How can I check whether a numpy array is empty or not?
I used the following code, but this fails if the array contains a zero.
if not self.Definition.all():
Is this the solution?
if self.Definition == array([]):
4 Answers
You can always take a look at the .size attribute. It is defined as an integer, and is zero (0) when there are no elements in the array:
import numpy as np
a = np.array([])
if a.size == 0:
# Do something when `a` is empty
3 Comments
eric
This is great for numpy, but is unfortunate that it is considered unpythonic for lists. Check out discussion for lists: stackoverflow.com/questions/53513/… It would be nice to use same pattern for numpy arrays and lists.
asmeurer
NumPy code in general doesn't work properly on lists, or visa-versa. You have to write code in a different way if you are using lists vs. NumPy arrays.
Tian
a quick equivalent
if not a.size:One caveat, though.
Note that np.array(None).size returns 1!
This is because a.size is equivalent to np.prod(a.shape),
np.array(None).shape is (), and an empty product is 1.
>>> import numpy as np
>>> np.array(None).size
1
>>> np.array(None).shape
()
>>> np.prod(())
1.0
Therefore, I use the following to test if a NumPy array has elements:
>>> def elements(array):
... return array.ndim and array.size
>>> elements(np.array(None))
0
>>> elements(np.array([]))
0
>>> elements(np.zeros((2,3,4)))
24
4 Comments
vidstige
@DrBwts that's not relevant to this answer.
Navin
@DrBwts His array does not contain the
None object. Look at its shape.asmeurer
Arrays with shape
() are scalar arrays, which do contain an element (the scalar). In this example, the scalar is None (None doesn't have any special meaning, this is just an object array). It depends on what you are doing but you most likely do want to consider scalar arrays as not being empty.
aerobiomat
np.array(None).size == np.array(3).size == 1 because both arrays contain one element.https://numpy.org/devdocs/user/quickstart.html (2020.04.08)
NumPy’s main object is the homogeneous multidimensional array. It is a table of elements (usually numbers), all of the same type, indexed by a tuple of non-negative integers. In NumPy dimensions are called axes. (...) NumPy’s array class is called ndarray. (...) The more important attributes of an ndarray object are:
ndarray.ndim
the number of axes (dimensions) of the array.ndarray.shape
the dimensions of the array. This is a tuple of integers indicating the size of the array in each dimension. For a matrix with n rows and m columns, shape will be (n,m). The length of the shape tuple is therefore the number of axes, ndim.ndarray.size the total number of elements of the array. This is equal to the product of the elements of shape.
Comments
Why would we want to check if an array is empty? Arrays don't grow or shrink in the same that lists do. Starting with a 'empty' array, and growing with np.append is a frequent novice error.
Using a list in if alist: hinges on its boolean value:
In [102]: bool([])
Out[102]: False
In [103]: bool([1])
Out[103]: True
But trying to do the same with an array produces (in version 1.18):
In [104]: bool(np.array([]))
/usr/local/bin/ipython3:1: DeprecationWarning: The truth value
of an empty array is ambiguous. Returning False, but in
future this will result in an error. Use `array.size > 0` to
check that an array is not empty.
#!/usr/bin/python3
Out[104]: False
In [105]: bool(np.array([1]))
Out[105]: True
and bool(np.array([1,2]) produces the infamous ambiguity error.
edit
The accepted answer suggests size:
In [11]: x = np.array([])
In [12]: x.size
Out[12]: 0
But I (and most others) check the shape more than the size:
In [13]: x.shape
Out[13]: (0,)
Another thing in its favor is that it 'maps' on to an empty list:
In [14]: x.tolist()
Out[14]: []
But there are other other arrays with 0 size, that aren't 'empty' in that last sense:
In [15]: x = np.array([[]])
In [16]: x.size
Out[16]: 0
In [17]: x.shape
Out[17]: (1, 0)
In [18]: x.tolist()
Out[18]: [[]]
In [19]: bool(x.tolist())
Out[19]: True
np.array([[],[]]) is also size 0, but shape (2,0) and len 2.
While the concept of an empty list is well defined, an empty array is not well defined. One empty list is equal to another. The same can't be said for a size 0 array.
The answer really depends on
- what do you mean by 'empty'?
- what are you really test for?
len(array( [] ))is 0.len(array( [0] )is 1.len(array([[]])is 1 too!len()gives the number of dimensions in the first axis. But an array can have a non-zero dimension in the first axis but still be empty if it has a zero dimension in another axis.sizeis better as it is the product of all axes.