# 5 Best Ways to Return the Cumulative Sum of Array Elements, Treating NaNs as Zero and Changing the Result Type in Python

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π‘ Problem Formulation: We need to compute the cumulative sum of a numeric array in Python where any occurrence of a not-a-number (NaN) is treated as zero. Moreover, after summing, the type of the cumulative sum array must be changed. For instance, given an input array like `[1, NaN, 3, 4]`, the desired output with type conversion to integer would be `[1, 1, 4, 8]`.

## Method 1: Using NumPy with Custom Type Conversion

This method employs the NumPy library’s `numpy.nancumsum()` function to compute the cumulative sum of the array, treating NaNs as zero. Afterward, the resulting array is cast to the desired type using `astype()` method. NumPy provides a high-performance implementation which is optimal for large data.

Here’s an example:

```import numpy as np

array = np.array([1, np.nan, 3, 4], dtype=float)
cumulative_sum = np.nancumsum(array).astype(int)
print(cumulative_sum)```

Output:

`[1 1 4 8]`

In this example, `np.nancumsum()` computes the cumulative sum of the given array while treating NaNs as zero, and `astype(int)` converts the resulting array into an integer array. This combination is efficient and ensures that the results are presented in the desired format.

## Method 2: Using pandas with In-place Type Conversion

Pandas library is a powerful tool for data manipulation, and it provides `pd.Series.cumsum()` for cumulative sum with automatic handling of NaN values as zero. The series object resulting from this operation is then converted in-place to the required type using the `astype()` method. This method is perfect when working with pandas data structures.

Here’s an example:

```import pandas as pd

series = pd.Series([1, np.nan, 3, 4])
cumulative_sum = series.cumsum().fillna(0).astype(int)
print(cumulative_sum)```

Output:

```0    1
1    1
2    4
3    8
dtype: int32```

The `pd.Series.cumsum()` returns a cumulative sum treating NaNs as zero, as Series automatically handles NaN as zero during cumulative operations. The `fillna(0)` method ensures all NaNs are zero in the result, followed by conversion to integer type with `astype()`.

## Method 3: Iterative Approach with a For-Loop

For scenarios where external libraries are not an option, a simple iterative approach using a for-loop can be employed. This method iterates through the input list, treating NaNs as zero, and builds a new list with the cumulative sum, manually converting each element to the desired type.

Here’s an example:

```input_array = [1, float('nan'), 3, 4]
cumulative_sum = []
current_sum = 0

for num in input_array:
current_sum += 0 if num != num else num  # NaN check
cumulative_sum.append(int(current_sum))

print(cumulative_sum)```

Output:

`[1, 1, 4, 8]`

This code initializes a current sum to zero and iterates over each element in the input array. It checks if the element is NaN by verifying if `num != num` (as NaN is not equal to itself) and adds zero in that case. Otherwise, it adds the actual number to the current sum. Each computed value is cast to an integer and appended to the new list `cumulative_sum`.

## Method 4: Using Comprehension and accumulate from itertools

Combining Python’s list comprehension and the `accumulate()` function from the itertools module provides a succinct way of calculating the cumulative sum of an array with NaNs treated as zero. This method is memory-efficient as it generates the cumulative sum on the fly without creating intermediate lists.

Here’s an example:

```from itertools import accumulate

input_array = [1, float('nan'), 3, 4]
cumulative_sum = list(accumulate(0 if i != i else int(i) for i in input_array))
print(cumulative_sum)```

Output:

`[1, 1, 4, 8]`

This example utilizes a generator expression within `accumulate()` to handle NaN values and type casting. The comprehension checks for NaN and replaces it with zero; meanwhile, it also casts the element to an integer. The `accumulate()` function then takes care of the cumulative sum process.

## Bonus One-Liner Method 5: Using NumPy with Inline Type Conversion

This one-liner approach uses NumPy to perform the cumulative sum and type conversion in a single expression by leveraging the `dtype` argument of `nancumsum`.

Here’s an example:

```import numpy as np

cumulative_sum = np.nancumsum([1, np.nan, 3, 4], dtype=int)
print(cumulative_sum)```

Output:

`[1 1 4 8]`

This succinct code leverages the `dtype` argument to specify the desired output type directly in the `nancumsum()` function call. This trick minimizes the number of operations and maintains NumPy’s performance advantages.

## Summary/Discussion

Here is a brief summary of the methods we have discussed:

• Method 1: NumPy with Custom Type Conversion. High performance on large arrays. Requires NumPy installation.
• Method 2: Pandas In-place Type Conversion. Fluent integration with pandas workflows. Overkill for simple tasks.
• Method 3: Iterative Approach with For-Loop. No dependencies required. Potentially slower and more verbose.
• Method 4: List Comprehension with accumulate. Elegant and efficient. Requires understanding of advanced Python features.
• Bonus Method 5: NumPy One-Liner. Fast and concise. Dependant on NumPy and less readable to newcomers.