Transpose a 1-dimensional array in Numpy without casting to matrix

Question

My goal is to to turn a row vector into a column vector and vice versa. The documentation for numpy.ndarray.transpose says:

For a 1-D array, this has no effect. (To change between column and row vectors, first cast the 1-D array into a matrix object.)

However, when I try this:

my_array = np.array([1,2,3])
my_array_T = np.transpose(np.matrix(myArray))

I do get the wanted result, albeit in matrix form (matrix([[66],[640],[44]])), but I also get this warning:

PendingDeprecationWarning: the matrix subclass is not the recommended way to represent matrices or deal with linear algebra (see https://docs.scipy.org/doc/numpy/user/numpy-for-matlab-users.html). Please adjust your code to use regular ndarray.
my_array_T = np.transpose(np.matrix(my_array))

How can I properly transpose an ndarray then?

It looks like that suggestion in the docstring should be removed or changed to not recommend using numpy.matrix. — Warren Weckesser
– Warren Weckesser, Commented Nov 14, 2018 at 16:28

Matthieu Brucher · Accepted Answer · 2018-11-14 15:50:32Z

14

A 1D array is itself once transposed, contrary to Matlab where a 1D array doesn't exist and is at least 2D.

What you want is to reshape it:

my_array.reshape(-1, 1)

Or:

my_array.reshape(1, -1)

Depending on what kind of vector you want (column or row vector).

The -1 is a broadcast-like, using all possible elements, and the 1 creates the second required dimension.

answered Nov 14, 2018 at 15:50

Matthieu Brucher

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2 Comments

hpaulj Over a year ago

my_array[:, None] also does the needed reshape.

Matthieu Brucher Over a year ago

Agreed, that's also one way of doing reshape.

Warren Weckesser · Accepted Answer · 2018-11-14 16:30:35Z

2

If your array is my_array and you want to convert it to a column vector you can do:

my_array.reshape(-1, 1)

For a row vector you can use

my_array.reshape(1, -1)

Both of these can also be transposed and that would work as expected.

edited Nov 14, 2018 at 16:30

Warren Weckesser

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answered Nov 14, 2018 at 15:50

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Comments

rafaelc · Accepted Answer · 2018-11-14 15:50:23Z

1

IIUC, use reshape

my_array.reshape(my_array.size, -1)

answered Nov 14, 2018 at 15:50

rafaelc

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4 Comments

Matthieu Brucher Over a year ago

There is an extra hing here, you probably mean (-1, 1), the -1 in this case is really a 1.

rafaelc Over a year ago

@MatthieuBrucher either 1 and -1 are ok ;)

Matthieu Brucher Over a year ago

Yes, except that -1 does an additional computation, the easiest is (-1, 1).

rafaelc Over a year ago

@MatthieuBrucher The time difference would be really really negligible, but still fair point ;)

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