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Smallest subarray with sum greater than a given value


An effective algorithmic technique for effectively resolving problems involving arrays and strings is the sliding window approach. Finding contiguous subarrays or substrings that meet specific requirements is a task that it is particularly well suited for.

The sliding window technique enables us to efficiently update the window as we traverse through the array in the context of locating the smallest subarray with a sum greater than a given value. We can efficiently find the desired subarray without the use of nested loops or exhaustive computations by modifying the window's size and position.

What is the Smallest Subarray Problem?

A target sum and an array of integers are provided to us in the classic programming puzzle known as The Smallest Subarray Problem. Finding the smallest contiguous subarray whose sum exceeds the specified target sum is the task at hand. Numerous situations, such as financial applications, data processing, and resource management, frequently involve this issue.


A brute force method is one way to tackle this issue. We can create every conceivable subarray and determine each one's sum separately. This allows us to identify the smallest subarray whose sum is greater than the desired value. However, this method is very inefficient because of its O(n3) time complexity, which makes it especially bad for large arrays.

Effective Sliding Window Method

The sliding window method can be used to optimise the solution. A popular algorithmic technique for effectively solving problems involving arrays or strings is the sliding window. In order to locate the desired subarray, the idea is to keep a window moving through the array while modifying its size and position as necessary.

Let's begin by using a for loop to implement the sliding window technique:


Using a For Loop in Python to Implement the Solution


Smallest subarray length: 3

Here's a brief explanation of the code:

  1. The function smallest_subarray_with_sum(arr, target_sum) takes two arguments: arr, which is the input array, and target_sum, the desired sum.
  2. The min_length variable is initialized to positive infinity to store the length of the smallest subarray found. The current_sum variable keeps track of the sum of elements in the current subarray.
  3. The start variable is initialized to 0, indicating the start index of the current subarray.
  4. A loop iterates through the array using the end pointer, which represents the ending index of the current subarray being considered.
  5. In each iteration, the value of the element at index end is added to the current_sum.
  6. A while loop runs as long as the current_sum is greater than the target_sum. Within this loop, the code updates the min_length by taking the minimum of the current min_length and the length of the current subarray (end - start + 1). It then subtracts the value of the element at index start from the current_sum and increments the start index to move the subarray's start to the right.
  7. The function continues iterating through the array and updating the min_length until the current_sum is no longer greater than the target_sum.
  8. The function returns min_length if it's not still positive infinity (indicating a valid subarray was found), otherwise it returns 0.
  9. An example usage is provided with the array arr = [1, 4, 45, 6, 0, 19] and target_sum = 51. The smallest_subarray_with_sum function is called with these values, and the resulting length of the smallest subarray whose elements sum up to at least the target sum is printed.

The While Loop

As an alternative, we can achieve the same objective by using a while loop:


Smallest subarray length: 1

The provided code defines a Python function called smallest_subarray_with_sum that finds the length of the smallest subarray whose elements sum up to at least a given target sum. It uses a sliding window approach with two pointers, start and end, to efficiently solve the problem.

The purpose of the code is to demonstrate how to find the length of the smallest subarray in an array that has a sum greater than or equal to a given target sum. This is achieved by using a sliding window technique with two pointers to efficiently adjust the subarray under consideration and track the current sum.

Efficiency and Complexity

When compared to brute-force methods, the sliding window technique offers significant efficiency gains. It enables us to solve issues in O(n), or linear time complexity, where n is the array's element count.

Additionally, because the sliding window technique uses a constant amount of extra space to maintain the window's pointers and other variables, its space complexity is typically O(1).

Handling Negative Values

The sliding window method still offers a useful solution when dealing with arrays that include negative values. By keeping track of the current sum and the beginning and ending positions of the subarray, the algorithm can adjust to handle negative numbers. The sum is updated as the window moves through the array.

Take into account the example below with negative values:


Take a look at an array of negative values:

[-1, 2, 4, -3, 5]

The desired amount is five. [2, 4] has a sum of 6 and is the smallest subarray whose sum is greater than 5. Consequently, this subarray has a length of 2.

By adjusting the window and taking into account subarrays with both positive and negative sums, the algorithm effectively handles the negative values.

Performance Analysis

Our solution performs significantly better thanks to the sliding window technique, which brings down the time complexity to O(n). It moves quickly through the array, avoiding pointless calculations, and quickly arrives at the desired outcome.


The sliding window method is incredibly flexible and has uses in a variety of situations, including:

  • identifying subarrays with sums that are higher or equal to a given value.
  • determining the substring with the most length and unique characters.
  • calculating rolling sums and moving averages from time series data.
  • real-time data processing resource utilisation optimisation.

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