Difference between Python List and Numpy array
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There are 2 major benefits of Numpy arrays over Python lists:
Broadcasting: when you need to perform calculations over all the items in an array / list
Indexing: when you have a 2D list /array, it is easier and more efficient to retrieve a single column using arrays
Broadcasting
For example, say we need to divide all student raw scores in a test by 2 to arrive at the final score. If we try to perform the following using a Python list:
raw_scores = [50, 60, 70]final_scores = raw_scores / 2final_scores
TypeError: unsupported operand type(s) for /: 'list' and 'int'
We get a TypeError as such operations are not supported by Python lists.
Instead using a Numpy Array:
raw_scores = np.array([50, 60, 70])final_scores = raw_scores / 2final_scores
array([25., 30., 35.])
As we can see, each element in the array has been divided by 2.
All elements in an array have to be of the same data type. (Not the case for Python list, which can handle elements of different types).
Indexing
To retrieve first column of 2D Numpy Array:
numpy_2d_array = np.array([[1,2,3], [4,5,6]])numpy_2d_array[:, 0]
array([1, 4])
To retrieve first column of 2D list we must use list comprehension:
list2d = [[1,2,3], [4,5,6]][row[0] for row in list2d ]
[1, 4]
