Hash map vs hash set in Data Structures Theory - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the sentence to describe a hash map.

Data Structures Theory

A hash map stores data as key[1]value pairs.

Drag options to blanks, or click blank then click option'

Ato

Band

C:

Dwith

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'and' or 'to' which do not represent the key-value relationship.

Confusing the connection symbol between key and value.

A hash map stores data as key:value pairs, where each key maps to a value.

2fill in blank

medium

Complete the sentence to describe a hash set.

Data Structures Theory

A hash set stores only unique [1] without any associated values.

Drag options to blanks, or click blank then click option'

Aindexes

Bvalues

Cpairs

Dkeys

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'values' which implies associated data, not correct for sets.

Choosing 'pairs' which applies to maps, not sets.

A hash set stores only unique keys (or elements) without associated values.

3fill in blank

hard

Fix the error in the statement about hash maps.

Data Structures Theory

In a hash map, each [1] must be unique, but values can repeat.

Drag options to blanks, or click blank then click option'

Avalue

Bkey

Cpair

Delement

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing keys and values uniqueness.

Thinking values must be unique in a hash map.

In a hash map, keys must be unique to identify values, but values can be duplicated.

4fill in blank

hard

Fill both blanks to compare hash map and hash set.

Data Structures Theory

A hash map stores [1] pairs, while a hash set stores only [2].

Drag options to blanks, or click blank then click option'

Akey-value

Bunique values

Cunique keys

Dindex-value

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up keys and values for sets.

Using 'index-value' which is not related to hash sets or maps.

A hash map stores key-value pairs, while a hash set stores only unique keys (elements).

5fill in blank

hard

Fill all three blanks to complete the differences between hash map and hash set.

Data Structures Theory

In a hash map, [1] are unique and map to [2], but in a hash set, only [3] are stored.

Drag options to blanks, or click blank then click option'

Akeys

Bvalues

Cunique elements

Dindexes

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing values with keys in sets.

Thinking sets store indexes.

In a hash map, keys are unique and map to values, but in a hash set, only unique elements (keys) are stored without values.

Practice

1. Which data structure is best suited for storing unique items without any associated values?

easy

A. Hash set

B. Linked list

C. Array

D. Hash map

Solution

1. Step 1: Understand the purpose of a hash set

   A hash set stores only unique values and does not associate keys with values.

2. Step 2: Compare with hash map

   A hash map stores key-value pairs, so it is not just for unique values but for mapping keys to values.

3. Final Answer:

   Hash set -> Option A

4. Quick Check:

   Unique values only = Hash set [OK]

Hint: Unique items without keys? Think hash set [OK]

Common Mistakes:

• Confusing hash map with hash set
• Thinking hash map stores unique values only
• Assuming arrays enforce uniqueness

2. Which of the following is the correct way to declare a hash map in Python?

easy

A. my_map = ["key1", "value1", "key2", "value2"]

B. my_map = ("key1", "value1", "key2", "value2")

C. my_map = {"key1": "value1", "key2": "value2"}

D. my_map = set("key1", "key2")

Solution

1. Step 1: Recognize Python dictionary syntax

   Python hash maps are dictionaries declared with curly braces and key-value pairs separated by colons.

2. Step 2: Check each option

   my_map = {"key1": "value1", "key2": "value2"} uses curly braces and key-value pairs correctly. Options A and C use lists or tuples, not dictionaries. my_map = set("key1", "key2") creates a set, not a map.

3. Final Answer:

   my_map = {"key1": "value1", "key2": "value2"} -> Option C

4. Quick Check:

   Curly braces with key:value = dict [OK]

Hint: Curly braces with colons mean hash map in Python [OK]

Common Mistakes:

• Using lists or tuples instead of dictionaries
• Confusing sets with maps
• Missing colons between keys and values

3. What will be the output of the following Python code?

my_set = {1, 2, 3}
my_set.add(2)
print(len(my_set))

medium

A. 4

B. 3

C. 2

D. Error

Solution

1. Step 1: Understand set behavior on adding duplicates

   Sets store unique values. Adding 2 again does not increase size.

2. Step 2: Calculate length after adding duplicate

   Initial set has 3 elements. Adding 2 again keeps it at 3.

3. Final Answer:

   3 -> Option B

4. Quick Check:

   Set ignores duplicates = length unchanged [OK]

Hint: Adding duplicate to set doesn't increase size [OK]

Common Mistakes:

• Assuming duplicates increase set size
• Confusing set with list behavior
• Expecting an error on duplicate add

4. Identify the error in this JavaScript code snippet that tries to use a hash set:

const mySet = new Set();
mySet.add('apple');
mySet.add('banana');
mySet.add('apple');
console.log(mySet.length);

medium

A. Using 'length' property instead of 'size' for Set

B. Adding duplicate values to Set causes error

C. Set cannot store strings

D. Missing parentheses in Set constructor

Solution

1. Step 1: Check Set properties in JavaScript

   JavaScript Sets use 'size' property to get number of elements, not 'length'.

2. Step 2: Analyze the code

   Code uses 'mySet.length' which is undefined, causing incorrect output.

3. Final Answer:

   Using 'length' property instead of 'size' for Set -> Option A

4. Quick Check:

   Set size = size property, not length [OK]

Hint: Use .size, not .length, to get Set size in JavaScript [OK]

Common Mistakes:

• Using length instead of size for Sets
• Thinking duplicates cause errors
• Believing Sets can't store strings

5. You have a list of student names with possible duplicates. You want to count how many unique students there are and also store their grades. Which data structures should you use together?

hard

A. Use two hash sets: one for names and one for grades

B. Use only a hash map with names as keys and grades as values

C. Use a hash set for grades and a hash map for names

D. Use a hash set for unique names and a hash map for name-grade pairs

Solution

1. Step 1: Identify need for uniqueness and mapping

   Counting unique students requires a hash set to store unique names. Storing grades requires a hash map to map each name to a grade.

2. Step 2: Evaluate options

   Use a hash set for unique names and a hash map for name-grade pairs uses both structures correctly. Use only a hash map with names as keys and grades as values does not separate uniqueness check explicitly. Options C and D misuse sets and maps.

3. Final Answer:

   Use a hash set for unique names and a hash map for name-grade pairs -> Option D

4. Quick Check:

   Uniqueness + mapping = set + map [OK]

Hint: Unique names = set; name to grade = map [OK]

Common Mistakes:

• Using only one data structure for both tasks
• Confusing sets for storing grades
• Not separating uniqueness from mapping