Why Hash Map Exists and What Problem It Solves in DSA C++ - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to insert a key-value pair into the hash map.

DSA C++

hashMap[[1]] = 10;

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

Akey

BhashMap

C"apple"

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Using a value instead of a key inside the brackets.

We use a key (like "apple") to store the value 10 in the hash map.

2fill in blank

medium

Complete the code to check if a key exists in the hash map.

DSA C++

if (hashMap.find([1]) != hashMap.end()) {
    // key exists
}

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

A"apple"

BhashMap

C5

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using a value or the whole map instead of a key.

We check if the key "apple" exists in the hash map using find().

3fill in blank

hard

Fix the error in the code to retrieve a value from the hash map safely.

DSA C++

int value = hashMap[[1]];

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

AhashMap

B"banana"

Cvalue

D"orange"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the map or value variable instead of a key.

We use the key "banana" to get its value from the hash map.

4fill in blank

hard

Fill both blanks to create a hash map and insert a key-value pair.

DSA C++

std::unordered_map<std::string, int> [1];
[2]["cat"] = 3;

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

AanimalCount

BhashMap

CcountMap

Dmap

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for declaration and access.

We declare a hash map named animalCount and insert the key "cat" with value 3.

5fill in blank

hard

Fill all three blanks to iterate over a hash map and print keys and values.

DSA C++

for (const auto& [1] : [2]) {
    std::cout << [1].first << " -> " << [1].second << std::endl;
}

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

Aentry

BmyMap

Citem

DhashMap

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

We use 'entry' as the loop variable and 'hashMap' as the map to print all key-value pairs.

Practice

1. Why do we use a hash map instead of a simple array for storing key-value pairs?

easy

A. Because arrays do not allow storing multiple values

B. Because arrays can only store numbers, not keys and values

C. Because hash maps use less memory than arrays

D. Because hash maps provide faster search, insert, and delete operations on average

Solution

1. Step 1: Understand array limitations for key-value storage

   Arrays store elements by index, so searching by key requires scanning all elements, which is slow (O(n)).

2. Step 2: Recognize hash map advantages

   Hash maps use a hash function to find the index quickly, allowing average O(1) time for search, insert, and delete.

3. Final Answer:

   Because hash maps provide faster search, insert, and delete operations on average -> Option D

4. Quick Check:

   Hash map speed advantage = D [OK]

Hint: Hash maps speed up key-based access compared to arrays [OK]

Common Mistakes:

• Thinking arrays can't store key-value pairs
• Assuming hash maps always use less memory
• Confusing hash maps with linked lists

2. Which of the following is the correct way to declare a hash map in C++ that maps strings to integers?

easy

A. std::unordered_map map;

B. std::unordered_map map;

C. std::map map;

D. std::unordered_map map;

Solution

1. Step 1: Identify correct template parameters for unordered_map

   The first template parameter is the key type, the second is the value type. Here, key is string, value is int.

2. Step 2: Check syntax correctness

   std::unordered_map map; uses std::unordered_map map; which is correct syntax and types.

3. Final Answer:

   std::unordered_map map; -> Option A

4. Quick Check:

   Correct unordered_map declaration = A [OK]

Hint: Key type first, value type second in unordered_map template [OK]

Common Mistakes:

• Swapping key and value types
• Missing std:: before unordered_map
• Incorrect template syntax

3. What will be the output of the following C++ code?

std::unordered_map map;
map["apple"] = 5;
map["banana"] = 3;
map["apple"] = 7;
std::cout << map["apple"] << std::endl;

medium

A. 5

B. 7

C. 3

D. Compilation error

Solution

1. Step 1: Understand insertion and update in unordered_map

   map["apple"] = 5 inserts key "apple" with value 5. Then map["apple"] = 7 updates the value to 7.

2. Step 2: Output the value for key "apple"

   After update, map["apple"] holds 7, so printing it outputs 7.

3. Final Answer:

   7 -> Option B

4. Quick Check:

   Updated value for "apple" = 7 [OK]

Hint: Last assigned value for a key is stored [OK]

Common Mistakes:

• Assuming values accumulate instead of overwrite
• Confusing keys and values
• Expecting compilation error due to syntax

4. Identify the error in the following code snippet that uses a hash map in C++:

std::unordered_map map;
map.insert(1, "one");
std::cout << map[1] << std::endl;

medium

A. insert() requires a pair, not two separate arguments

B. map[1] cannot be used to access elements

C. unordered_map keys must be strings, not integers

D. std::cout syntax is incorrect

Solution

1. Step 1: Check usage of insert() method in unordered_map

   insert() expects a std::pair, not two separate arguments.

2. Step 2: Identify correct way to insert key-value pair

   Use map.insert({1, "one"}); or map[1] = "one"; to insert correctly.

3. Final Answer:

   insert() requires a pair, not two separate arguments -> Option A

4. Quick Check:

   insert() needs pair argument [OK]

Hint: insert() needs pair, not separate key and value [OK]

Common Mistakes:

• Passing key and value as separate arguments to insert()
• Thinking map[1] cannot access elements
• Believing keys must be strings

5. You want to count the frequency of each word in a large text using a hash map in C++. Which problem does using a hash map solve best compared to other data structures?

hard

A. It sorts the words alphabetically automatically

B. It uses less memory than arrays for storing counts

C. It allows counting frequencies in O(n) time by direct key access

D. It prevents duplicate words from being stored

Solution

1. Step 1: Understand frequency counting requirements

   Counting frequencies requires fast lookup and update of counts by word keys.

2. Step 2: Recognize hash map advantage for frequency counting

   Hash maps provide average O(1) access to keys, enabling counting in O(n) time over all words.

3. Step 3: Eliminate incorrect options

   Hash maps do not sort keys automatically, may use more memory than arrays, and allow duplicates as keys with updated counts.

4. Final Answer:

   It allows counting frequencies in O(n) time by direct key access -> Option C

5. Quick Check:

   Fast key access enables O(n) frequency count [OK]

Hint: Hash maps give fast key lookup for counting [OK]

Common Mistakes:

• Thinking hash maps sort keys automatically
• Assuming hash maps use less memory than arrays
• Believing hash maps prevent duplicate keys