Duplicate elements in a list

Question

I have a list in Python:

l = [\'a\', \'c\', \'e\', \'b\']

I want to duplicate each element immediately next to the original.

Answer 1

import itertools

ll = list(itertools.chain.from_iterable((e, e) for e in l))

At work:

>>> import itertools
>>> l = ['a', 'c', 'e', 'b']
>>> ll = list(itertools.chain.from_iterable((e, e) for e in l))
>>> ll
['a', 'a', 'c', 'c', 'e', 'e', 'b', 'b']

As Lattyware pointed out, in case you want more than just double the element:

from itertools import chain, repeat

ll = list(chain.from_iterable(repeat(e, 2) for e in l))

Answer 2

>>> l = ['a', 'c', 'e', 'b']
>>> [x for pair in zip(l,l) for x in pair]
['a', 'a', 'c', 'c', 'e', 'e', 'b', 'b']

Or

>>> from itertools import repeat
>>> [x for item in l for x in repeat(item, 2)]
['a', 'a', 'c', 'c', 'e', 'e', 'b', 'b']

Answer 3

Here's a pretty easy way:

sum(zip(l, l), tuple())

It duplicates each item, and adds them to a tuple. If you don't want a tuple (as I suspect), you can call list on the the tuple:

list(sum(zip(l, l), tuple()))

---

A few other versions (that yield lists):

list(sum(zip(l, l), ()))

sum([list(i) for i in zip(l, l)], [])

sum(map(list, zip(l, l)), [])

Answer 4

Try this

for i in l:
    ll.append(i)
    ll.append(i)

Demo

It will just do your work but it's not an optimized way of doing this.

use the ans. posted by @Steven Rumbalski

Answer 5

This is old but I can't see the straightforward option here (IMO):

[ item for item in l for repetitions in range(2) ]

So for the specific case:

>>> l = ['a', 'c', 'e', 'b']
l = ['a', 'c', 'e', 'b']
>>> [ i for i in l for r in range(2) ]
[ i for i in l for r in range(2) ]
['a', 'a', 'c', 'c', 'e', 'e', 'b', 'b']
>>> 

And generalizing:

[ item for item in l for _ in range(r) ] 

Where r is the quantity of repetitions you want.

So this has a O(n.r) space and time complexity, is short, with no dependencies and also idiomatic.

Answer 6

Pandas gives a method for duplicated elements:

import pandas as pd
l = pd.Series([2, 1, 3, 1])
print(l.duplicated())
>>>0    False
   1    False
   2    False
   3     True
   dtype: bool

print('Has list duplicated ? :', any(l.duplicated()))
>>>Has list duplicated ? : True