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Python: Is there a one-liner to create a list with repeated elements?

0 votes

I can create a list that has repeated elements of another list as follows:

xx = ["a","b"]
nrep = 3
print xx
yy = []
for aa in xx:
 for i in range(nrep):
print yy


['a', 'b']
['a', 'a', 'a', 'b', 'b', 'b']

Is there a one-liner to create a list with repeated elements?

posted Mar 26, 2016 by Luv Kumar

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3 Answers

+1 vote

yy = [aa for aa in xx for _ in range(nrep)]

I suggest that you try this sort of the thing at an interactive prompt, it's a great way to learn.

You might also want to take a look at the itertools module

answer Mar 26, 2016 by Sridharan
0 votes

Sure. As with all one liners, there comes a degree of complexity when it gets in the way of readability; you must decide what is better in your use case.

Look up the chain() function from the itertools module. Generate 2 (or nrep) length lists from each element of the original list and chain() them together. That gets you an iterable of all the elements. If you really need a list out the end instead of the iterable of the elements, convert the iterable to a list (hint: lists can be initialized with iterables).

answer Mar 26, 2016 by Majula Joshi
0 votes
yy = reduce(lambda a, b: a + b, ([i] * nrep for i in xx), [])

Or, if you want to "import operator" first, you can use 'operator.add' instead of the lambda (but you _did_ ask for a one-liner ;)).

answer Mar 26, 2016 by Majula Joshi
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How can I flatten just a specific sublist of each list in a list of lists?

So if I had this data

[ ['46295', 'Montauk', '3', '60', '85', ['19', '5', '1', '0 $277790.00']],
 ['46295', 'Dark Eyes', '5', '59', '83', ['6', '4', '1', '0 $105625.00']],
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[ ['46295', 'Montauk', '3', '60', '85', '19', '5', '1', '0 $277790.00'],
 ['46295', 'Dark Eyes', '5', '59', '83', '6', '4', '1', '0 $105625.00'],
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Been looking around but most solutions just entirely flatten everything. This was popular on SO but yeah it flattens everything I want to be more selective

def flatten(lst):
 for elem in lst:
 if type(elem) in (tuple, list):
 for i in flatten(elem):
 yield i
 yield elem

What I am thinking is that if for each list the sublist should be at index 1, so


for item in list:
 item[1] - somehow flatten.

Any Idea or pointer?

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