[python] 关于sum函数:sum(-1)、sum(1)、sum(0)以及keepdims=True的说明

sum(-1)和sum(1)

用途：求数组每一列的和，等价于 sum(axis=1) #关于axis的理解可以参考博主的另一篇文章【python】axis的理解

>>> import numpy as np

>>> x = np.array([[0, 1], [1, 1], [2, 2]])
>>> x.sum(1)
array([1, 2, 4])
>>> x.sum(-1)
array([1, 2, 4])
>>> x.sum(axis=1)
array([1, 2, 4])
>>> print(x,axis=1)

sum(0)

用途：求数组每一行的和，等价于sum(axis=0)

>>> x.sum(0)
array([3, 4])
>>> x.sum(axis=0)
array([3, 4])

 sum(x,axis=1,keepdims=True)

keepdims=True    用途：保持原数组的维度

x.sum(axis=1,keepdims=True)
array([[1],
[2],
[4]])