Subarray Sum Equals K Using Hash Map in DSA Python - 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 to a given target k using a hash map for efficient lookup.

📋 What You'll Learn

Create a list called sales with the exact values [3, 4, 7, 2, -3, 1, 4, 2]

Create a variable called k and set it to 7

Use a hash map (dictionary) called prefix_sums to store counts of prefix sums

Use a variable called count to keep track of the number of subarrays summing to k

Use a variable called current_sum to store the running sum while iterating through sales

Print the final count of subarrays that sum to k

💡 Why This Matters

🌍 Real World

Finding continuous periods where sales or other metrics meet exact targets helps businesses analyze trends and make decisions.

💼 Career

This technique is useful for software engineers and data analysts working on time series data, financial analysis, or any scenario requiring efficient subarray sum calculations.

1

Create the sales data list

Create a list called sales with these exact values: [3, 4, 7, 2, -3, 1, 4, 2]

DSA Python

# Create the sales list with exact values
# Your code here

Hint

Use square brackets to create a list and separate numbers with commas.

2

Set the target sum k

Create a variable called k and set it to the integer 7

DSA Python

sales = [3, 4, 7, 2, -3, 1, 4, 2]
# Set the target sum k to 7
# Your code here

Hint

Use = to assign the value 7 to the variable k.

3

Count subarrays summing to k using a hash map

Create a dictionary called prefix_sums with initial entry {0: 1}. Create variables count and current_sum set to 0. Use a for loop with variable num to iterate over sales. Inside the loop, add num to current_sum. Check if current_sum - k is in prefix_sums and if yes, add its value to count. Update prefix_sums[current_sum] by adding 1 if it exists or setting it to 1 if not.

DSA Python

sales = [3, 4, 7, 2, -3, 1, 4, 2]
k = 7
# Initialize prefix_sums, count, current_sum
# Use a for loop to iterate over sales and update count
# Your code here

Hint

Use a dictionary to store counts of sums seen so far. Update count when you find a matching prefix sum.

4

Print the count of subarrays summing to k

Print the variable count to display the number of continuous subarrays in sales that sum to k

DSA Python

sales = [3, 4, 7, 2, -3, 1, 4, 2]
k = 7
prefix_sums = {0: 1}
count = 0
current_sum = 0
for num in sales:
    current_sum += num
    if current_sum - k in prefix_sums:
        count += prefix_sums[current_sum - k]
    prefix_sums[current_sum] = prefix_sums.get(current_sum, 0) + 1
# Print the count variable
# Your code here

Hint

The output should be the total count of subarrays that sum to 7.