Combining Three 1D arrays into a 2D array?

Question

I have three numpy arrays of shape (250L,) called c , I and error . I want to be able to save the three arrays to a text file so that each array is one column in the file. So far I have tried the following:

DataArray = np.concatenate((c,I,Error),axis=1)
np.savetxt('VaryingC.txt',DataArray)

This returns an error since the initial arrays are 1D:

IndexError: axis 1 out of bounds [0, 1)

How can I combine the three arrays to make a shape (250,3) array?

Answer 1

concatenate连接现有的轴， stack插入新的轴：

DataArray = np.stack((c,I,Error),axis=1)

Answer 2

It seems you are looking for:

DataArray = np.column_stack((c,I,Error))

Timing:

In [201]: a1 = np.random.randint(10**6, size=10**6)

In [202]: a2 = np.random.randint(10**6, size=10**6)

In [203]: a3 = np.random.randint(10**6, size=10**6)

In [204]: %timeit np.column_stack((a1,a2,a3))
100 loops, best of 3: 14.1 ms per loop

In [205]: %timeit np.stack((a1,a2,a3),axis=1)
100 loops, best of 3: 14.2 ms per loop

In [206]: %timeit np.transpose([a1,a2,a3])
100 loops, best of 3: 10.7 ms per loop

Answer 3

In your case you could just create a new array and then transpose it:

np.transpose([c, I, Error])

np.transpose automatically creates a new array, so you don't need to create one yourself.

For example:

>>> import numpy as np

>>> a = np.array([1, 1, 1, 1, 1])
>>> b = np.array([2, 2, 2, 2, 2])
>>> c = np.array([3, 3, 3, 3, 3])

>>> DataArray = np.transpose([a, b, c])
>>> DataArray
array([[1, 2, 3],
       [1, 2, 3],
       [1, 2, 3],
       [1, 2, 3],
       [1, 2, 3]])

>>> DataArray.shape
(5, 3)