Subarray Sum Equals K Using Hash Map in DSA C# - Build from Scratch

Subarray Sum Equals K Using Hash Map

📖 Scenario: You are analyzing daily sales data for a small store. You want to find how many periods (continuous days) had total sales exactly equal to a target number.

🎯 Goal: Build a program that counts the number of continuous subarrays in a list of daily sales that sum up to a given target k using a hash map for efficient lookup.

📋 What You'll Learn

Create an integer array called sales with the exact values: 1, 2, 3, 4, 2

Create an integer variable called k and set it to 5

Use a Dictionary<int, int> called prefixSums to store cumulative sums and their counts

Use a for loop with variable i to iterate over sales

Calculate cumulative sums and update prefixSums to count subarrays summing to k

Print the final count of subarrays that sum to k

💡 Why This Matters

🌍 Real World

Finding periods of exact sales targets helps businesses analyze performance and plan promotions.

💼 Career

This technique is useful in data analysis, software development, and coding interviews where efficient subarray sum problems are common.

1

Create the sales data array

Create an integer array called sales with these exact values: 1, 2, 3, 4, 2

DSA C#

# Your code here

Hint

Use int[] sales = {1, 2, 3, 4, 2}; to create the array.

2

Set the target sum k

Create an integer variable called k and set it to 5

DSA C#

int[] sales = {1, 2, 3, 4, 2};
# Your code here

Hint

Use int k = 5; to set the target sum.

3

Count subarrays summing to k using a hash map

Create a Dictionary<int, int> called prefixSums and initialize it with 0 key and value 1. Then create integer variables count and currentSum set to 0. Use a for loop with variable i to iterate over sales. Inside the loop, add sales[i] to currentSum. Check if prefixSums contains key currentSum - k and if yes, add its value to count. Then update prefixSums by adding or incrementing the count for currentSum.

DSA C#

int[] sales = {1, 2, 3, 4, 2};
int k = 5;
# Your code here

Hint

Use a dictionary to store counts of prefix sums. Update count when currentSum - k is found.

4

Print the count of subarrays summing to k

Print the value of the variable count using Console.WriteLine(count);

DSA C#

int[] sales = {1, 2, 3, 4, 2};
int k = 5;

var prefixSums = new Dictionary {{0, 1}};
int count = 0;
int currentSum = 0;

for (int i = 0; i < sales.Length; i++)
{
    currentSum += sales[i];
    if (prefixSums.ContainsKey(currentSum - k))
    {
        count += prefixSums[currentSum - k];
    }
    if (prefixSums.ContainsKey(currentSum))
    {
        prefixSums[currentSum]++;
    }
    else
    {
        prefixSums[currentSum] = 1;
    }
}
# Your code here

Hint

Use Console.WriteLine(count); to print the final count.

Practice

1. What is the main purpose of using a hash map in the Subarray Sum Equals K problem?

easy

A. To store cumulative sums and their frequencies for quick lookup

B. To sort the array elements before processing

C. To store the original array elements as keys

D. To reverse the array for easier traversal

Solution

1. Step 1: Understand the problem goal

   The problem wants to find subarrays whose sums equal a target value k.

2. Step 2: Role of hash map in solution

   The hash map stores cumulative sums seen so far and how many times they occurred, enabling quick checks for needed sums.

3. Final Answer:

   To store cumulative sums and their frequencies for quick lookup -> Option A

4. Quick Check:

   Hash map stores sums = A [OK]

Hint: Hash map tracks sums to find subarrays fast [OK]

Common Mistakes:

• Thinking hash map stores array elements directly
• Assuming sorting is needed
• Using hash map to reverse array

2. Which of the following is the correct way to declare a hash map (Dictionary) in C# to store cumulative sums and their counts?

easy

A. Dictionary sumCounts = new Dictionary();

B. HashMap sumCounts = new HashMap();

C. Map sumCounts = new Map();

D. Dictionary sumCounts = new Dictionary();

Solution

1. Step 1: Identify correct C# dictionary syntax

   C# uses Dictionary<TKey, TValue> with new Dictionary<TKey, TValue>() to declare hash maps.

2. Step 2: Check each option

   Dictionary sumCounts = new Dictionary(); uses correct generic syntax. Options B and C use Java-like syntax. Dictionary sumCounts = new Dictionary(); misses generic types.

