python - IndexError: too many indices. Numpy Array with 1 row and 2 columns

Question

When I try to get just the first element of an array like this

import numpy

a = numpy.array([1,2])

a[:,0]

I get this error

---------------------------------------------------------------------------
 IndexError                                Traceback (most recent call last)
<ipython-input-3-ed371621c46c> in <module>()
----> 1 a[:,0]

IndexError: too many indices

I would like to find a way to do this while still using slicing because the full code opens and reads many different files using numpy.loadtxt() all having two columns which vary from 1 to some N.

Answer 1

Your array a = numpy.array([1,2]) only has one dimension: its shape is (2,). However, your slice a[:,0] specifies selections for two dimensions. This causes NumPy to raise the error.

To get the first element from a you only need to write a[0] (a selection for only one dimension is being made here).

---

Looking at your other question, if you want to ensure that the syntax a[:,0] always works, you could ensure that a always has two dimensions. When loading an array with np.loadtxt use the ndmin parameter, e.g.:

np.loadtxt(F, skiprows=0, ndmin=2)