numpy.sum(a, axis=None, dtype=None, out=None, keepdims=, initial=)[source])用於計算array元素的和.

python中常用的numpy進行數學計算，其中array的求和運算分為兩種，一種是呼叫numpy.array自身的sum()方法,另一種是利用numpy的內建函式numpy.sum()使用。(tips:python從0開始計數axis=0對應第一個維度)

#example
import numpy as np
x = np.random.rand(2,3,4)    #生成三維隨array
 

print("This is a 2*3rows 4cols array\n",x)
'''
>>> This is a 2*3rows 4cols array
[[[ 0.88264952  0.12446208  0.82166137  0.31747846]
  [ 0.51436626  0.03051283  0.46987831  0.64086531]
  [ 0.14819094  0.6395191   0.21753309  0.61340538]]

 [[ 0.0878878   0.5317064   0.65523138  0.704961  ]
  [ 0.51081521  0.88710145  0.92958269  0.89587262]
  [ 0.60233393  0.26146419  0.26584161  0.0823285 ]]]
'''
 

print('instance method:',x.sum())
print('numpy function:',np.sum(x))
'''
>>> 'instance method:', 11.835649415181807
>>> 'numpy function:', 11.835649415181807
'''

#對特定的軸求和
print('First axis sum',x.sum(axis=0))
'''
>>> 'First axis sum',   #第一維是軸0，對前後兩頁array加和得到3*4的array輸出
array([[ 0.97053732,  0.65616848,  1.47689275,  1.02243946],
       [ 1.02518147,  0.91761428,  1.399461  ,  1.53673793],
       [ 0.75052486,  0.90098329,  0.4833747 ,  0.69573388]])
'''
 

print('Second axis sum',x.sum(axis=1))
'''
>>> 'Second axis sum',   #第二維是軸1，將每列的三行進行sum（對行求sum），得到2*4的輸出
array([[ 1.54520672,  0.79449401,  1.50907277,  1.57174914],
       [ 1.20103694,  1.68027204,  1.85065568,  1.68316212]])
'''
print('Third axis sum',x.sum(axis=2))
'''
>>> 'Third axis sum',    #第三維是軸2，有4個數，將每行四個數加和（對列求sum），得到2*3輸出
array([[ 2.14625142,  1.65562271,  1.6186485 ],
       [ 1.97978658,  3.22337197,  1.21196823]])
'''


#對多個軸求和
print('axis1,2 sum:',x.sum(axis=(1,2)))
'''
>>> 'axis1,2 sum:      #對第二維(axis=1)和第三維(axis=2)求和，歸在第一維(axis=0)上
array([ 5.42052263,  6.41512678]) 
'''

pic from pexels

ref:
https://docs.scipy.org/doc/numpy/reference/generated/numpy.ndarray.sum.html
https://docs.scipy.org/doc/numpy/reference/generated/numpy.sum.html
https://blog.csdn.net/Leekingsen/article/details/76242244
https://blog.csdn.net/addmana/article/details/78472608