python - 如何计算 2D numpy 数组的所有列的总和(有效)

Question

假设我有以下由四行三列组成的二维 numpy 数组:

>>> a = numpy.arange(12).reshape(4,3)
>>> print(a)
[[ 0  1  2]
 [ 3  4  5]
 [ 6  7  8]
 [ 9 10 11]]

生成包含所有列之和的一维数组(如 [18,22,26])的有效方法是什么？这可以在不需要遍历所有列的情况下完成吗？

Answer 1

查看 numpy.sum 的文档，特别注意 axis 参数。总结列:

>>> import numpy as np
>>> a = np.arange(12).reshape(4,3)
>>> a.sum(axis=0)
array([18, 22, 26])

或者，对行求和:

>>> a.sum(axis=1)
array([ 3, 12, 21, 30])

其他聚合函数，如 numpy.mean , numpy.cumsum和 numpy.std ，例如，也带 axis 参数。

来自 Tentative Numpy Tutorial :

Many unary operations, such as computing the sum of all the elements in the array, are implemented as methods of the ndarray class. By default, these operations apply to the array as though it were a list of numbers, regardless of its shape. However, by specifying the axis parameter you can apply an operation along the specified axis of an array: