Merging/adding lists in Python

Question

I'm pretty sure there should be a more Pythonic way of doing this - but I can't think of one: How can I merge a two-dimensional list into a one-dimensional list? Sort of like zip/map but with more than two iterators.

Example - I have the following list:

array = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] 

I want to have

result = [12, 15, 18] # [1+4+7, 2+5+8, 3+6+9] 

So far what I've come up with is:

def add_list(array):     number_items = len(array[0])     result = [0] * number_items     for index in range(number_items):         for line in array:             result[index] += line[index]     return result 

But that doesn't look very elegant/Pythonic to me. Aside from not checking if all the "lines" in the 2D array are of the same length, can be added to each other, etc. What would be a better way to do it?

Answer 1

[sum(a) for a in zip(*array)] 

Answer 2

[sum(value) for value in zip(*array)] is pretty standard.

This might help you understand it:

In [1]: array=[[1, 2, 3], [4, 5, 6], [7, 8, 9]]  In [2]: array Out[2]: [[1, 2, 3], [4, 5, 6], [7, 8, 9]]  In [3]: *array ------------------------------------------------------------    File "<ipython console>", line 1      *array      ^ <type 'exceptions.SyntaxError'>: invalid syntax 

The unary star is not an operator by itself. It unwraps array elements into arguments into function calls.

In [4]: zip(*array) Out[4]: [(1, 4, 7), (2, 5, 8), (3, 6, 9)] 

zip() is a built-in function

In [5]: zip(*array)[0] Out[5]: (1, 4, 7) 

each element for the list returned by zip is a set of numbers you want.

In [6]: sum(zip(*array)[0]) Out[6]: 12  In [7]: [sum(values) for values in zip(*array)] Out[7]: [12, 15, 18]