Use a function such as np.zeros. No need to worry about the value it generates since there are no values in a 0 x 2 array:
arr = np.zeros((0, 2))
arr.shape
(0, 2)
Answer from busybear on Stack Overflowpython - How can I create a numpy array with shape (0, 2)? - Stack Overflow
what's the difference between a numpy array with shape (x,) and (x, 1)?
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Use a function such as np.zeros. No need to worry about the value it generates since there are no values in a 0 x 2 array:
arr = np.zeros((0, 2))
arr.shape
(0, 2)
It can be done by np.empty((0,2)) or np.zeros((0,2)).
np.empty(), unlike np.zeros(), does not set the array values to zero and it is slightly faster.
Extra information:
In your code np.array([]).shape outputs a rank-1 array which is equal to its transpose.
hey, im doing an ai thing in school and my code didnt work as expected, and after 5 hours i found out i reshaped an array from (206,) to (206,1) and that made the results wrong. and from what i understand, the shape means the length of each dimension, and length is not 0 indexed so a size of 1 would be equal to just 1D no?
yourarray.shape or np.shape() or np.ma.shape() returns the shape of your ndarray as a tuple; And you can get the (number of) dimensions of your array using yourarray.ndim or np.ndim(). (i.e. it gives the n of the ndarray since all arrays in NumPy are just n-dimensional arrays (shortly called as ndarrays))
For a 1D array, the shape would be (n,) where n is the number of elements in your array.
For a 2D array, the shape would be (n,m) where n is the number of rows and m is the number of columns in your array.
Please note that in 1D case, the shape would simply be (n, ) instead of what you said as either (1, n) or (n, 1) for row and column vectors respectively.
This is to follow the convention that:
For 1D array, return a shape tuple with only 1 element (i.e. (n,))
For 2D array, return a shape tuple with only 2 elements (i.e. (n,m))
For 3D array, return a shape tuple with only 3 elements (i.e. (n,m,k))
For 4D array, return a shape tuple with only 4 elements (i.e. (n,m,k,j))
and so on.
Also, please see the example below to see how np.shape() or np.ma.shape() behaves with 1D arrays and scalars:
# sample array
In [10]: u = np.arange(10)
# get its shape
In [11]: np.shape(u) # u.shape
Out[11]: (10,)
# get array dimension using `np.ndim`
In [12]: np.ndim(u)
Out[12]: 1
In [13]: np.shape(np.mean(u))
Out[13]: () # empty tuple (to indicate that a scalar is a 0D array).
# check using `numpy.ndim`
In [14]: np.ndim(np.mean(u))
Out[14]: 0
P.S.: So, the shape tuple is consistent with our understanding of dimensions of space, at least mathematically.
Unlike it's most popular commercial competitor, numpy pretty much from the outset is about "arbitrary-dimensional" arrays, that's why the core class is called ndarray. You can check the dimensionality of a numpy array using the .ndim property. The .shape property is a tuple of length .ndim containing the length of each dimensions. Currently, numpy can handle up to 32 dimensions:
a = np.ones(32*(1,))
a
# array([[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[[ 1.]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]]])
a.shape
# (1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1)
a.ndim
# 32
If a numpy array happens to be 2d like your second example, then it's appropriate to think about it in terms of rows and columns. But a 1d array in numpy is truly 1d, no rows or columns.
If you want something like a row or column vector you can achieve this by creating a 2d array with one of its dimensions equal to 1.
a = np.array([[1,2,3]]) # a 'row vector'
b = np.array([[1],[2],[3]]) # a 'column vector'
# or if you don't want to type so many brackets:
b = np.array([[1,2,3]]).T