How to add a calculated/computed column in numpy?

Question

Suppose I have a numpy array:

1 10
2 20
3 0
4 30

and I want to add a third column where each row is the sum (or some arbitrary calculation) of the first two columns in that row:

1 10 11
2 20 22
3 0  3
4 30 34

How do I do that?

Answer 1

For these kinds of calculations the built-in map function is very useful. It only remains to add the result of the calculation to the third column. For summing:

>>> import numpy as np
>>> my_arr = np.array([[1, 10], [2, 20], [3, 0], [4, 30]])
>>> np.vstack( (my_arr.T, map(sum, my_arr) )).T
array([[ 1, 10, 11],
       [ 2, 20, 22],
       [ 3,  0,  3],
       [ 4, 30, 34]])

It also works with other functions:

>>> my_func = lambda x: 2*x[0] + x[1]
>>> np.vstack( (my_arr.T, map(my_func, my_arr) )).T
array([[ 1, 10, 12],
       [ 2, 20, 24],
       [ 3,  0,  6],
       [ 4, 30, 38]])