Python 根据两列的值选择numpy ndarray中的行
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selecting rows in numpy ndarray based on the value of two columns
提问by user3584444
I have a big np.ndarray (3600000,3), the HUE, the VALUE, and an associated CLASSnumber. For each pairs of HUEand VALUEI would like to find, using this array the corresponding Classnumber. I'm a very beginner in Python and have a hard time doing it. Do you know a way to do it?
我有一个 big np.ndarray (3600000,3)、 the HUE、 theVALUE和一个关联的CLASS数字。对于每个对HUE和VALUE我想找到，使用该阵列对应的Class号码。我是 Python 的初学者，很难做到。你知道一种方法吗？
Thank you in advance!
先感谢您！
采纳答案by Kei Minagawa
I assume your array looks like:
我假设你的数组看起来像：
|(HUE)(VALUE)(CLASS)
row/col| 0 1 2
-------+-----------------
0 | 0 1 2
1 | 3 4 5
2 | 6 7 8
. | . . .
. | . . .
3599999| . . .
And here is the sample code. For simplicity I changed the size 3600000 to 5.
这是示例代码。为简单起见，我将大小 3600000 更改为 5。
a = np.array(xrange(5 * 3))
a.shape = (5, 3)
Now array alook like this:
现在数组a看起来像这样：
array([[ 0, 1, 2],
[ 3, 4, 5],
[ 6, 7, 8],
[ 9, 10, 11],
[12, 13, 14]])
If you want row with HUE=9, do like this:
如果你想用 排HUE=9，这样做：
a[np.where(a[:,0] == 9)]
#array([[ 9, 10, 11]])
If you want row with VALUE=4, do like this:
如果你想用 排VALUE=4，这样做：
a[np.where(a[:,1] == 4)]
#array([[3, 4, 5]])
If you want row with HUE=0and VALUE=1, do like this:
如果你想用HUE=0and排VALUE=1，这样做：
a[np.where((a[:,0] == 0) * (a[:,1] == 1))]
#array([[0, 1, 2]])
回答by emeth
Try this code:
试试这个代码：
x[x[:, 2] == class_number[:, :2]
where xis np.ndarray
这里x是np.ndarray
x[:, 2] == class_number
contains true/falsethat means whether the last is class_numberor not.
包含true/false这意味着最后一个是否是class_number。
You need to take a look at: Boolean indexingin http://wiki.scipy.org/Cookbook/Indexing
你需要看看：Boolean indexing在http://wiki.scipy.org/Cookbook/Indexing
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