Python 二维 numpy 数组中的阈值
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threshold in 2D numpy array
提问by physics_for_all
I have an array of shape 512x512 which contains numbers between 0 and 100 at ith and jth position. Now I want to select array[i,j] < 25 and zero at other places. I have tried with array = array[where(array<25)], which gives me a 1D array, but I want 2D. Please help me to solve this.
我有一个形状为 512x512 的数组，它在第 i 个和第 j 个位置包含 0 到 100 之间的数字。现在我想在其他地方选择 array[i,j] < 25 和零。我试过array = array[where(array<25)]，它给了我一个一维数组，但我想要二维。请帮我解决这个问题。
回答by kazemakase
One solution:
一种解决方案：
result = (array < 25) * array
The first part array < 25gives you an array of the same shape that is 1 (True) where values are less than 25 and 0 (False) otherwise. Element-wise multiplication with the original array retains the values that are smaller than 25 and sets the rest to 0. This does not change the original array
第一部分array < 25为您提供一个形状相同的数组，该数组为 1（真），其中值小于 25，否则为 0（假）。与原始数组的逐元素乘法保留小于 25 的值并将其余值设置为 0。这不会更改原始数组
Another possibility is to set all values that are >= 25 to zero in the original array:
另一种可能性是将原始数组中 >= 25 的所有值设置为零：
array[array >= 25] = 0
回答by hLk
I also wanted to add that you can take advantage of numpy views to achieve this:
我还想补充一点，您可以利用 numpy 视图来实现这一点：
>>> a = np.asarray([ [1,2], [3,4], [4,1], [6,2], [5,3], [0,4] ])
>>> b = a[:, 1] # lets say you only care about the second column
>>> b[b > 3] = 0
>>> print(a)
[[1 2]
[3 0]
[4 1]
[6 2]
[5 3]
[0 0]]
This is nice when you want the values to be something other than 0.
当您希望值不是 0 时，这很好。
