Creating a list of items with multiplicities in Python

Question

I have an issue with the creation of lists.

I have a function that returns the roots of a polynomial (see below). What I obtain is a list of roots (R.keys()) and the times each one appears in the solution (R.values()).

When I obtain R.items() I am given a set of pairs of roots and their multiplicities: [(-2, 2), (-1, 1), (-3, 3)] as returned from the example below.

But what I want is to obtain a list with each root repeated by the number of times it appears, that is, [-2, -2, -1, -3, -3, -3].

I guess this is not difficult but I'm getting stuck with finding a solution.

pol=Lambda((y), y**6 + 14*y**5 + 80*y**4 + 238*y**3 + 387*y**2 + 324*y + 108)
poli=Poly(pol(y))
R=roots(poli)
R.keys()
R.values()
R.items()

def list_of_roots(poli):
    return(R.items())
list_of_roots(poli)

What type is `R`? I.e., what is `type(R)`? And are you using Python 2 or Python 3? — Stefan Pochmann, Jan 14 '17 at 23:52
Yes, `[-2, -2, -1, -3, -3, -3]`, I've corrected it, thanks for this. I'm using Python 2.7 — NS1, Jan 15 '17 at 10:34

score 0 · Answer 1 · answered Jan 14 '17 at 23:46

0

If you can get the list of items in the form of an Array<Tuple>, then you can create a list like this:

items = [(-2, 2), (-1, 1), (-3, 3)]
listOfRoots = []
for x in items:
    for y in range(x[1]):
        listOfRoots.append(x[0])
print(listOfRoots)

answered Jan 14 '17 at 23:46

A. Sokol

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score 0 · Answer 2 · answered Jan 14 '17 at 23:47

0

roots = [(-2, 2), (-1, 1), (-3, 3)]
[ r for (root, mult) in roots for r in [root] * mult]

[-2, -2, -1, -3, -3, -3]

answered Jan 14 '17 at 23:47

Rich L

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ooknosi · Accepted Answer · 2017-01-14T23:59:53.347

0

def get_list_of_roots(poli):
    # initialize an empty list 
    list_of_roots = []
    # process your poli object to get roots
    R = roots(poli)
    # we obtain the key value pairs using R.items()
    for root, multiplicity in R.items():
        # extend the list_of_roots with each root by multiplicity amount
        list_of_roots += [root] * multiplicity
    return list_of_roots

EDIT: Processed poli within function since you seem to want to pass poli to it.

EDIT: Added explanation of code.

edited Jan 14 '17 at 23:59

answered Jan 14 '17 at 23:51

ooknosi

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Thanks for this, this is what I needed! – NS1 Jan 15 '17 at 10:49

score 0 · Answer 4 · answered Jan 14 '17 at 23:59

0

items = [(-2, 2), (-1, 1), (-3, 3)]
result = []
for val, mult in items:
  result.extend(mult * [val])

answered Jan 14 '17 at 23:59

gipsy

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score 0 · Answer 5 · edited May 23 '17 at 12:09

0

I'm assuming you mean [-2, -2, -1, -3, -3, -3].

roots1 = [(-2, 2), (-1, 1), (-3, 3)]
roots2 = [i for (r, m) in roots1 for i in [r] * m]

This might be of interest: Making a flat list out of list of lists in Python

edited May 23 '17 at 12:09

Community

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answered Jan 15 '17 at 00:04

Spherical Cowboy

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Creating a list of items with multiplicities in Python

5 Answers5