Combine lists by joining strings with matching index values

Question

I have two lists that I would like to combine, but instead of increasing the number of items in the list, I\'d actually like to join the items that have a matching index. Fo

Answer 1

map(''.join,zip(List1,List2,List3))
>>> 
['A1A1', 'B2B2', 'C3C3']

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Explanation:

zip(List1,List2,List3)

Returns

[('A', '1', 'A1'), ('B', '2', 'B2'), ('C', '3', 'C3')]

Each tuple repesents the elements associated with an index N in the zipped lists. We want to combine the elements in each tuple into a single string. For a single tuple we can go:

>>> ''.join(('A', '1', 'A1'))
'A1A1'

To produce the desired result, hence to get a list of all the desired strings, we map this join function to all the tuples like so:

map(''.join,zip(List1,List2,List3))

Resulting in

['A1A1', 'B2B2', 'C3C3']

---

So if you wanted to add only List1 and List2

map(''.join,zip(List1,List2))
>>> 
['A1', 'B2', 'C3']

Some timeings:

Microsoft Windows [Version 6.2.9200]
(c) 2012 Microsoft Corporation. All rights reserved.

C:\Users\Henry>python -m timeit -s "List1 = ['A', 'B', 'C']*10**5; List2 = ['1', '2','3']*10**5" "map(lambda x, y: x + y, List1, List2)"
10 loops, best of 3: 44 msec per loop

C:\Users\Henry>python -m timeit -s "List1 = ['A', 'B', 'C']*10**5; List2 = ['1', '2', '3']*10**5" "[x + y for x, y in zip(List1, List2)]"
10 loops, best of 3: 44 msec per loop

C:\Users\Henry>python -m timeit -s "List1 = ['A', 'B', 'C']*10**5; List2 = ['1', '2', '3']*10**5" "map(''.join,zip(List1,List2))"
10 loops, best of 3: 42.6 msec per loop

C:\Users\Henry>python -m timeit -s "from operator import add" "List1 = ['A', 'B', 'C']*10**5; List2 = ['1', '2', '3']*10**5" "map(add, List1, List2)"
10 loops, best of 3: 28.6 msec per loop

And using izip instead of zip

Microsoft Windows [Version 6.2.9200]
(c) 2012 Microsoft Corporation. All rights reserved.

C:\Users\Henry>python -m timeit -s "List1 = ['A', 'B', 'C']*10**5; List2 = ['1', '2','3']*10**5" "map(lambda x, y: x + y, List1, List2)"
10 loops, best of 3: 44.1 msec per loop

C:\Users\Henry>python -m timeit -s "from itertools import izip" "List1 = ['A', 'B', 'C']*10**5; List2 = ['1', '2', '3']*10**5" "[x + y for x, y in izip(List1, List2)]"
10 loops, best of 3: 31.3 msec per loop

C:\Users\Henry>python -m timeit -s "from itertools import izip" "List1 = ['A', 'B', 'C']*10**5; List2 = ['1', '2', '3']*10**5" "map(''.join,izip(List1,List2))"
10 loops, best of 3: 36.2 msec per loop

C:\Users\Henry>python -m timeit -s "from operator import add" "List1 = ['A', 'B', 'C']*10**5; List2 = ['1', '2', '3']*10**5" "map(add, List1, List2)"
10 loops, best of 3: 28.6 msec per loop