numpy indices的用法

Suppose you have a matrix M whose (i,j)-th element equals



M_ij = 2*i + 3*j

One way to define this matrix would be



i, j = np.indices((2,3))

M = 2*i + 3*j

which yields



array([[0, 3, 6],

       [2, 5, 8]])

In other words, np.indices returns arrays which can be used as indices. The elements in i indicate the row index:



In [12]: i

Out[12]: 

array([[0, 0, 0],

       [1, 1, 1]])

The elements in j indicate the column index:



In [13]: j

Out[13]: 

array([[0, 1, 2],

       [0, 1, 2]])

 
上面是Stack Overflow的解释。 翻译一下：

np.indices((2,3))
 
返回的是一个行列的索引，然后可以用这个索引快速的创建二维数据。
 
比如我要画一个圆：
img = np.zeros((400,400))
ir,ic = np.indices(img.shape)
circle = (ir-135)**2+(ic-150)**2 < 30**2 # 半径30，圆心在135,150
img[circle]=1
 
img现在就是一个圆啦