3. Final Answer:

   Dictionary sumCounts = new Dictionary(); -> Option A

4. Quick Check:

   C# Dictionary syntax = A [OK]

Hint: Use Dictionary in C# for hash maps [OK]

Common Mistakes:

• Using Java HashMap syntax in C#
• Omitting generic type parameters
• Using non-existent Map type in C#

3. What is the output of the following C# code snippet?

int[] nums = {1, 2, 3};
int k = 3;
int count = 0;
Dictionary sumCounts = new Dictionary();
sumCounts[0] = 1;
int sum = 0;
foreach (int num in nums) {
    sum += num;
    if (sumCounts.ContainsKey(sum - k)) {
        count += sumCounts[sum - k];
    }
    if (sumCounts.ContainsKey(sum)) {
        sumCounts[sum]++;
    } else {
        sumCounts[sum] = 1;
    }
}
Console.WriteLine(count);

medium

A. 1

B. 0

C. 3

D. 2

Solution

1. Step 1: Calculate cumulative sums and check counts

   nums = [1,2,3], k=3. sumCounts starts with {0:1}. - sum=1: sum-k= -2 (not in sumCounts), count=0, sumCounts={0:1,1:1} - sum=3: sum-k=0 (in sumCounts=1), count=1, sumCounts={0:1,1:1,3:1} - sum=6: sum-k=3 (in sumCounts=1), count=2, sumCounts={0:1,1:1,3:1,6:1}

2. Step 2: Final count of subarrays with sum k

   Count is 2 because subarrays [1,2] and [3] sum to 3.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Subarrays count = 2 [OK]

Hint: Track cumulative sums and check sum-k in map [OK]

Common Mistakes:

• Not initializing sumCounts with {0:1}
• Forgetting to update count correctly
• Misunderstanding sum-k lookup

4. Identify the bug in this C# code snippet for counting subarrays summing to k:

int[] nums = {1, 1, 1};
int k = 2;
int count = 0;
Dictionary sumCounts = new Dictionary();
int sum = 0;
foreach (int num in nums) {
    sum += num;
    if (sumCounts.ContainsKey(sum - k)) {
        count += sumCounts[sum - k];
    }
    sumCounts[sum] = 1;
}
Console.WriteLine(count);

medium

A. nums array is empty

B. sumCounts is not initialized with sum 0 count 1

C. count is not incremented correctly

D. sum variable is not updated inside the loop

Solution

1. Step 1: Check initialization of sumCounts

   sumCounts should start with {0:1} to count subarrays starting at index 0, but here it is empty.

2. Step 2: Check updating sumCounts inside loop

   sumCounts[sum] = 1 overwrites counts instead of incrementing, but main bug is missing initial {0:1}.

3. Final Answer:

   sumCounts is not initialized with sum 0 count 1 -> Option B

4. Quick Check:

   Initialize sumCounts[0] = 1 [OK]

Hint: Always initialize sumCounts with {0:1} before loop [OK]

Common Mistakes:

• Not initializing sumCounts with {0:1}
• Overwriting counts instead of incrementing
• Ignoring sum-k lookup logic

5. Given the array nums = {3, 4, 7, 2, -3, 1, 4, 2} and k = 7, how many subarrays sum to k using the hash map approach?

hard

A. 5

B. 3

C. 4

D. 6

Solution

1. Step 1: Calculate cumulative sums and track counts

   Initialize sumCounts = {0:1}, sum=0, count=0. - i=0: sum=3, sum-k=-4(not found), count=0, sumCounts={0:1,3:1} - i=1: sum=7, sum-k=0(found=1), count=1, sumCounts={0:1,3:1,7:1} - i=2: sum=14, sum-k=7(found=1), count=2, sumCounts={0:1,3:1,7:1,14:1} - i=3: sum=16, sum-k=9(not found), count=2, sumCounts={0:1,3:1,7:1,14:1,16:1} - i=4: sum=13, sum-k=6(not found), count=2, sumCounts={0:1,3:1,7:1,14:1,16:1,13:1} - i=5: sum=14, sum-k=7(found=1), count=3, sumCounts[14]=2 - i=6: sum=18, sum-k=11(not found), count=3, sumCounts={0:1,3:1,7:1,14:2,16:1,13:1,18:1} - i=7: sum=20, sum-k=13(found=1), count=4, sumCounts={0:1,3:1,7:1,14:2,16:1,13:1,18:1,20:1}

2. Step 2: Verify the counted subarrays

   Subarrays summing to 7 are: - [3,4] - [7] - [7,2,-3,1] - [1,4,2] Count is 4 total.

3. Final Answer:

   4 -> Option C

4. Quick Check:

   Count subarrays with sum k = 4 [OK]

Hint: Track sums and counts carefully to find all subarrays [OK]

Common Mistakes:

• Missing subarrays with negative numbers
• Not updating counts when sum repeats
• Forgetting to initialize sumCounts with {0:1}