python - 将前 n 行作为列添加到 NumPy 数组

Question

我想将前面的 n 行作为列添加到 NumPy 数组。

例如，如果n=2，下面的数组...

[[ 1, 2]
 [ 3, 4]
 [ 5, 6]
 [ 7, 8]
 [ 9, 10]
 [11, 12]]

...应该变成下面的:

[[ 1, 2, 0, 0, 0, 0]
 [ 3, 4, 1, 2, 0, 0]
 [ 5, 6, 3, 4, 1, 2]
 [ 7, 8, 5, 6, 3, 4]
 [ 9, 10, 7, 8, 5, 6]
 [11, 12, 9, 10, 7, 8]]

关于如何在 for 循环中遍历整个数组的情况下如何做到这一点，我有什么想法吗？

Answer 1

这是一个矢量化的方法-

def vectorized_app(a,n):   
    M,N = a.shape
    idx = np.arange(a.shape[0])[:,None] - np.arange(n+1)
    out = a[idx.ravel(),:].reshape(-1,N*(n+1))
    out[N*(np.arange(1,M+1))[:,None] <= np.arange(N*(n+1))] = 0
    return out

sample 运行-

In [255]: a
Out[255]: 
array([[ 1,  2,  3],
       [ 4,  5,  6],
       [ 7,  8,  9],
       [10, 11, 12],
       [13, 14, 15],
       [16, 17, 18]])

In [256]: vectorized_app(a,3)
Out[256]: 
array([[ 1,  2,  3,  0,  0,  0,  0,  0,  0,  0,  0,  0],
       [ 4,  5,  6,  1,  2,  3,  0,  0,  0,  0,  0,  0],
       [ 7,  8,  9,  4,  5,  6,  1,  2,  3,  0,  0,  0],
       [10, 11, 12,  7,  8,  9,  4,  5,  6,  1,  2,  3],
       [13, 14, 15, 10, 11, 12,  7,  8,  9,  4,  5,  6],
       [16, 17, 18, 13, 14, 15, 10, 11, 12,  7,  8,  9]])

运行时测试-

我在计时@Psidom's loop-comprehension based method以及这篇文章中列出的矢量化方法，是问题中发布的样本的 100x 放大版本(就大小而言):

In [246]: a = np.random.randint(0,9,(600,200))

In [247]: n = 200

In [248]: %timeit np.column_stack(mypad(a, i) for i in range(n + 1))
1 loops, best of 3: 748 ms per loop

In [249]: %timeit vectorized_app(a,n)
1 loops, best of 3: 224 ms per loop