CODEWITHSUNDEEP
pythonpairwise
Fred
Fred

Reputation: 23

make pairwise difference of array with python

I have an array with numbers like this, in this case 5 numbers:

a = array([2.88581812, 2.8930633 , 2.85603976, 2.86739916, 2.85736707])

I would like to create an array of 10 elements in which the pairwise difference of all the numbers of the array is present. For now i used nested loop for like this:

diffmean = []
for i in range(len(a)-1):
    for j in range(i+1, len(a)):
        diffmean.append(a[i]-a[j])

Obtaining a list with 10 elements of pairwise difference

    [-0.007245185215707384,
 0.029778354907735505,
 0.018418952746142025,
 0.0284510446999775,
 0.03702354012344289,
 0.02566413796184941,
 0.035696229915684885,
 -0.01135940216159348,
 -0.0013273102077580035,
 0.010032091953835476]

there is a "pythonic" way to perform this? without loopfor or nested loop for?

Upvotes: 2

Views: 1080

Answers (2)

Laputa
Laputa

Reputation: 199

You can use combinations in build-in itertools library. Like:

from itertools import combinations
a = [2.88581812, 2.8930633 , 2.85603976, 2.86739916, 2.85736707]
diffmean = []
for b,c in combinations(a, 2):
    diffmean.append(b - c)

The second argument of the function is the number of elements you want to combine. The function is order-free and the order-based version is permutations which returns 20 values in this case.

Upvotes: 2

Ashok Kumar
Ashok Kumar

Reputation: 89

Assuming a is is numpy array. The below should work, but perhaps a more efficient solution exists as this calculates differences twices

np.expand_dims(a, 1) - a
d[np.tril_indices(a.size, k=-1)]

Upvotes: 1

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