Concise vector adding in Python? [duplicate]

Answer 1

Or, if you're willing to use an external library (and fixed-length arrays), use numpy, which has "+=" and related operations for in-place operations.

import numpy as np
a = np.array([0, 1, 2])
b = np.array([3, 4, 5])
a += b

Answer 2

While Numeric is excellent, and list-comprehension solutions OK if you actually wanted to create a new list, I'm surprised nobody suggested the "one obvious way to do it" -- a simple for loop! Best:

for i, bi in enumerate(b): a[i] += bi

Also OK, kinda sorta:

for i in xrange(len(a)): a[i] += b[i]

Answer 3

I don't think you will find a faster solution than the 3 sums proposed in the question. The advantages of numpy are visible with larger vectors, and also if you need other operators. numpy is specially useful with matrixes, witch are trick to do with python lists.

Still, yet another way to do it :D

In [1]: a = [1,2,3]

In [2]: b = [2,3,4]

In [3]: map(sum, zip(a,b))
Out[3]: [3, 5, 7]

Edit: you can also use the izip from itertools, a generator version of zip

In [5]: from itertools import izip

In [6]: map(sum, izip(a,b))
Out[6]: [3, 5, 7]

Answer 4

If you think Numpy is overkill, this should be really fast, because this code runs in pure C (map() and __add__() are both directly implemented in C):

a = [1.0,2.0,3.0]
b = [4.0,5.0,6.0]

c = map(float.__add__, a, b)

Or alternatively, if you don't know the types in the list:

import operator
c = map(operator.add, a, b)

Answer 5

list(map(lambda x:x[0]+x[1], zip(a,b)))

Answer 6

How about this:

a = [x+y for x,y in zip(a,b)]