3D列表索引-如何获取特定x，y的max（z）

Question

I have a list with 3D index and for each combination (x,y,z) I have a value. I would like to know if there is a way to get the max z for a specific x,y.

Example:

 a=defaultdict(list)
 ...
 a = {(0,0,1): 5, (0,0,2): 1, (0,0,3): 4}

How to get the max(z) for (x=0,y=0) (which should be 3)?

I need to use this kind of data structure, because I have a dynamic 3 dimensional matrix with unknown z (heterogenous data over Z).

Answer 1

How about: max(b[2] for b in a.keys() if b[0] == 0 and b[1] == 0)

This uses a generator expression to return the z-coordinate of those values where x and y is 0.

Answer 2

If you have access to numpy , this could be much easier:

>>> import numpy as np
>>> a = np.arange(1, 9).reshape((2, 2, 2))
>>> a
array([[[1, 2],
        [3, 4]],

       [[5, 6],
        [7, 8]]])
>>> a[0, 0].max()
2

Answer 3

In case you need this for any of all possible combinations of x and y, i suggest that you should reorder your data, such that a tuple of x and y is a key, whereas the corresponding z is the value. Then you can simply find the max z:

from collections import defaultdict
a = {(0,0,1): 5, (0,0,2): 1, (0,0,3): 4, (1,0,2): 5}
b = defaultdict(list)
for xyz in a:
    xy = xyz[:-1]
    z = xyz[-1]
    b[xy].append(z)

max(b[(0,0)])