multiple list generation from a list based on condition in python

Question

I have a Pandas series (which could be a list, this is not very important) of lists which contains (to simplify, but that could also be letters of words) positive and negative number, such as

0 [12,-13,0,6]  
1 [2,-3,8,233]  
2 [0,6,8,3]

for each of these, i want to fill a row in a three columns data frame, with a list of all positive values, a list of all negative values, and a list of all values comprised in some interval. Such as:

 [[12,6],[-13],[0,6]]   
 [[2,8,233],[-3],[2,8]]   
 [[6,8,3],[],[6,8,3]]

What I first thought was using a list comprehension to generate a list of triadic lists of lists, which would be converted using pd.DataFrame to the right form. This was because i don't want to loop over the list of lists 3 times to apply each time a new choice heuristics, feels slow and dull.

But the problem is that I can't actually generate well the lists of the triad [[positive],[negative], [interval]]. I was using a syntax like

[[[positivelist.extend(number)],[negativelist], [intervalist.extend(number)]]\

for listofnumbers in listoflists for number in listofnumbers\
if number>0 else [positivelist],[negativelist.extend(number)], [intervalist.extend(number)]]

but let be honest, this is unreadable, and anyway it doesn't do what I want since extend yields none.
So how could I go about that without looping three times (I could have many millions elements in the list of lists, and in the sublists, and I might want to apply more complexe formulae to these numbers, too, it is a first approach)?

I thought about using functional programming, map/lambda; but it is unpythonic. The catch is: what in python may help to do it right?

My guess would be something as:

newlistoflist=[]
for list in lists:
     positive=[]
     negative=[]
     interval=[]
     for element in list:
         positive.extend(element) if element>0
         negative.extend(element) if element<0
         interval.extend(element) if n<element<m
     triad=[positive, negative,interval]
 newlistoflist.append(triad)

what do you think?

Nuageux · Accepted Answer · 2017-04-27 10:06:36Z

1

You can do:

import numpy
l = [[12,-13,0,6], [2,-3,8,233], [0,6,8,3]]
l = numpy.array([x for e in l for x in e])
positive = l[l>0]
negative = l[l<0]
n,m = 1,5
interval = l[((l>n) & (l<m))]
print positive, negative, interval

Output: [ 12 6 2 8 233 6 8 3] [-13 -3] [2 3]

Edit: Triad version:

import numpy
l = numpy.array([[12,-13,0,6], [2,-3,8,233], [0,6,8,3]])
n,m = 1,5
triad = numpy.array([[e[e>0], e[e<0], e[((e>n) & (e<m))]] for e in l])
print triad

Output:

[[array([12,  6]) array([-13]) array([], dtype=int64)]
 [array([  2,   8, 233]) array([-3]) array([2])]
 [array([6, 8, 3]) array([], dtype=int64) array([3])]]

edited Apr 27, 2017 at 10:06

answered Apr 27, 2017 at 9:23

Nuageux

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5 Comments

Ando Jurai Over a year ago

That's a start but it doesn't do what I want. With your exemple, for each list, I'd like a triad : [[12,6],[-13],[0,6]] [[2,8,233],[-3],[2,8]] [[6,8,3],[],[6,8,3]]

Nuageux Over a year ago

Can you give n,mfor interval?

Nuageux Over a year ago

@AndoJurai: If you change n,m values it should be it.

Ando Jurai Over a year ago

Thanks. What if I want to use the same principle on list containing words instead? I guess that the masking use may be reproduced (and eventually transposed to pandas, which would be more practical for me) but I can't be sure.

Nuageux Over a year ago

You can use the same principle e.g. l = numpy.array([['alpha', 'beta', 'gamma'], ['theta', 'epislon', 'zeta'], ['delta', 'sigma', 'omega']], dtype=str) then triad = numpy.array([[e[e>'a'], e[e<'h'], e[((e>'a') & (e<'g'))]] for e in l])

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