Numpy:排成一排

Question

我想弄清楚如何对数组的每两行求和.EG在此示例中将a转换为b:

a=array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [ 8,  9, 10, 11],
       [12, 13, 14, 15]])

b=array([[ 4,  6,  8, 10],
       [20, 22, 24, 26]])

当前代码看起来像这样:

b=[]
for num in range(len(a)/2):
    b.append(a[num*2]+a[num*2+1])

当然必须有一个更快的方式.感谢您的时间.

回答如下:

b=a[::2,:]+a[1::2,:]

这实际上帮助我扩展了如何跳过最初两行的次要问题.

>>> a=np.arange(24).reshape(6,-1)
>>> a
array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [ 8,  9, 10, 11],
       [12, 13, 14, 15],
       [16, 17, 18, 19],
       [20, 21, 22, 23]])
>>> b=np.vstack((a[:2],a[2::2,:]+a[3::2,:]))
>>> b
array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [20, 22, 24, 26],
       [36, 38, 40, 42]])

非常感谢你的帮助.

Answer 1

切片救援!我们只需要指定切片的步幅,我们都已设置好.

b = a[::2,:] + a[1::2,:]

演示:

>>> from numpy import *
>>> a=array([[ 0,  1,  2,  3],
...        [ 4,  5,  6,  7],
...        [ 8,  9, 10, 11],
...        [12, 13, 14, 15]])
>>>
>>> a[::2,:]
array([[ 0,  1,  2,  3],
       [ 8,  9, 10, 11]])
>>>
>>> a[1::2,:]
array([[ 4,  5,  6,  7],
       [12, 13, 14, 15]])
>>>
>>> a[::2,:] + a[1::2,:]
array([[ 4,  6,  8, 10],
       [20, 22, 24, 26]])