🦄 Rune Ledger: Character Frequency

Difficulty: Beginner Tags: strings, hash-map, counting, dictionary Series: CS 101: Fundamentals

Problem

In the ancient library, scholars maintain a ledger of rune frequencies. Count how many times each character appears in the mystical text.

Given a string, return a dictionary mapping each character to its frequency (number of occurrences). The counting is case-sensitive.

Real-World Application

Character frequency counting appears in: - Text analysis - word clouds, sentiment analysis, stylometry - Cryptography - frequency analysis, cipher breaking - Compression algorithms - Huffman coding, LZ77 - Spam detection - analyzing character distributions - Anagram detection - comparing character frequencies - Data deduplication - identifying similar strings - Natural language processing - n-gram analysis - Search engines - term frequency calculations

Input

data = str  # String to analyze

Output

dict[str, int]  # Character → frequency mapping

Constraints

• String length ≤ 100,000
• ASCII text (letters, digits, punctuation, spaces, etc.)
• Case-sensitive counting

Examples

Example 1: Basic Frequency Count

Input: "aba!"

Output: {'a': 2, 'b': 1, '!': 1}

Explanation: - 'a' appears 2 times - 'b' appears 1 time - '!' appears 1 time

Example 2: Empty String

Input: ""

Output: {}

Explanation: - No characters to count - Return empty dictionary

Example 3: Case Sensitivity

Input: "Aa"

Output: {'A': 1, 'a': 1}

Explanation: - 'A' and 'a' are different characters - Each counted separately

Example 4: Repeated Characters

Input: "aaaaaa"

Output: {'a': 6}

Explanation: - Only one unique character - Appears 6 times

Example 5: With Spaces and Punctuation

Input: "Hello, World!"

Output: {'H': 1, 'e': 1, 'l': 3, 'o': 2, ',': 1, ' ': 1, 'W': 1, 'r': 1, 'd': 1, '!': 1}

Explanation: - All characters counted including spaces and punctuation - 'l' appears 3 times, 'o' appears 2 times

What You'll Learn

• Building frequency tables with dictionaries
• Iterating over strings
• Using get() method for safe dictionary access
• Understanding hash maps for O(1) lookups
• Difference between mutable (dict) and immutable (string) types

Why This Matters

Character frequency counting is: - Fundamental pattern in programming - Building block for many algorithms - Interview favorite - tests hash map understanding - Practical skill used in real applications

Starter Code

def challenge_function(data):
    """
    Count frequency of each character in string.

    Args:
        data: str to analyze

    Returns:
        dict[str, int]: character → count mapping
    """
    # Your implementation here
    pass