) parentheses (). Example: (1, 2, 3).

2. A dictionary stores data as

   1. key-value pairs
   2. index-value pairs only
   3. only keys
   4. only values
   Show answer

   Answer: (a) key-value pairs. Example: {'a': 1}.

3. Tuples are

   1. mutable
   2. only numeric
   3. unordered
   4. immutable
   Show answer

   Answer: (d) immutable. Their elements cannot be reassigned.

4. Which of these creates a tuple with one element

   1. (5,)
   2. (5)
   3. [5]
   4. {5}
   Show answer

   Answer: (a) (5,). The comma makes it a tuple.

5. For D = {'a': 1, 'b': 2}, D.get('c') returns

   1. 0
   2. None
   3. an error
   4. 'c'
   Show answer

   Answer: (b) None. get() does not raise KeyError.

6. For D = {'x': 10}, the statement D['x'] = 20

   1. adds a second key 'x'
   2. updates the value of 'x' to 20
   3. raises an error
   4. deletes 'x'
   Show answer

   Answer: (b) updates the value of 'x' to 20. Keys are unique.

7. Which cannot be a dictionary key

   1. 10
   2. 'name'
   3. (1, 2)
   4. [1, 2]
   Show answer

   Answer: (d) [1, 2]. Lists are mutable and unhashable.

8. After a, b = 3, 7 and then a, b = b, a, the values are

   1. a = 3, b = 7
   2. a = 7, b = 7
   3. a = 7, b = 3
   4. an error
   Show answer

   Answer: (c) a = 7, b = 3. Tuple assignment swaps the values.