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

Frequency Counter Pattern Using Hash Map

📖 Scenario: You work at a bookstore. You want to count how many times each book title appears in a list of sold books to understand which books are popular.

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

📋 What You'll Learn

Create a dictionary to store book titles and their counts

Add a list of sold book titles

Use a loop to count how many times each book title appears

Print the frequency of each book title

💡 Why This Matters

🌍 Real World

Counting frequencies of items is useful in sales analysis, inventory management, and data summarization.

💼 Career

Understanding how to use hash maps (dictionaries) to count frequencies is a common task in software development, data analysis, and backend programming.

1

Create the list of sold book titles

Create a list of strings called soldBooks with these exact titles: "The Alchemist", "1984", "The Alchemist", "Brave New World", "1984"

DSA C#

// Create the list soldBooks with the given book titles
// Your code here

Hint

Use List<string> and initialize it with the exact book titles inside curly braces.

2

Create the dictionary to count frequencies

Create a dictionary called bookCounts that maps strings to integers to store the count of each book title.

DSA C#

using System.Collections.Generic;

List soldBooks = new List { "The Alchemist", "1984", "The Alchemist", "Brave New World", "1984" };

// Create the dictionary bookCounts to store frequencies
// Your code here

Hint

Use Dictionary<string, int> to create bookCounts.

3

Count the frequency of each book title

Use a foreach loop with variable book to iterate over soldBooks. Inside the loop, if book is already a key in bookCounts, increase its value by 1. Otherwise, add book to bookCounts with value 1.

DSA C#

using System.Collections.Generic;

List soldBooks = new List { "The Alchemist", "1984", "The Alchemist", "Brave New World", "1984" };

Dictionary bookCounts = new Dictionary();

// Use foreach loop to count frequencies
// Your code here

Hint

Use ContainsKey to check if the book is already counted.

4

Print the frequency of each book title

Use a foreach loop with variables title and count to iterate over bookCounts. Inside the loop, print the book title and its count in the format: "{title}: {count}".

DSA C#

using System.Collections.Generic;

List soldBooks = new List { "The Alchemist", "1984", "The Alchemist", "Brave New World", "1984" };

Dictionary bookCounts = new Dictionary();

foreach (string book in soldBooks)
{
    if (bookCounts.ContainsKey(book))
    {
        bookCounts[book] += 1;
    }
    else
    {
        bookCounts[book] = 1;
    }
}

// Print each book title and its count
// Your code here

Hint

Use foreach (KeyValuePair<string, int> entry in bookCounts) and Console.WriteLine with string interpolation.

Practice

1. What is the main purpose of using a frequency counter pattern with a hash map in algorithms?

easy

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

B. To sort elements in ascending order

C. To reverse the order of elements

D. To find the maximum value in a list

Solution

1. Step 1: Understand frequency counter pattern

   The frequency counter pattern uses a hash map to store counts of each element in a collection.

2. Step 2: Identify the purpose

   This pattern helps to quickly find how many times each element appears, which is useful for comparisons or validations.

3. Final Answer:

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

4. Quick Check:

   Frequency counter = count elements [OK]

Hint: Frequency counter counts occurrences using a hash map [OK]

Common Mistakes:

• Confusing frequency counting with sorting
• Thinking it reverses elements
• Assuming it finds max value

2. Which of the following C# code snippets correctly initializes a frequency counter using a Dictionary for integers?

easy

A. var freq = new HashSet<int>();

B. var freq = new List<int>();

C. var freq = new Dictionary();

D. var freq = new int[10];

Solution

1. Step 1: Identify correct data structure for frequency counting

   A Dictionary maps each integer to its count, perfect for frequency counting.

2. Step 2: Check other options

   List<int> stores values but not counts, HashSet<int> stores unique values without counts, int[] is fixed size and less flexible.

