在花哨的索引时使用范围? [英] Using range while fancy indexing?
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问题描述
有人可以简单地向我解释一下这个表达吗?你怎么能取两个值的范围并将它们设置为零?
Can someone explain this expression to me in simple terms? How could you take a range of two values and set them to zero?
lena_image[range(xmax), range(ymax)] = 0
推荐答案
range(n)
生成范围从 0
到 n
的列表> 独家:
range(n)
generates a list ranging from 0
inclusive to n
exclusive:
In [847]: range(3)
Out[847]: [0, 1, 2]
和 numpy 花式索引获取索引 X
s 和 Y
s 处的元素,并为它们分配一个新值:
and numpy fancy indexing takes the elements at indices X
s and Y
s , and assign a new value to them:
In [856]: d
Out[856]:
array([[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.],
[ 0., 0., 0., 0., 0.]])
In [857]: d[range(5), range(5)]=2 #assigns elems on diagonal d[0,0], d[1,1], d[2,2], etc.
...: print d
[[ 2. 0. 0. 0. 0.]
[ 0. 2. 0. 0. 0.]
[ 0. 0. 2. 0. 0.]
[ 0. 0. 0. 2. 0.]
[ 0. 0. 0. 0. 2.]]
注意在这种情况下必须有相同数量的X
和Y
Note that there must be the same numbers of X
s and Y
s in this case
如果你想修改一个block
(这次不是对角线),使用broadcasting:
If you want to modify a block
(not diagonal this time), use broadcasting:
In [878]: arange(3)[:, None] #get a 2D array of shape (3, 1)
Out[878]:
array([[0],
[1],
[2]])
In [874]: d[arange(3)[:, None], arange(2)] #get the sub array of row 0~2, col 0~1
Out[874]:
array([[ 2., 0.],
[ 0., 2.],
[ 0., 0.]])
In [875]: d[arange(3)[:, None], arange(2)]=100 #assign new values element-wise
In [876]: d
Out[876]:
array([[ 100., 100., 0., 0., 0.],
[ 100., 100., 0., 0., 0.],
[ 100., 100., 2., 0., 0.],
[ 0., 0., 0., 2., 0.],
[ 0., 0., 0., 0., 2.]])
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