[FIXED] numpy - how to add a value to every element in the first column of an array?

Issue

I have an array like this:

array([('6506', 4.6725971801473496e-25, 0.99999999995088695),
       ('6601', 2.2452745388799898e-27, 0.99999999995270605),
       ('21801', 1.9849650921836601e-31, 0.99999999997999001), ...,
       ('45164194', 1.0413482803123399e-24, 0.99999999997453404),
       ('45164198', 1.09470356446595e-24, 0.99999999997635303),
       ('45164519', 3.7521365799080699e-24, 0.99999999997453404)], 
      dtype=[('pos', '|S100'), ('par1', '<f8'), ('par2', '<f8')])

And I want to turn it into this: (adding a prefix '2R' onto each value in the first column)

array([('2R:6506', 4.6725971801473496e-25, 0.99999999995088695),
       ('2R:6601', 2.2452745388799898e-27, 0.99999999995270605),
       ('2R:21801', 1.9849650921836601e-31, 0.99999999997999001), ...,
       ('2R:45164194', 1.0413482803123399e-24, 0.99999999997453404),
       ('2R:45164198', 1.09470356446595e-24, 0.99999999997635303),
       ('2R:45164519', 3.7521365799080699e-24, 0.99999999997453404)], 
      dtype=[('pos', '|S100'), ('par1', '<f8'), ('par2', '<f8')])

I looked up some stuff about nditer (but I want to support earlier versions of numpy.) Also I'm reading one should avoid iteration.

Solution

Using numpy.core.defchararray.add:

>>> from numpy import array
>>> from numpy.core.defchararray import add
>>>
>>> xs = array([('6506', 4.6725971801473496e-25, 0.99999999995088695),
...             ('6601', 2.2452745388799898e-27, 0.99999999995270605),
...             ('21801', 1.9849650921836601e-31, 0.99999999997999001),
...             ('45164194', 1.0413482803123399e-24, 0.99999999997453404),
...             ('45164198', 1.09470356446595e-24, 0.99999999997635303),
...             ('45164519', 3.7521365799080699e-24, 0.99999999997453404)],
...            dtype=[('pos', '|S100'), ('par1', '<f8'), ('par2', '<f8')])
>>> xs['pos'] = add('2R:', xs['pos'])
>>> xs
array([('2R:6506', 4.67259718014735e-25, 0.999999999950887),
       ('2R:6601', 2.24527453887999e-27, 0.999999999952706),
       ('2R:21801', 1.98496509218366e-31, 0.99999999997999),
       ('2R:45164194', 1.04134828031234e-24, 0.999999999974534),
       ('2R:45164198', 1.09470356446595e-24, 0.999999999976353),
       ('2R:45164519', 3.75213657990807e-24, 0.999999999974534)],
      dtype=[('pos', 'S100'), ('par1', '<f8'), ('par2', '<f8')])

UPDATE: You can use num.char.add instead of numpy.core.defchararray.add (commented by @joel-buursma):

>>> import numpy
>>> numpy.char == numpy.core.defchararray
True

Answered By - falsetru