numpy cumsum()函数失效？求正确累加求和实现方法

Hey there! Let's figure out why you're seeing this discrepancy and how to fix it.

First, let's break down the core issue: You're expecting a cumulative sum result of [4, 10, 22], but instead getting back what looks like your original array [4, 6, 12]. Here's what might be going on, plus alternative methods to get the result you want.

Why isn't cumsum() working as expected? #

Chances are, either you're not using cumsum() correctly, or your input array isn't what you think it is:

• If your input array is [4, 6, 12], a proper cumsum() should return [4, 4+6=10, 10+12=22]. If you're seeing the original array, you might be printing the input instead of the cumsum() result.
• If your input array is actually [4, 2, 6], then cumsum() would correctly return [4, 6, 12]—double-check your source data!

Alternative Methods to Get Your Desired Cumulative Sum #

If for some reason cumsum() isn't working for you (or you're in an environment without it), here are reliable ways to implement the cumulative sum you need:

1. Manual Loop (Works in Any Language) #

This is the most straightforward approach—track a running total and build your result list step by step:

# Example in Python
input_arr = [4, 6, 12]
cumulative_sum = []
running_total = 0

for num in input_arr:
    running_total += num
    cumulative_sum.append(running_total)

print(cumulative_sum)  # Output: [4, 10, 22]

2. Use Built-in Tools (Python-Specific) #

If you're using Python, the itertools module has a ready-made accumulate() function that does exactly this:

from itertools import accumulate

input_arr = [4, 6, 12]
cumulative_sum = list(accumulate(input_arr))

print(cumulative_sum)  # Output: [4, 10, 22]

3. Fix Your cumsum() Usage #

If you want to stick with cumsum(), make sure you're assigning its result to a variable and using it correctly. For example, with NumPy:

import numpy as np

input_arr = np.array([4, 6, 12])
cumulative_sum = input_arr.cumsum()

print(cumulative_sum)  # Output: [ 4 10 22 ]

Double-check that you're not accidentally printing the original input array instead of the cumulative sum result—this is a super common mistake!

内容的提问来源于stack exchange，提问作者Nick X Tsui