快速汇总python列表列表的第i项的方法

Question

I am looking for an alternative approach of summing the ith item of a list of lists that is feasible over large data sets. Below is an example list and my approach.

j=[[1,2,3],[3,2,1],[2,1,3]]

My attempt:
h_0=zip(j[0],j[1],j[2])
h_1=[sum(x) for x in h]
print h_1

Output: [6, 5, 7]

Desired output:[6, 5, 7] ....the same as the output I got but would prefer a different approach as my approach is not feasible given the size of my actual data.

Thanks for your suggestions.

Answer 1

Use zip with * -operator (unpacking argument list) :

>>> j = [[1,2,3],[3,2,1],[2,1,3]]
>>> zip(*j)
[(1, 3, 2), (2, 2, 1), (3, 1, 3)]

with map :

>>> map(sum, zip(*j))
[6, 5, 7]
>>> list(map(sum, zip(*j))) # In Python 3.x
[6, 5, 7]

using list comprehension:

>>> [sum(cols) for cols in zip(*j)]
[6, 5, 7]

---

Using numpy :

>>> a = np.array(j)
>>> a.sum(axis=0)
array([6, 5, 7])