Frequency Counter Pattern Using Hash Map in DSA C - Build from Scratch

Frequency Counter Pattern Using Hash Map

📖 Scenario: You are working in a small grocery store. You want to count how many times each fruit appears in a list of fruits sold today.

🎯 Goal: Build a program that counts the frequency of each fruit sold using a hash map (dictionary) and prints the counts.

📋 What You'll Learn

Create an array of fruits with exact values

Create a hash map (dictionary) to store fruit counts

Use a loop to count the frequency of each fruit

Print the frequency of each fruit in the format: fruit: count

💡 Why This Matters

🌍 Real World

Counting frequencies is useful in stores to track sales, in text analysis to count words, and in many other data processing tasks.

💼 Career

Understanding frequency counting with hash maps is a key skill for software developers, data analysts, and anyone working with data structures.

1

Create the list of fruits sold

Create a char* array called fruits with these exact values: "apple", "banana", "apple", "orange", "banana", "apple".

DSA C

# Create the array fruits with the given fruit names
// Your code here

Hint

Use an array of char* strings and count the number of fruits in a separate variable.

2

Set up the hash map for counting

Create a struct called FruitCount with two fields: char* fruit and int count. Then create an array called counts of size 3 to store FruitCount elements. Initialize all count fields to 0.

DSA C

#include 

// Define struct FruitCount and create counts array
// Your code here

Hint

Use a struct to hold fruit name and count. Initialize counts for each fruit to zero.

3

Count the frequency of each fruit

Write a for loop with variable i from 0 to fruits_count - 1. Inside it, write another for loop with variable j from 0 to 2 to compare fruits[i] with counts[j].fruit using strcmp. If they match, increase counts[j].count by 1.

DSA C

// Count the frequency of each fruit
// Your code here

Hint

Use nested loops and strcmp to compare strings and update counts.

4

Print the frequency of each fruit

Write a for loop with variable k from 0 to 2. Inside the loop, print the fruit and its count in the format: fruit: count using printf.

DSA C

// Print the frequency of each fruit
// Your code here

Hint

Use a loop and printf to display each fruit and its count on a new line.

Practice

1. What is the main purpose of using a hash map in the Frequency Counter Pattern?

easy

A. To reverse the order of elements in the collection

B. To sort the elements in ascending order

C. To remove duplicate elements from the collection

D. To count how many times each element appears in a collection

Solution

1. Step 1: Understand the role of a hash map

   A hash map stores key-value pairs, allowing quick lookup and update of counts.

2. Step 2: Apply to frequency counting

   Using a hash map, we can track how many times each element appears by incrementing counts.

3. Final Answer:

   To count how many times each element appears in a collection -> Option D

4. Quick Check:

   Frequency counting = Counting elements [OK]

Hint: Hash map keys track elements, values track counts [OK]

Common Mistakes:

• Thinking hash map sorts elements
• Confusing frequency counting with removing duplicates
• Using arrays instead of hash maps for counting

2. Which of the following is the correct syntax to declare a hash map in C for counting frequencies of characters?

easy

A. map freq;

B. int freq[256] = {0};

C. unordered_map freq;

D. HashMap freq = new HashMap();

Solution

1. Step 1: Recognize C language limitations

   C does not have built-in hash map types like C++ STL's map or unordered_map.

2. Step 2: Use array for frequency counting in C

   We use an integer array of size 256 (for ASCII chars) initialized to zero to count frequencies.

3. Final Answer:

   int freq[256] = {0}; -> Option B

4. Quick Check:

   C uses arrays for frequency counting [OK]

Hint: In C, use arrays for frequency counts, not map types [OK]

Common Mistakes:

• Using C++ STL syntax in C code
• Trying to declare map or unordered_map in C
• Not initializing frequency array to zero

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

char str[] = "aabbcc";
int freq[256] = {0};
for (int i = 0; str[i] != '\0'; i++) {
    freq[(int)str[i]]++;
}
printf("%d", freq['b']);

medium

A. 0

B. 1

C. 2

D. 3

Solution

1. Step 1: Count occurrences of 'b' in the string

   The string "aabbcc" has two 'b' characters at positions 2 and 3.

2. Step 2: Check frequency array value for 'b'

   freq['b'] is incremented twice, so freq['b'] = 2.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Count of 'b' = 2 [OK]

Hint: Count characters manually, then check freq array [OK]

Common Mistakes:

• Confusing character ASCII codes
• Forgetting null terminator in loop condition
• Printing freq for wrong character

4. Identify the error in the following C code that tries to count character frequencies:

char str[] = "hello";
int freq[256];
for (int i = 0; str[i] != '\0'; i++) {
    freq[(int)str[i]]++;
}
printf("%d", freq['l']);

medium

A. freq array is not initialized to zero

B. Loop condition should be i < strlen(str)

C. Casting str[i] to int is incorrect

D. printf format specifier should be %c

Solution

1. Step 1: Check initialization of freq array

   The freq array is declared but not initialized, so it contains garbage values.

2. Step 2: Effect on frequency counting

   Incrementing uninitialized values leads to incorrect counts; freq should be zeroed before use.

3. Final Answer:

   freq array is not initialized to zero -> Option A

4. Quick Check:

   Initialize freq array to zero before counting [OK]

Hint: Always zero frequency array before counting [OK]

Common Mistakes:

• Assuming uninitialized arrays start at zero
• Using wrong printf format for integers
• Misunderstanding loop conditions with null terminator

5. Given two strings s1 = "listen" and s2 = "silent", which approach using frequency counters correctly checks if they are anagrams in C?

hard

A. Count frequencies of characters in both strings using arrays, then compare arrays element-wise

B. Sort both strings and compare them character by character

C. Check if lengths are equal, then check if first characters match

D. Use a hash map to store characters of s1 only and check if s2 contains them

Solution

1. Step 1: Use frequency arrays for both strings

   Create two arrays freq1 and freq2 of size 256, initialize to zero, count characters in s1 and s2 respectively.

2. Step 2: Compare frequency arrays element-wise

   If all counts match for every character, s1 and s2 are anagrams; otherwise, they are not.

3. Final Answer:

   Count frequencies of characters in both strings using arrays, then compare arrays element-wise -> Option A

4. Quick Check:

   Frequency arrays equal means anagrams [OK]

Hint: Compare frequency arrays to confirm anagrams [OK]

Common Mistakes:

• Only counting one string's characters
• Comparing only string lengths or first characters
• Assuming sorting is the only way to check anagrams