np.shape()

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This tutorial explains NumPy’s shape() function.

numpy.shape(a)

Return the shape of an array or array_like object a.

ArgumentData TypeDescription
aarray_likeNumPy array or Python list for which the shape should be returned. If it is a NumPy array, it returns the attribute a.shape. If it is a Python list, it returns a tuple of integer values defining the number of elements in each dimension if you would’ve created a NumPy array from it.

Return Value: shape — a tuple of integers that are set to the lengths of the corresponding array dimensions.

Examples

The straightforward example is when applied to a NumPy array:

>>> import numpy as np
>>> a = np.array([[1, 2], [3, 4]])
>>> np.shape(a)
(2, 2)

You import the NumPy library and create a two-dimensional array from a list of lists. If you pass the NumPy array into the shape function, it returns a tuple with two values (=dimensions). Each dimension stores the number of elements in this dimension (=axis). As it is a 2×2 quadratic matrix, the result is (2,2).

The following shape is another example of a multi-dimensional array:

>>> b = np.array([[1, 2, 3, 4], [5, 6, 7, 8]])
>>> b
array([[1, 2, 3, 4],
       [5, 6, 7, 8]])
>>> b.shape
(2, 4)
>>> np.shape(b)
(2, 4)

The shape is now (2, 4) with two rows and four columns.

np.shape() vs array.shape

Note that the result of np.shape(b) and b.shape is the same if b is a NumPy array. If b isn’t a NumPy array but a list, you cannot use b.shape as lists don’t have the shape attribute. Let’s have a look at this example:

>>> b = [[1, 2, 3, 4], [5, 6, 7, 8]]
>>> np.shape(b)
(2, 4)

The np.shape() function returns the same shape tuple—even if you pass a nested list into the function instead of a NumPy array.

But if you try to access the list.shape attribute, NumPy throws the following error:

>>> b.shape
Traceback (most recent call last):
  File "<pyshell#9>", line 1, in <module>
    b.shape
AttributeError: 'list' object has no attribute 'shape'

So, the difference between np.shape() and array.shape is that the former can be used for all kinds of array_like objects while the latter can only be used for NumPy arrays with the shape attribute.

Recap NumPy Shape

The shape attribute always returns a tuple that tells us the length of each dimension. The one-dimensional array is a row vector and its shape is a single value iterable followed by a comma. One-dimensional arrays don’t have rows and columns, so the shape attribute returns a single value tuple.

Let’s look at an example:

import numpy as np

#an one-dimensional NumPy array
arr = np.arange(10)

#print an array
print(arr)
# [0 1 2 3 4 5 6 7 8 9]

print(arr.shape)
# (10, )

The code snippet also uses the NumPy arange function to create an initial array of subsequent values between 0 and 9. Please find a detailed discussion of the NumPy arange function in this Finxter blog article: https://blog.finxter.com/numpy-arange/.

The shape attribute of a two-dimensional array (also called a matrix) gives us a tuple. The shape returns the number of elements along each dimension, which is the number of rows and columns in the two-dimensional array.

# A two-dimensional NumPy array
import numpy as np

arr = np.array([[1,2,3,4,5], [5,4,3,2,1]])
print(arr.shape)
# (2, 5)

The following example is for the shape of three-dimensional arrays.

# A three-dimensional array
import numpy as np

arr = np.array([ [ [0, 11, 15, 16], [3, 7, 10, 34], [44, 99, 5, 67] ],[ [52, 8, 11, 13], [0, 4, 5, 6], [4, 4, 4, 4] ] ])
print(arr.shape)
# (2, 3, 4)

It takes some practice to understand the shape tuple for multidimensional arrays. The dimensions represented by a tuple are read from the outside-in. If you observe the brackets, the outmost bracket is a part of the basic syntax for the whole array. In the shape tuple 2 represents the second set of brackets. If you count them you will see that there are 2 elements in this dimension.

1st element [ [0, 11, 15, 16], [3, 7, 10, 34], [44, 99, 5, 67] ]

2nd element [ [52, 8, 11, 13], [0, 4, 5, 6], [4, 4, 4, 4] ]

Each element contains 3 more elements in the second dimension. If you think about nested lists, you can draw the analogy. These elements are:

1st element [0, 11, 15, 16]

2nd element [3, 7, 10, 34]

3rd element [44, 99, 5, 67]

Finally, number 4 represents the number of elements in the third dimension. Those are the innermost elements. For example 0, 11, 15 and 16.

The Ultimate Guide to NumPy Reshape() in Python

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References