Indexing and Selection in 2d with Numpy Arrays

2021-01-31

Indexing and Selection in 2d with Numpy Arrays

Matrices or one of the most useful structures to use in mathematics. While numpy does have a matrix data structure, it is more common to use a 2d array using the baisc np.array method. In this article, we will see how to select items from 2d arrays in numpy.

In our example below, we start by creating a 2d array with 3 rows and 3 columns.

2d = np.array([
	[1, 2, 3],
	[4, 5, 6],
	[7, 8, 9],
])

No, we see how to select rows. We can us normal array index notation, but we will receive rows back instead of indiviual elements.

# Select the first row
print(2d[0])

# Select the second row
print(2d[1])

# Select up to the first row
print(2d[:1])

Finally, we see how to select rows and columns. The notation is similar, but we add an extra selector after a comma. We can also use the slice operator to select multiple rows and columns.

# Select the first row, second column
print(2d[0, 1])

# Select the first two rows, second and third columns
print(2d[:2, 1:])
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