一个又一个有效率的人

Question

我有一个3xN的三维坐标数组，我想有效地计算所有项目的距离矩阵。 有没有比嵌套循环更有效的循环策略呢？在

当前伪代码实现：

for i,coord in enumerate(coords):
    for j,coords2 in enumerate(coords):
        if i != j:
             dist[i,j] = numpy.norm(coord - coord2)

Answer 1

要准确再现结果：

>>> import scipy.spatial as sp
>>> import numpy as np
>>> a=np.random.rand(5,3) #Note this is the transpose of your array.
>>> a
array([[ 0.83921304,  0.72659404,  0.50434178],  #0
       [ 0.99883826,  0.91739731,  0.9435401 ],  #1
       [ 0.94327962,  0.57665875,  0.85853404],  #2
       [ 0.30053567,  0.44458829,  0.35677649],  #3
       [ 0.01345765,  0.49247883,  0.11496977]]) #4
>>> sp.distance.cdist(a,a)
array([[ 0.        ,  0.50475862,  0.39845025,  0.62568048,  0.94249268],
       [ 0.50475862,  0.        ,  0.35554966,  1.02735895,  1.35575051],
       [ 0.39845025,  0.35554966,  0.        ,  0.82602847,  1.1935422 ],
       [ 0.62568048,  1.02735895,  0.82602847,  0.        ,  0.3783884 ],
       [ 0.94249268,  1.35575051,  1.1935422 ,  0.3783884 ,  0.        ]])

要在不重复计算且只计算唯一对的情况下更有效地执行此操作：

^{pr2}$

注意两个数组之间的关系。cdist数组可以通过以下方式复制：

>>> out=np.zeros((a.shape[0],a.shape[0]))
>>> dists=sp.distance.pdist(a)
>>> out[np.triu_indices(a.shape[0],1)]=dists
>>> out+=out.T

>>> out
array([[ 0.        ,  0.50475862,  0.39845025,  0.62568048,  0.94249268],
       [ 0.50475862,  0.        ,  0.35554966,  1.02735895,  1.35575051],
       [ 0.39845025,  0.35554966,  0.        ,  0.82602847,  1.1935422 ],
       [ 0.62568048,  1.02735895,  0.82602847,  0.        ,  0.3783884 ],
       [ 0.94249268,  1.35575051,  1.1935422 ,  0.3783884 ,  0.        ]])

---

有些令人惊讶的时间安排-

设置：

def pdist_toarray(a):
    out=np.zeros((a.shape[0],a.shape[0]))
    dists=sp.distance.pdist(a)

    out[np.triu_indices(a.shape[0],1)]=dists
    return out+out.T

def looping(a):
    out=np.zeros((a.shape[0],a.shape[0]))
    for i in xrange(a.shape[0]):
        for j in xrange(a.shape[0]):
            out[i,j]=np.linalg.norm(a[i]-a[j])
    return out

时间安排：

arr=np.random.rand(1000,3)

%timeit sp.distance.pdist(arr)
100 loops, best of 3: 4.26 ms per loop

%timeit sp.distance.cdist(arr,arr)
100 loops, best of 3: 9.31 ms per loop

%timeit pdist_toarray(arr)
10 loops, best of 3: 66.2 ms per loop

%timeit looping(arr)
1 loops, best of 3: 16.7 s per loop

因此，如果你想要回平方数组，你应该使用cdist，如果你只想让对使用pdist。与cdist相比，具有1000个元素的数组的循环速度要慢~4000x，对于有10个元素的数组，循环速度要慢~70x。在