在 numpy 中,[:,None] 的选择有什么作用? [英] In numpy, what does selection by [:,None] do?
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问题描述
我正在学习关于深度学习的 Udacity 课程,我遇到了以下代码:
I'm taking the Udacity course on deep learning and I came across the following code:
def reformat(dataset, labels):
dataset = dataset.reshape((-1, image_size * image_size)).astype(np.float32)
# Map 0 to [1.0, 0.0, 0.0 ...], 1 to [0.0, 1.0, 0.0 ...]
labels = (np.arange(num_labels) == labels[:,None]).astype(np.float32)
return dataset, labels
labels[:,None]
在这里实际上做了什么?
What does labels[:,None]
actually do here?
推荐答案
http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html
numpy.newaxis
numpy.newaxis
newaxis 对象可用于所有切片操作以创建长度为 1 的轴.:const: newaxis 是None"的别名,可以使用None"代替它,结果相同.
The newaxis object can be used in all slicing operations to create an axis of length one. :const: newaxis is an alias for ‘None’, and ‘None’ can be used in place of this with the same result.
http://docs.scipy.org/doc/numpy-1.10.1/reference/generated/numpy.expand_dims.html
使用您的部分代码进行演示
Demonstrating with part of your code
In [154]: labels=np.array([1,3,5])
In [155]: labels[:,None]
Out[155]:
array([[1],
[3],
[5]])
In [157]: np.arange(8)==labels[:,None]
Out[157]:
array([[False, True, False, False, False, False, False, False],
[False, False, False, True, False, False, False, False],
[False, False, False, False, False, True, False, False]], dtype=bool)
In [158]: (np.arange(8)==labels[:,None]).astype(int)
Out[158]:
array([[0, 1, 0, 0, 0, 0, 0, 0],
[0, 0, 0, 1, 0, 0, 0, 0],
[0, 0, 0, 0, 0, 1, 0, 0]])
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