CODEWITHSUNDEEP
pythonlistpython-2.7mutable
xenoid
xenoid

Reputation: 8869

Best way to extend a list with itself N times

So I have a list [a,b,c] and I want to obtain [a,b,c,a,b,c,...a,b,c].

I can of course do this with two nested loops, but there must be a better way? itertools.cycle() would have been solution if I could provide a count.

Two constraints:

  1. it should work in 2.7 (but for the sake of curiosity I'm interested in a 3.x solution)
  2. list elements should be independent copies (they are mutable types)

Upvotes: 4

Views: 3680

Answers (3)

Gonçalo Peres
Gonçalo Peres

Reputation: 13582

To start with, if one has a list = [a, b, c], if one prints it, one will get the following error

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-17-eb02c90df247> in <module>
----> 1 list = [a, b, c]
      2 list

NameError: name 'c' is not defined

The list should be defined as follows

>>> list = ['a', 'b', 'c']
>>> print(list)
['a', 'b', 'c']

For your case, simply multiplying the list with N will do the work. For the examples, let's consider N=3

>>> n = 3
>>> list = list*n
['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b', 'c']

Now, and giving an alternative answer, as the need may arise, if one wants to extend the list (considering, for the example, N=3) as follows

['a' 'a' 'a' 'b' 'b' 'b' 'c' 'c' 'c']

One can use np.repeat do that

>>> n = 3
>>> list= np.repeat(list, n)
array(['a', 'a', 'a', 'b', 'b', 'b', 'c', 'c', 'c'], dtype='<U1')

This retrieved an array. If that doesn't fit one's needs and one wants a list (use np.ndarray.tolist), as follows

>>> list = list.tolist()
['a', 'a', 'a', 'b', 'b', 'b', 'c', 'c', 'c']

Upvotes: -1

xenoid
xenoid

Reputation: 8869

Eventually I came up with:

[copy.copy(e) for _ in range(N) for e in sourceList]

Upvotes: 7

Jean-Fran&#231;ois Fabre
Jean-Fran&#231;ois Fabre

Reputation: 140168

For immutable types, you can use multiplication operator on the list:

>>> [1,2,3]*5
[1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3]

for mutable types, you need to perform a copy of the elements prior to inserting.

I would chain a repeated copy of the items of the list.

import copy,itertools

a=[12]
b=[13]
c=[14]

l = [a,b,c]

new_l = list(itertools.chain.from_iterable(map(copy.copy,l) for _ in range(5)))

in new_l, all lists are independent (references are not copies from each other). You may need copy.deepcopy in some cases.

Upvotes: 4

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