Using zip function for combining long list of lists

Question

suppose I have a list as

mylist = [[1,2,3],[4,5,6],[7,8,9],[10,11,12],[13,14,15],[16,17,18]]

If I want to add element-by-element then I can use logic as:

[x+y for x,y in zip(mylist[0],mylist[1],mylist[2],mylist[3],mylist[4],mylist[6]]

and it will give sum of columnwise elements. But the problem is , writing each index of mylist inside zip function is somewhat awkward and vague if mylist happens to be list of say 1000 lists and I need to do the element by element operation.

I tried putting loop inside the zip function but it doesn't work. So any idea for that??

It should work like

zip(for k in mylist) # or something else

Thanks

Your example has a syntax error and a logic error in it, just try to run it. But I think what you are looking for is zip(*mylist). — fjarri
– fjarri, Commented Oct 15, 2013 at 10:30
Look at 'reduce': docs.python.org/2/library/functions.html#reduce — Don
– Don, Commented Oct 15, 2013 at 10:30

Lev Levitsky · Accepted Answer · 2013-10-15 10:31:06Z

3

To sum all the first elements, all the second elements and so on:

In [2]: [sum(k) for k in zip(*mylist)]
Out[2]: [51, 57, 63]

Note that each number in the output is a sum of len(mylist) numbers:

In [3]: zip(*mylist)
Out[3]: [(1, 4, 7, 10, 13, 16), (2, 5, 8, 11, 14, 17), (3, 6, 9, 12, 15, 18)]

The * operator unpacks mylist so that its items become separate arguments to zip.

answered Oct 15, 2013 at 10:31

Lev Levitsky

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Abhijit · Accepted Answer · 2013-10-15 10:52:32Z

2

Another approach, that looks more obvious here, is to use the built-in map with sum along side zip on an unpacked list

>>> map(sum, zip(*mylist))
[51, 57, 63]

And if you are a fan of generators, the following might add some benefits

>>> from itertools import izip, imap
>>> list(imap(sum, izip(*mylist)))
[51, 57, 63]

answered Oct 15, 2013 at 10:52

Abhijit

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