Character Frequency Counting in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a map that counts character frequencies.

DSA C++

std::map<char, int> freq[1];

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

A{}

B()

C[]

D<>

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets [] or parentheses () instead of empty braces {} for map initialization.

Using angle brackets <> which are for template parameters.

The map is declared with empty braces {} to default construct it using uniform initialization.

2fill in blank

medium

Complete the code to increase the frequency count of character c in the map.

DSA C++

freq[1]++;

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

A(c)

B[c]

C{c}

D<c>

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses () instead of square brackets [] for map access.

Using braces {} or angle brackets <> which are invalid here.

Square brackets are used to access or insert elements in a map by key.

3fill in blank

hard

Fix the error in the loop to iterate over the map entries correctly.

DSA C++

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

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

Apair

Bnode

Citem

Dentry

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name different from the one used inside the loop body.

Using reserved keywords as variable names.

The loop variable name can be any valid identifier; 'entry' is clear and used here.

4fill in blank

hard

Fill both blanks to check if character c is in the map and print its frequency.

DSA C++

if (freq.find([1]) != freq.[2]()) {
    std::cout << c << ": " << freq[c] << std::endl;
}

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

Ac

Bend

Cbegin

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using freq.begin() instead of freq.end() for comparison.

Using freq.count() in place of find() incorrectly.

Use freq.find(c) to search and compare with freq.end() to check presence.

5fill in blank

hard

Fill all three blanks to create a map of character frequencies from string s.

DSA C++

std::map<char, int> freq;
for (char [1] : [2]) {
    freq[3]++;
}

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

Ach

Bs

C[ch]

D(ch)

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of square brackets to access map elements.

Using wrong variable names inconsistent with the loop.

Loop variable is ch, loop over string s, access map with freq[ch].

Practice

1. What does character frequency counting in a string help us find?

easy

A. The length of the string

B. The position of a character in the string

C. The first character of the string

D. How many times each character appears in the string

Solution

1. Step 1: Understand character frequency counting

   It means counting how many times each character shows up in the string.

2. Step 2: Compare with options

   Only How many times each character appears in the string matches this meaning exactly.

3. Final Answer:

   How many times each character appears in the string -> Option D

4. Quick Check:

   Frequency counting = count characters [OK]

Hint: Frequency means count of each character [OK]

Common Mistakes:

• Confusing frequency with string length
• Thinking frequency means position
• Mixing frequency with substring extraction

2. Which of the following C++ code snippets correctly declares a map to count character frequencies?

easy

A. std::map freq;

B. std::vector freq;

C. std::map freq;

D. std::unordered_map<int> freq;

Solution

1. Step 1: Identify correct map declaration

   We need a map with key as char and value as int to count frequencies.

2. Step 2: Check each option

   std::map freq; uses std::map, which is correct. Others have wrong types or missing template parameters.

3. Final Answer:

   std::map freq; -> Option A

4. Quick Check:

   Map key=char, value=int for frequency [OK]

Hint: Map keys = characters, values = counts [OK]

Common Mistakes:

• Using vector instead of map
• Swapping key and value types
• Missing template parameters in map

3. What is the output of the following C++ code?

std::string s = "hello";
std::map freq;
for (char c : s) {
    freq[c]++;
}
for (auto &p : freq) {
    std::cout << p.first << ":" << p.second << " ";
}

medium

A. h:1 e:1 l:2 o:1

B. e:1 h:1 l:2 o:1

C. h:1 e:1 o:1 l:2

D. h:1 l:2 e:1 o:1

Solution

1. Step 1: Count characters in "hello"

   h=1, e=1, l=2, o=1

2. Step 2: Understand map iteration order

   std::map orders keys ascending by character: e, h, l, o

3. Final Answer:

   e:1 h:1 l:2 o:1 -> Option B

4. Quick Check:

   Map sorts keys alphabetically [OK]

Hint: Map prints keys sorted, not insertion order [OK]

Common Mistakes:

• Assuming insertion order output
• Mixing character counts
• Ignoring map key sorting

4. Identify the error in this code snippet for counting character frequencies:

std::string s = "abcabc";
std::map freq;
for (int i = 0; i <= s.size(); i++) {
    freq[s[i]]++;
}

medium

A. freq[s[i]]++ should be freq[s[i]] = 1

B. Map should be unordered_map for frequency counting

C. Loop condition should be i < s.size(), not <= s.size()

D. String s should be a vector<char>

Solution

1. Step 1: Check loop boundary

   Loop runs from i=0 to i <= s.size(), which accesses s[s.size()] out of bounds.

2. Step 2: Correct loop condition

   It should be i < s.size() to avoid out-of-range access.

3. Final Answer:

   Loop condition should be i < s.size(), not <= s.size() -> Option C

4. Quick Check:

   Index must be less than string length [OK]

Hint: Use <, not <=, for string indexing loops [OK]

Common Mistakes:

• Off-by-one error in loop
• Wrong map type confusion
• Incorrect frequency increment logic

5. Given a string, how can you find the character with the highest frequency using C++ STL containers?

hard

A. Count frequencies in a map, then iterate to find max frequency character

B. Sort the string and pick the first character

C. Use a vector<char> to store characters and pick the last one

D. Use a queue to enqueue characters and dequeue the max frequency

Solution

1. Step 1: Count character frequencies using a map

   Use std::map to count how many times each character appears.

2. Step 2: Iterate map to find character with max frequency

   Loop through map entries to track character with highest count.

3. Final Answer:

   Count frequencies in a map, then iterate to find max frequency character -> Option A

4. Quick Check:

   Map + iteration finds max frequency char [OK]

Hint: Use map to count, then find max by scanning map [OK]

Common Mistakes:

• Sorting string doesn't give max frequency
• Using vector or queue wrongly
• Not iterating to find max frequency