[英]Assign values to numpy array using row indices
Suppose I have two arrays, a=np.array([0,0,1,1,1,2]), b=np.array([1,2,4,2,6,5])
.假设我有两个 arrays,
a=np.array([0,0,1,1,1,2]), b=np.array([1,2,4,2,6,5])
。 Elements in a
mean the row indices of where b
should be assigned. a
元素表示应分配b
的行索引。 And if there are multiple elements in the same row, the values should be assigned in order.如果同一行中有多个元素,则应按顺序分配值。 So the result is a 2D array
c
:所以结果是一个二维数组
c
:
c = np.zeros((3, 4))
counts = {k:0 for k in range(3)}
for i in range(a.shape[0]):
c[a[i], counts[a[i]]]=b[i]
counts[a[i]]+=1
print(c)
Is there a way to use some fancy indexing method in numpy to get such results faster (without a for loop) in case these arrays are big.如果这些 arrays 很大,有没有办法在 numpy 中使用一些花哨的索引方法来更快地获得这样的结果(没有 for 循环)。
I had to run your code to actually see what it produced.我必须运行你的代码才能真正看到它产生了什么。 There are limits to what I can 'run' in my head.
我可以在脑海中“奔跑”的东西是有限度的。
In [230]: c
Out[230]:
array([[1., 2., 0., 0.],
[4., 2., 6., 0.],
[5., 0., 0., 0.]])
In [231]: counts
Out[231]: {0: 2, 1: 3, 2: 1}
Omitting this information may be delaying possible answers.省略此信息可能会延迟可能的答案。 'vectorization' requires thinking in whole-array terms, which is easiest if I can visualize the result, and look for a pattern.
“向量化”需要从整个数组的角度进行思考,如果我可以可视化结果并寻找模式,这是最简单的。
padding
problem.padding
问题。In [260]: u, c = np.unique(a, return_counts=True)
In [261]: u
Out[261]: array([0, 1, 2])
In [262]: c
Out[262]: array([2, 3, 1]) # cf with counts
Load data with rows of different sizes into Numpy array 将不同大小行的数据加载到 Numpy 数组中
Working from previous padding questions, I can construct a mask:根据之前的填充问题,我可以构建一个掩码:
In [263]: mask = np.arange(4)<c[:,None]
In [264]: mask
Out[264]:
array([[ True, True, False, False],
[ True, True, True, False],
[ True, False, False, False]])
and use that to assign the b
values to c
:并使用它将
b
值分配给c
:
In [265]: c = np.zeros((3,4),int)
In [266]: c[mask] = b
In [267]: c
Out[267]:
array([[1, 2, 0, 0],
[4, 2, 6, 0],
[5, 0, 0, 0]])
Since a
is already sorted we might get the counts faster than with unique
.由于
a
已经排序,我们可能会比使用unique
更快地获得计数。 Also it will have problems if a
doesn't have any values for some row(s).如果
a
某些行没有任何值,也会出现问题。
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