Accessing an elements within a array of list of arrays in Python 2.x

Question

Considering the following dataset extracted using numpy.genfromtxt():

    data[0:3]
array([('T', 2,  8, 3, 5, 1,  8, 13, 0, 6,  6, 10, 8, 0, 8, 0,  8),
       ('I', 5, 12, 3, 7, 2, 10,  5, 5, 4, 13,  3, 9, 2, 8, 4, 10),
       ('D', 4, 11, 6, 8, 6, 10,  6, 2, 6, 10,  3, 7, 3, 7, 3,  9)], 
      dtype=[('f0', 'S1'), ('f1', '<i8'), ('f2', '<i8'), ('f3', '<i8'), ('f4', '<i8'), ('f5', '<i8'), ('f6', '<i8'), ('f7', '<i8'), ('f8', '<i8'), ('f9', '<i8'), ('f10', '<i8'), ('f11', '<i8'), ('f12', '<i8'), ('f13', '<i8'), ('f14', '<i8'), ('f15', '<i8'), ('f16', '<i8')])

I am trying to retrieve the letters from the first 2 arrays using the following code but it's giving the first complete array instead of the first element from each array.

data[:2][0]

Ishan Manchanda · Accepted Answer · 2017-09-24 11:27:31Z

1

The issue is that the contents of the array are tuples. So, when you ask for data[:2][0] it returns the 0'th item in a list of tuples, and not the 0th item of each tuple. Use the following snippet:

output = []
for i in range(2):
    output += data[:2][i][0]

Hope I helped.

answered Sep 24, 2017 at 11:27

Ishan Manchanda

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1 Comment

hpaulj Over a year ago

They display like tuples, but are actually records of a structured array. Your output will be a list like [84, 73, 68]. Those are the numerical byte values of the bytestrings, [b'T', b'I', b'D'].

hpaulj · Accepted Answer · 2017-09-24 15:50:17Z

This is a structured array, 1d with many fields. Fields are accessed by name, not 'column' number:

In [618]: data
Out[618]: 
array([(b'T', 2,  8, 3, 5, 1,  8, 13, 0, 6,  6, 10, 8, 0, 8, 0,  8),
       (b'I', 5, 12, 3, 7, 2, 10,  5, 5, 4, 13,  3, 9, 2, 8, 4, 10),
       (b'D', 4, 11, 6, 8, 6, 10,  6, 2, 6, 10,  3, 7, 3, 7, 3,  9)],
      dtype=[('f0', 'S1'), ('f1', '<i8'), ('f2', '<i8'), ('f3', '<i8'), ('f4', '<i8'), ('f5', '<i8'), ('f6', '<i8'), ('f7', '<i8'), ('f8', '<i8'), ('f9', '<i8'), ('f10', '<i8'), ('f11', '<i8'), ('f12', '<i8'), ('f13', '<i8'), ('f14', '<i8'), ('f15', '<i8'), ('f16', '<i8')])

One element or record (displays like a tuple)

In [619]: data[0]
Out[619]: (b'T', 2, 8, 3, 5, 1, 8, 13, 0, 6, 6, 10, 8, 0, 8, 0, 8)

One field, an array:

In [620]: data['f0']
Out[620]: 
array([b'T', b'I', b'D'],
      dtype='|S1')

Several fields, returning another structured array (as illustrated in your next question, Access range of elements from an array Python)

In [621]: data[['f1','f2']]
Out[621]: 
array([(2,  8), (5, 12), (4, 11)],
      dtype=[('f1', '<i8'), ('f2', '<i8')])

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