Concatenate two lists of different length in between each other

Question

I have two list of different sizes, n and n-1. I have to concatenate two lists that look like this

list1 = ['a','b','c']
list2 = ['-','-']

They have to be concatenated to get s.th like this

str_out = 'a-b-c'

I have tried to figure out an elegant way to do this but only managed to come up with this solution

list1 = ['a','b','c']
list2 = ['-','-']
string2 = ''

for index,item in enumerate(list1):
    string2 = string2 + item + list2[index-1]

print(string2)

which prints

'a-b-c-'

I am looking for a nicer implementation or how I can get rid of the final dash (-)

EDIT: To clarify, the lists will be dynamic and list2 can contain arbitrary characters.

e.g: list2 = ['*','-']

Will one list always be one shorter than the other? And if so, do you know which? — tobias_k, Dec 07 '16 at 12:07
`''.join(chain.from_iterable(zip_longest(list1,list2,fillvalue='')))` (`izip_longest` in Python2) — Chris_Rands, Dec 07 '16 at 12:17
Possible duplicate of [Combining two lists to string](http://stackoverflow.com/questions/15703590/combining-two-lists-to-string) — Chris_Rands, Dec 07 '16 at 12:21

derM - not here for BOT dreams · Answer 1 · 2016-12-07T12:22:19.183

2

You might use the itertools Many posibilities, e.g.

list1 = ['a', 'b', 'c']
list2 = ['-', '*']
''.join(map(''.join, itertools.izip_longest(list1, list2, fillvalue='')))
''.join(itertools.chain(*itertools.izip_longest(list1, list2, fillvalue='')))

edited Dec 07 '16 at 12:22

answered Dec 07 '16 at 12:14

derM - not here for BOT dreams

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Gennady Kandaurov · Accepted Answer · 2021-03-09T14:43:33.097

1

Try following:

from itertools import chain
"".join(x for x in chain(*map(None, list1, list2)) if x is not None)

Update add izip_longest version:

from itertools import chain, izip_longest
"".join(x for x in chain(*izip_longest(list1, list2)) if x is not None)

Update py3 version:

from itertools import chain, zip_longest
"".join(x for x in chain(*zip_longest(list1, list2)) if x is not None)

edited Mar 09 '21 at 14:43

answered Dec 07 '16 at 12:07

Gennady Kandaurov

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If you already use the itertools, why not the `izip_longest()`? – derM - not here for BOT dreams Dec 07 '16 at 12:17
Yes, `izip_longest` can be used here too: `"".join(x for x in chain(*izip_longest(list1, list2)) if x is not None)` – Gennady Kandaurov Dec 07 '16 at 12:31
2

I have `TypeError: 'NoneType' object is not callable` for the first solution in Python 3, so it's a Python 2-only solution. – Ilya V. Schurov Dec 07 '16 at 12:54
In python3 `"".join(x for x in chain(*zip_longest(list1, list2)) if x is not None)` will work. – Gennady Kandaurov Dec 07 '16 at 13:40
@GennadyKandaurov I got this error when run in python 3: TypeError: sequence item 0: expected str instance, numpy.uint8 found – SteveTz Mar 09 '21 at 13:45
@Joe_Tz it's from `"".join()` since `"".join` requires an iterable of strings, if elements are not strings, convert them into strings with additional step: `"".join(map(str, x for x in chain(...)...))` – Gennady Kandaurov Mar 09 '21 at 14:41

Rahul K P · Answer 3 · 2016-12-07T12:20:11.453

1

Try this,

In [32]: ''.join(i+j for i,j in zip(list1,list2+['']))
Out[32]: 'a-b-c'

Just add a black ('') element at end of list2. Then just apply zip and join.

Tried with another example,

In [36]: list2 = ['*','-']
In [37]: ''.join(i+j for i,j in zip(list1,list2+['']))
Out[37]: 'a*b-c'

edited Dec 07 '16 at 12:20

answered Dec 07 '16 at 12:13

Rahul K P

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score 0 · Answer 4 · answered Dec 07 '16 at 12:07

Assuming your lists always correct, you can do:

list1 = ['a','b','c']
list2 = ['-','-']

res = []
for i1, i2 in zip(list1, list2):
    res.append(i1)
    res.append(i2)

res.append(list1[-1])

print ''.join(res)

Iterate on the two lists simultaneously, and add an item from list1 and then from list2. When the loop terminates, you have one more item in list1, which you append manually.

Another solution would be having a separate counter for each list:

list1 = ['a','b','c']
list2 = ['-','-']

res = []
j = 0
for i1 in list1:
    res.append(i1)
    if j < len(list2):
        res.append(list2[j])
        j += 1

print ''.join(res)

score 0 · Answer 5 · answered Dec 07 '16 at 12:14

You can use NumPy arrays, as their indexing tools are very useful for the purpose of the OP:

list1 = np.array(['a','b','c'])
list2 = np.array(['*','-'])
final_list = np.zeros(len(l1) + len(l2)).astype('S1')
list3[0::2] = list1
list3[1::2] = list2

result_string = ''.join(list3)

The result will be:

'a*b-c'

score 0 · Answer 6 · edited May 23 '17 at 10:30

0

Borrowing from this answer regarding interleaving lists:

''.join(val for pair in zip(list1, list2) for val in pair) + list1[-1]

edited May 23 '17 at 10:30

Community

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answered Dec 07 '16 at 12:17

Billy

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score 0 · Answer 7 · answered Dec 07 '16 at 12:20

You can slice the first list to get a sublist with the same length as the second list, then apply zip() to the result. extend is used to add the other elements of the first list:

list1 = ['a','b','c']
list2 = ['-','-']

my_list = [''.join(item) for item in zip(list1[:len(list2)], list2)]
my_list.extend(list1[len(list2):])
str_out = ''.join(my_list)
print(str_out)
# Output: 'a-b-c'

MSeifert · Answer 8 · 2017-04-04T19:46:08.153

There are several external packages that have builtin functions for this kind of "interleaving" of iterables, just to show one of them: iteration_utilities.roundrobin (note, that I'm the author of this library):

>>> from iteration_utilities import ManyIterables
>>> ManyIterables(['a','b','c'], ['-','-']).roundrobin().as_string()
'a-b-c'
>>> ManyIterables(['a','b','c'], ['-','*']).roundrobin().as_string()
'a-b*c'

The as_string is just a wrapped ''.join call.

Just to name a few alternatives:

These are generalized solutions that work on an arbitary number of sequences and iterables. With only two iterables and if you don't want to use external packages using a zip or itertools.zip_longest approach (see other answers) is probably easier.

宏杰李 · Answer 9 · 2016-12-07T12:43:11.170

-3

from itertools import zip_longest,chain
list1 = ['a','b','c']    
list2 = ['*','-']
''.join(i+j for i,j in zip_longest(list1, list2, fillvalue=''))

or:

list1 = ['a','b','c']    
list2 = ['*','-']
def item_gen(list1, list2):
    for i,j in zip(list1, list2):
        yield i
        yield j
    yield list1[-1]

each = item_gen(list1, list2)
''.join(each)

edited Dec 07 '16 at 12:43

answered Dec 07 '16 at 12:05

宏杰李

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thanks for your answer if have just updated my question, join won't work unfortunately – ThatQuantDude Dec 07 '16 at 12:07

Concatenate two lists of different length in between each other

9 Answers9