Subarray Sum Equals K Using Hash Map in DSA Javascript - 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 continuous days had sales that add up exactly 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 value using a hash map.

📋 What You'll Learn

Create an array called sales with the exact values [1, 2, 3, 4, 2]

Create a variable called target and set it to 5

Use a for loop with index i to iterate over sales

Use a hash map (JavaScript object) called prefixSums to store counts of prefix sums

Calculate the number of continuous subarrays that sum to target

Print the final count

💡 Why This Matters

🌍 Real World

Finding continuous periods where sales meet a target helps businesses analyze performance and plan promotions.

💼 Career

This technique is useful in data analysis, software development, and coding interviews to solve subarray sum problems efficiently.

1

Create the sales data array

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

DSA Javascript

# Create the sales array with the given values
// Your code here

Hint

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

2

Set the target sum value

Create a variable called target and set it to 5

DSA Javascript

const sales = [1, 2, 3, 4, 2];
// Create the target variable and set it to 5
// Your code here

Hint

Use const target = 5; to set the target sum.

3

Calculate the count of subarrays summing to target

Create a variable called count and set it to 0. Create a variable called currentSum and set it to 0. Create an object called prefixSums and initialize it with {0: 1}. Use a for loop with variable i to iterate over sales. Inside the loop, add sales[i] to currentSum. Check if currentSum - target exists as a key in prefixSums. If yes, add its value to count. Then, add or update currentSum in prefixSums by increasing its count by 1.

DSA Javascript

const sales = [1, 2, 3, 4, 2];
const target = 5;
// Calculate the count of subarrays that sum to target
// Your code here

Hint

Use a hash map to store counts of prefix sums and update count when currentSum - target is found.

4

Print the result

Print the value of count using console.log(count)

DSA Javascript

const sales = [1, 2, 3, 4, 2];
const target = 5;

let count = 0;
let currentSum = 0;
const prefixSums = {0: 1};

for (let i = 0; i < sales.length; i++) {
    currentSum += sales[i];
    if (prefixSums.hasOwnProperty(currentSum - target)) {
        count += prefixSums[currentSum - target];
    }
    prefixSums[currentSum] = (prefixSums[currentSum] || 0) + 1;
}
// Print the count
// Your code here

Hint

Use console.log(count); to display 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 reverse the array for easier traversal

B. To sort the array elements before processing

C. To store the original array elements as keys

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

Solution

1. Step 1: Understand the problem goal

   The problem requires finding subarrays whose sums equal a target value k.

2. Step 2: Role of hash map in solution

   A hash map stores cumulative sums and their counts to quickly check if a previous sum exists that forms a subarray summing to k.

3. Final Answer:

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

4. Quick Check:

   Hash map stores sums = D [OK]

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

Common Mistakes:

• Thinking hash map sorts elements
• Using hash map to store array elements directly
• Ignoring cumulative sums concept

2. Which of the following is the correct syntax to initialize a JavaScript Map to store cumulative sums in the Subarray Sum Equals K problem?

easy

A. const map = new Map();

B. const map = Map();

C. const map = {};

D. const map = new map();

Solution

1. Step 1: Identify correct Map initialization syntax

   In JavaScript, a Map is created using new Map() constructor.

2. Step 2: Check options for correct syntax

   const map = {}; uses object literal {} which is not a Map. const map = Map();: Map() misses new. const map = new Map();: new Map() is correct. const map = new map();: new map() uses invalid lowercase constructor.

3. Final Answer:

   const map = new Map(); -> Option A

4. Quick Check:

   Map initialization = new Map() [OK]

Hint: Use 'new Map()' to create a Map in JavaScript [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using object literal instead of Map
• Incorrect capitalization of Map

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

function subarraySum(nums, k) {
  let count = 0, sum = 0;
  const map = new Map();
  map.set(0, 1);
  for (let num of nums) {
    sum += num;
    if (map.has(sum - k)) {
      count += map.get(sum - k);
    }
    map.set(sum, (map.get(sum) || 0) + 1);
  }
  return count;
}

console.log(subarraySum([1, 2, 3], 3));

medium

A. 3

B. 1

C. 2

D. 0

Solution

1. Step 1: Trace cumulative sums and map updates

   nums = [1,2,3], k=3
   sum=0 initially, map={0:1}
   Iteration 1: sum=1, sum-k= -2 (not in map), map={0:1,1:1}
   Iteration 2: sum=3, sum-k=0 (in map with count 1), count=1, map={0:1,1:1,3:1}
   Iteration 3: sum=6, sum-k=3 (in map with count 1), count=2, map={0:1,1:1,3:1,6:1}

2. Step 2: Calculate final count

   Count of subarrays summing to 3 is 2: [1,2] and [3].

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Subarrays with sum 3 = 2 [OK]

Hint: Track sums and counts in map to find subarrays fast [OK]

Common Mistakes:

• Not initializing map with (0,1)
• Forgetting to add counts from map
• Incorrect sum-k calculation

4. Identify the bug in the following code for Subarray Sum Equals K and choose the correct fix.

function subarraySum(nums, k) {
  let count = 0, sum = 0;
  const map = new Map();
  for (let num of nums) {
    sum += num;
    if (map.has(sum - k)) {
      count += map.get(sum - k);
    }
    map.set(sum, (map.get(sum) || 0) + 1);
  }
  return count;
}

medium

A. Change map.set to map.add inside loop

B. Initialize map with map.set(0, 1) before loop

C. Remove the if condition checking map.has(sum - k)

D. Initialize count to 1 instead of 0

Solution

1. Step 1: Identify missing initialization

   The map must start with sum 0 having count 1 to handle subarrays starting at index 0.

2. Step 2: Fix by adding map.set(0, 1)

   Adding map.set(0, 1) before the loop ensures correct counting of subarrays.

3. Final Answer:

   Initialize map with map.set(0, 1) before loop -> Option B

4. Quick Check:

   Start map with (0,1) fixes bug [OK]

Hint: Always initialize map with (0,1) before loop [OK]

Common Mistakes:

• Not initializing map with (0,1)
• Using map.add instead of map.set
• Removing essential if condition

5. Given the array nums = [3, 4, 7, 2, -3, 1, 4, 2] and k = 7, how many subarrays sum to k? Use the hash map approach to find the answer.

hard

A. 4

B. 5

C. 3

D. 6

Solution

1. Step 1: Calculate cumulative sums and track counts

   Initialize map with {0:1}, sum=0, count=0
   3: sum=3, sum-k=-4 (not), map={0:1,3:1}
   4: sum=7, 0 (1), count=1, map+={7:1}
   7: sum=14, 7 (1), count=2, map+={14:1}
   2: sum=16, 9 (not), map+={16:1}
   -3: sum=13, 6 (not), map+={13:1}
   1: sum=14, 7 (1), count=3, map 14:2
   4: sum=18, 11 (not), map+={18:1}
   2: sum=20, 13 (1), count=4, map+={20:1}

2. Step 2: Identify all subarrays summing to k

   Subarrays: [3,4], [7], [7,2,-3,1], [1,4,2]. Total count=4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Count subarrays with sum 7 = 4 [OK]

Hint: Count repeated sums in map for total subarrays [OK]

Common Mistakes:

• Ignoring repeated cumulative sums
• Not initializing map with (0,1)
• Miscounting subarrays overlapping