CODEWITHSUNDEEP
pythonlistnumpylist-comprehension
piccolo
piccolo

Reputation: 2217

python mean of list of lists

I want to find the means of all the negative numbers from a list that has a mix of positive and negative numbers. I can find the mean of the lists as 

import numpy as np

listA = [ [2,3,-7,-4] , [-2,3,4,-5] , [-5,-6,-8,2] , [9,5,13,2]  ]
listofmeans = [np.mean(i) for i in listA ]

I want to create a similar one line code that only takes the mean of the negative numbers in the list. So for example the first element of the new list would be (-7 + -4)/2 = -5.5

My complete list would be:

listofnegativemeans = [ -5.5, -3.5, -6.333333, 0 ]

Upvotes: 6

Views: 15995

Answers (4)

B. M.
B. M.

Reputation: 18628

The pure numpy way : 

In [2]: np.ma.masked_greater(listA,0).mean(1).data
Out[2]: array([-5.5       , -3.5       , -6.33333333,  0.        ])

Upvotes: 1

Juan Leni
Juan Leni

Reputation: 7588

That would be something like:

listA = np.array( [ [2,3,-7,-4] , [-2,3,4,-5] , [-5,-6,-8,2] , [9,5,13,2]  ] )

listofnegativemeans = [np.mean(i[i<0]) for i in listA ]

output:

[-5.5, -3.5, -6.333333333333333, nan]

Zero is misleading, I definitely prefer nan if you don't have any elements that are negative.

Upvotes: 0

wim
wim

Reputation: 362647

If you're using numpy at all, you should strive for numpythonic code rather than falling back to python logic. That means using numpy's ndarray data structure, and the usual indexing style for arrays, rather than python loops.

For the usual means:

>>> listA
[[2, 3, -7, -4], [-2, 3, 4, -5], [-5, -6, -8, 2], [9, 5, 13, 2]]
>>> A = np.array(listA)
>>> np.mean(A, axis=1)
array([-1.5 ,  0.  , -4.25,  7.25])

Negative means:

>>> [np.mean(row[row<0]) for row in A]
[-5.5, -3.5, -6.333333333333333, nan]

Upvotes: 2

gtlambert
gtlambert

Reputation: 11961

You could use the following:

listA = [[2,3,-7,-4], [-2,3,4,-5], [-5,-6,-8,2], [9,5,13,2]]
means = [np.mean([el for el in sublist if el < 0] or 0) for sublist in listA]
print(means)

Output

[-5.5, -3.5, -6.3333, 0.0]

If none of the elements in sublist are less than 0, the list comprehension will evaluate to []. By including the expression [] or 0 we handle your scenario where you want to evaluate the mean of an empty list to be 0.

Upvotes: 5

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