Calculating Mean of arrays with different lengths

问题:

Is it possible to calculate the mean of multiple arrays, when they may have different lengths? I am using numpy. So let's say I have:

numpy.array([[1, 2, 3, 4, 8],    [3, 4, 5, 6, 0]])
numpy.array([[5, 6, 7, 8, 7, 8], [7, 8, 9, 10, 11, 12]])
numpy.array([[1, 2, 3, 4],       [5, 6, 7, 8]])

Now I want to calculate the mean, but ignoring elements that are 'missing' (Naturally, I can not just append zeros as this would mess up the mean)

Is there a way to do this without iterating through the arrays?

PS. These arrays are all 2-D, but will always have the same amount of coordinates for that array. I.e. the 1st array is 5 and 5, 2nd is 6 and 6, 3rd is 4 and 4.

An example:

np.array([[1, 2],    [3, 4]])
np.array([[1, 2, 3], [3, 4, 5]])
np.array([[7],       [8]])

This must give

(1+1+7)/3  (2+2)/2   3/1
(3+3+8)/3  (4+4)/2   5/1

And graphically:

[1, 2]    [1, 2, 3]    [7]
[3, 4]    [3, 4, 5]    [8]

Now imagine that these 2-D arrays are placed on top of each other with coordinates overlapping contributing to that coordinate's mean.

回答1:

numpy.ma.mean allows you to compute the mean of non-masked array elements. However, to use numpy.ma.mean, you have to first combine your three numpy arrays into one masked array:

import numpy as np
x = np.array([[1, 2], [3, 4]])
y = np.array([[1, 2, 3], [3, 4, 5]])
z = np.array([[7], [8]])

arr = np.ma.empty((2,3,3))
arr.mask = True
arr[:x.shape[0],:x.shape[1],0] = x
arr[:y.shape[0],:y.shape[1],1] = y
arr[:z.shape[0],:z.shape[1],2] = z
print(arr.mean(axis = 2))

yields

[[3.0 2.0 3.0]
 [4.66666666667 4.0 5.0]]

回答2:

OP, I know you were looking for a non-iterative built-in solution, but the following really only takes 3 lines (2 if you combine transpose and means but then it just gets messy):

arrays = [
    np.array([1,2], [3,4]),
    np.array([1,2,3], [3,4,5]),
    np.array([7], [8])
    ]

mean = lambda x: sum(x)/float(len(x)) 

transpose = [[item[i] for item in arrays] for i in range(len(arrays[0]))]

means = [[mean(j[i] for j in t if i < len(j)) for i in range(len(max(t, key = len)))] for t in transpose]

Outputs:

>>>means
[[3.0, 2.0, 3.0], [4.666666666666667, 4.0, 5.0]]

回答3:

The below function also works by adding columns of arrays of different lengths:

def avgNestedLists(nested_vals):
    """
    Averages a 2-D array and returns a 1-D array of all of the columns
    averaged together, regardless of their dimensions.
    """
    output = []
    maximum = 0
    for lst in nested_vals:
        if len(lst) > maximum:
            maximum = len(lst)
    for index in range(maximum): # Go through each index of longest list
        temp = []
        for lst in nested_vals: # Go through each list
            if index < len(lst): # If not an index error
                temp.append(lst[index])
        output.append(np.nanmean(temp))
    return output

Going off of your first example:

avgNestedLists([[1, 2, 3, 4, 8], [5, 6, 7, 8, 7, 8], [1, 2, 3, 4]])

Outputs:

[2.3333333333333335,
 3.3333333333333335,
 4.333333333333333,
 5.333333333333333,
 7.5,
 8.0]

The reason np.amax(nested_lst) or np.max(nested_lst) was not used in the beginning to find the max value is because it will return an array if the nested lists are of different sizes.