CODEWITHSUNDEEP
pythonlistpython-2.7
jayelm
jayelm

Reputation: 7657

Python: duplicating each element in a list

What's the most pythonic way to do the following? It can either modify the old list or create a new list.

oldList = [1, 2, 3, 4]

newList = [1, 1, 2, 2, 3, 3, 4, 4]

Upvotes: 2

Views: 148

Answers (6)

Meenakshiyegu
Meenakshiyegu

Reputation: 51

a=[1,2,3,4]
print a*2

This will print [1,2,3,4,1,2,3,4]

This can be used if you want duplicate of elements

if you want the duplicate to be next to the original the below code can be used

a=[1,2,3,4]
b=[]
for i in range(0,len(a)):
    b.append(a[i])
    b.append(a[i])
print b

this will print b=[1,1,2,2,3,3,4,4]

Upvotes: 0

John La Rooy
John La Rooy

Reputation: 304117

>>> oldList = [1, 2, 3, 4]
>>> (4*(oldList+[0]+oldList))[::5]
[1, 1, 2, 2, 3, 3, 4, 4]

Strangely enough

$ python -m timeit -s  "oldList = [1, 2, 3, 4]" "[x for x in oldList for _ in range(2)]"
1000000 loops, best of 3: 1.65 usec per loop
$ python -m timeit -s  "oldList = [1, 2, 3, 4]" "(4*(oldList+[0]+oldList))[::5]"
1000000 loops, best of 3: 0.995 usec per loop

Upvotes: 1

John La Rooy
John La Rooy

Reputation: 304117

Since nobody has done an answer that modifies oldList in place. Here's a fun way

>>> oldList = [1, 2, 3, 4]
>>> oldList += oldList
>>> oldList[::2] = oldList[1::2] = oldList[-4:]
>>> oldList
[1, 1, 2, 2, 3, 3, 4, 4]

Upvotes: 2

Thorsten Kranz
Thorsten Kranz

Reputation: 12755

Two more approaches using functional concepts:

>>> list_ = [1,2,3,4]
>>> from itertools import chain
>>> list(chain(*[[x,x] for x in list_]))
[1, 1, 2, 2, 3, 3, 4, 4]
>>> reduce(lambda x,y: x+y, [[x,x] for x in list_])
[1, 1, 2, 2, 3, 3, 4, 4]

And one using numpy:

list(np.array([[x,x] for x in list_]).flatten())

And one last as list comprehension:

[x for y in zip(list_, list_) for x in y]

Upvotes: 1

roippi
roippi

Reputation: 25954

If you're really into functional programming:

from itertools import chain,izip
newList = list(chain(*izip(oldList,oldList)))

@falsetru's answer is more readable, so that likely meets the "most pythonic" litmus test.

Upvotes: 4

falsetru
falsetru

Reputation: 368894

Using list comprehension:

>>> oldList = [1, 2, 3, 4]
>>> newList = [x for x in oldList for _ in range(2)]
>>> newList
[1, 1, 2, 2, 3, 3, 4, 4]

Above list comprehension is similar to following nested for loop.

newList = []
for x in oldList:
    for _ in range(2):
        newList.append(x)

Upvotes: 9

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