# 5 Best Ways to Find Running Sum of 1D Array in Python

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π‘ Problem Formulation: We need to calculate the running sum of a one-dimensional array in Python, which means the sum of elements at every index should include the sum of all the previous elements. For instance, given an input array [1, 2, 3, 4], the desired output would be [1, 3, 6, 10]. Let’s explore the most efficient ways to achieve this in Python.

## Method 1: Iterative Approach

The iterative approach involves using a for-loop to iterate over the elements and summing them as we go, storing the result in a new array. This method is simple and straightforward but might not be the most efficient for very large arrays.

Here’s an example:

```def running_sum(nums):
result = []
current_sum = 0
for num in nums:
current_sum += num
result.append(current_sum)
return result

print(running_sum([1, 2, 3, 4]))
```

Output:

`[1, 3, 6, 10]`

This code defines a function `running_sum()` that takes a list of numbers as input and returns a new list containing the running sums. Each element in the output list is computed by adding the current number to the sum of all previous elements iteratively.

## Method 2: Using List Comprehension

Python’s list comprehension can be combined with the built-in `sum()` function to create a concise and elegant way to compute running sums. It’s more pythonic but less efficient than the iterative approach for large arrays since it recalculates the sum at each iteration.

Here’s an example:

```def running_sum(nums):
return [sum(nums[:i+1]) for i in range(len(nums))]

print(running_sum([1, 2, 3, 4]))
```

Output:

`[1, 3, 6, 10]`

The function `running_sum()` now leverages list comprehension to generate the running sums. Note that for each index `i`, the `sum()` is calculated from the start of the list up to the current element, which may lead to performance issues for long lists.

## Method 3: Using itertools.accumulate()

Python’s `itertools.accumulate()` function is a memory-efficient tool to calculate cumulative sums or running totals, and other aggregate functions. This method is suitable for larger datasets and is part of Python’s standard library.

Here’s an example:

```from itertools import accumulate

def running_sum(nums):
return list(accumulate(nums))

print(running_sum([1, 2, 3, 4]))
```

Output:

`[1, 3, 6, 10]`

In this code snippet, we’ve imported the `accumulate()` function from the `itertools` module and used it to return a list of running sums. This is the recommended way to handle running sum problems in Python as it’s both elegant and efficient.

## Method 4: Using NumPy Library

If we’re handling large numerical arrays, NumPy’s `numpy.cumsum()` could be the most efficient solution. It is highly optimized for numerical operations and is the go-to library for numeric computing in Python.

Here’s an example:

```import numpy as np

def running_sum(nums):
return np.cumsum(nums).tolist()

print(running_sum([1, 2, 3, 4]))
```

Output:

`[1, 3, 6, 10]`

This code utilizes the `numpy.cumsum()` function, which returns the cumulative sum of the elements along a given axis, effectively calculating the running sum. The result is then converted back to a Python list using the `tolist()` method.

## Bonus One-Liner Method 5: Using Python’s functools.reduce()

The `functools.reduce()` function can also be used to calculate the running sum by cumulatively applying an add operation. It is more of a functional programming approach and less intuitive for those unfamiliar with reduce functions.

Here’s an example:

```from functools import reduce

def running_sum(nums):
return list(reduce(lambda acc, x: acc + [acc[-1] + x] if acc else [x], nums, []))

print(running_sum([1, 2, 3, 4]))
```

Output:

`[1, 3, 6, 10]`

In this one-liner, `reduce()` is used with a lambda function to accumulate the sums. It starts with an empty list and appends the running total of the numbers. This is a more advanced method and may be less readable for novice Python programmers.

## Summary/Discussion

• Method 1: Iterative Approach. Easy to understand. Not the most efficient for large arrays.
• Method 2: Using List Comprehension. Pythonic and concise. Recalculates sum at each step which could be inefficient for large arrays.
• Method 3: Using itertools.accumulate(). Memory-efficient and part of the standard library. Highly recommended for handling running sum calculations.
• Method 4: Using NumPy Library. Highly optimized for numeric operations. Best for large numerical datasets but introduces a dependency on NumPy.
• Bonus Method 5: Using functools.reduce(). Harnesses functional programming concepts. Less readable but compact and powerful for one-liners.