Question

I would like to combine several lists or arrays into a single record array.
In the following code I want to created a record array with two colums: "a" and "b". The first column will contain letters from "a" to "j", the second one will contain numbers from 0 to 9

In [22]: a = list('abcdefghij'); b = range(10); c = numpy.vstack((a, b)).T

In [23]: desc = {'names': ('a', 'b'), 'formats': ('S4', 'f4')}

In [24]: d = numpy.array(c, dtype=desc)

In [25]: d
Out[25]: 
array([[('a', 0.0), ('0', 0.0)],
       [('b', 0.0), ('1', 0.0)],
       [('c', 0.0), ('2', 0.0)],
       [('d', 0.0), ('3', 0.0)],
       [('e', 0.0), ('4', 0.0)],
       [('f', 0.0), ('5', 0.0)],
       [('g', 0.0), ('6', 0.0)],
       [('h', 0.0), ('7', 0.0)],
       [('i', 0.0), ('8', 0.0)],
       [('j', 0.0), ('9', 0.0)]], 
      dtype=[('a', '|S4'), ('b', '<f4')])

In [26]: d['a']
Out[26]: 
array([['a', '0'],
       ['b', '1'],
       ['c', '2'],
       ['d', '3'],
       ['e', '4'],
       ['f', '5'],
       ['g', '6'],
       ['h', '7'],
       ['i', '8'],
       ['j', '9']], 
      dtype='|S4')

In [27]: d['b']
Out[27]: 
array([[ 0.,  0.],
       [ 0.,  0.],
       [ 0.,  0.],
       [ 0.,  0.],
       [ 0.,  0.],
       [ 0.,  0.],
       [ 0.,  0.],
       [ 0.,  0.],
       [ 0.,  0.],
       [ 0.,  0.]], dtype=float32)

The result is completely not what I would expect. What I want is:

In [XX]: d['a']
Out[XX]: array(['a', 'b', 'c', ..., 'j'])
In [XX]: d['b']
Out[XX]: array([1., 2., 3., ..., 9.])

EDIT

My goal was to be able to create record arrays from already existing numpy.array, and not only from the individual lists. Following the answer by Sven Marnach, I had several tries and errors and this is what I got:

d = array(map(lambda l: tuple(l[0]), zip(c)), dtype=desc)

This seems to be a pretty ugly solution. Does anyone has a better one?

Answer 1

Following from your comment on Sven’s answer, if you don’t have a and b, then do the following and populate c later,

from numpy import recarray

desc = {'names': ('a', 'b'), 'formats': ('S4', 'f4')}
c = recarray((10,),dtype=desc)

a = list('abcdefghij'); b = range(10)

c['a'] = a
c['b'] = b

print r

gives,

[('a', 0.0) ('b', 1.0) ('c', 2.0) ('d', 3.0) ('e', 4.0) ('f', 5.0)
 ('g', 6.0) ('h', 7.0) ('i', 8.0) ('j', 9.0)]