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Transposing a 1D NumPy array

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How do you transpose 1 dimensional array NumPy?

To transpose an array or matrix in NumPy, we have to use the T attribute that stores the transposed array or matrix. T attribute is exclusive to NumPy arrays, that is, ndarray only.

Can you transpose a 1D array?

The transpose() method can transpose the 2D arrays; on the other hand, it does not affect 1D arrays. Thus, the transpose() method transposes the 2D numpy array.

How do you transpose a NumPy array?

Use transpose(a, argsort(axes)) to invert the transposition of tensors when using the axes keyword argument. Transposing a 1-D array returns an unchanged view of the original array.

How do I transpose a column in NumPy?

To transpose NumPy array ndarray (swap rows and columns), use the T attribute ( . T ), the ndarray method transpose() and the numpy. transpose() function.


It's working exactly as it's supposed to. The transpose of a 1D array is still a 1D array! (If you're used to matlab, it fundamentally doesn't have a concept of a 1D array. Matlab's "1D" arrays are 2D.)

If you want to turn your 1D vector into a 2D array and then transpose it, just slice it with np.newaxis (or None, they're the same, newaxis is just more readable).

import numpy as np
a = np.array([5,4])[np.newaxis]
print(a)
print(a.T)

Generally speaking though, you don't ever need to worry about this. Adding the extra dimension is usually not what you want, if you're just doing it out of habit. Numpy will automatically broadcast a 1D array when doing various calculations. There's usually no need to distinguish between a row vector and a column vector (neither of which are vectors. They're both 2D!) when you just want a vector.


Use two bracket pairs instead of one. This creates a 2D array, which can be transposed, unlike the 1D array you create if you use one bracket pair.

import numpy as np    
a = np.array([[5, 4]])
a.T

More thorough example:

>>> a = [3,6,9]
>>> b = np.array(a)
>>> b.T
array([3, 6, 9])         #Here it didn't transpose because 'a' is 1 dimensional
>>> b = np.array([a])
>>> b.T
array([[3],              #Here it did transpose because a is 2 dimensional
       [6],
       [9]])

Use numpy's shape method to see what is going on here:

>>> b = np.array([10,20,30])
>>> b.shape
(3,)
>>> b = np.array([[10,20,30]])
>>> b.shape
(1, 3)

For 1D arrays:

a = np.array([1, 2, 3, 4])
a = a.reshape((-1, 1)) # <--- THIS IS IT

print a
array([[1],
       [2],
       [3],
       [4]])

Once you understand that -1 here means "as many rows as needed", I find this to be the most readable way of "transposing" an array. If your array is of higher dimensionality simply use a.T.


You can convert an existing vector into a matrix by wrapping it in an extra set of square brackets...

from numpy import *
v=array([5,4]) ## create a numpy vector
array([v]).T ## transpose a vector into a matrix

numpy also has a matrix class (see array vs. matrix)...

matrix(v).T ## transpose a vector into a matrix

numpy 1D array --> column/row matrix:

>>> a=np.array([1,2,4])
>>> a[:, None]    # col
array([[1],
       [2],
       [4]])
>>> a[None, :]    # row, or faster `a[None]`
array([[1, 2, 4]])

And as @joe-kington said, you can replace None with np.newaxis for readability.


To 'transpose' a 1d array to a 2d column, you can use numpy.vstack:

>>> numpy.vstack(numpy.array([1,2,3]))
array([[1],
       [2],
       [3]])

It also works for vanilla lists:

>>> numpy.vstack([1,2,3])
array([[1],
       [2],
       [3]])