3. Final Answer:

   var freq = new Dictionary(); -> Option C

4. Quick Check:

   Dictionary for frequency map [OK]

Hint: Use Dictionary to map element to count [OK]

Common Mistakes:

• Using List instead of Dictionary for counts
• Using HashSet which stores only unique elements
• Using fixed-size array without mapping keys

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

var arr = new int[] {1, 2, 2, 3, 3, 3};
var freq = new Dictionary();
foreach (var num in arr) {
  if (freq.ContainsKey(num)) freq[num]++;
  else freq[num] = 1;
}
foreach (var kvp in freq) {
  Console.Write(kvp.Key + ":" + kvp.Value + " ");
}

medium

A. 3:3 2:2 1:1

B. 1:1 2:2 3:3

C. 1:1 2:1 3:1

D. 1:3 2:2 3:1

Solution

1. Step 1: Count frequencies in array

   1 appears once, 2 appears twice, 3 appears three times.

2. Step 2: Output order in Dictionary foreach

   Dictionary preserves insertion order in recent C# versions, so output is 1:1 2:2 3:3.

3. Final Answer:

   1:1 2:2 3:3 -> Option B

4. Quick Check:

   Count each element correctly [OK]

Hint: Count each element, print in insertion order [OK]

Common Mistakes:

• Mixing counts of elements
• Assuming Dictionary outputs sorted keys
• Forgetting to increment counts

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

string text = "hello";
var freq = new Dictionary();
foreach (char c in text) {
  freq[c] += 1;
}

medium

A. Trying to increment a key that may not exist causes a KeyNotFoundException

B. The dictionary is not initialized

C. The foreach loop syntax is incorrect

D. Characters cannot be used as dictionary keys

Solution

1. Step 1: Analyze dictionary usage

   freq[c] += 1 assumes key c exists, but keys are not added before incrementing.

2. Step 2: Identify exception cause

   Accessing freq[c] when c is not a key throws KeyNotFoundException.

3. Final Answer:

   Trying to increment a key that may not exist causes a KeyNotFoundException -> Option A

4. Quick Check:

   Increment without key check causes error [OK]

Hint: Check key exists before incrementing frequency [OK]

Common Mistakes:

• Assuming keys auto-initialize in Dictionary
• Ignoring exception on missing keys
• Misunderstanding foreach syntax

5. Given two integer arrays arr1 and arr2, which C# code snippet correctly uses the frequency counter pattern to check if arr2 contains the squares of all elements in arr1 with matching frequencies?

hard

A. var freq1 = new Dictionary();
var freq2 = new Dictionary();
foreach(var num in arr1) freq1[num * num] = freq1.GetValueOrDefault(num * num,0) + 1;
foreach(var num in arr2) freq2[num] = freq2.GetValueOrDefault(num,0) + 1;
return freq1.SequenceEqual(freq2);

B. var freq1 = new Dictionary();
foreach(var num in arr1) freq1[num * num] = freq1.GetValueOrDefault(num * num,0) + 1;
foreach(var num in arr2) { if(!freq1.ContainsKey(num)) return false; freq1[num]--; } return freq1.Values.All(v => v == 0);

C. var freq1 = new Dictionary();
var freq2 = new Dictionary();
foreach(var num in arr1) freq1[num] = freq1.GetValueOrDefault(num,0) + 1;
foreach(var num in arr2) freq2[num * num] = freq2.GetValueOrDefault(num * num,0) + 1;
return freq1.SequenceEqual(freq2);

D. var freq1 = new Dictionary();
var freq2 = new Dictionary();
foreach(var num in arr1) freq1[num] = freq1.GetValueOrDefault(num,0) + 1;
foreach(var num in arr2) freq2[num] = freq2.GetValueOrDefault(num,0) + 1;
foreach(var kvp in freq1) { if(!freq2.ContainsKey(kvp.Key * kvp.Key) || freq2[kvp.Key * kvp.Key] != kvp.Value) return false; } return true;

Solution

1. Step 1: Build frequency maps for arr1 and arr2

   freq1 counts elements in arr1, freq2 counts elements in arr2.

2. Step 2: Check if freq2 has squares of freq1 keys with matching counts

   For each key in freq1, check if freq2 contains key squared with same frequency.

3. Final Answer:

   Code snippet C correctly implements the frequency counter pattern for this problem -> Option D

4. Quick Check:

   Compare freq1 keys squared with freq2 frequencies [OK]

Hint: Map arr1 counts, verify squared keys in arr2 counts [OK]

Common Mistakes:

• Comparing frequency dictionaries without squaring keys
• Decrementing counts without checking all keys
• Using SequenceEqual without sorting or matching keys