Indexing
Index into NumPy arrays to extract data and array slices.
We'll cover the following...
Chapter Goals:
- Learn about indexing arrays in NumPy
- Write code for indexing and slicing arrays
A. Array accessing
Accessing NumPy arrays is identical to accessing Python lists. For multi-dimensional arrays, it is equivalent to accessing Python lists of lists.
The code below shows example accesses of NumPy arrays.
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arr = np.array([1, 2, 3, 4, 5])print(arr[0])print(arr[4])arr = np.array([[6, 3], [0, 2]])# Subarrayprint(repr(arr[0]))
B. Slicing
NumPy arrays also support slicing. Similar to Python, we use the colon operator (i.e. arr[:]
) for slicing. We can also use negative indexing to slice in the backwards direction.
The code below shows example slices of a 1-D NumPy array.
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arr = np.array([1, 2, 3, 4, 5])print(repr(arr[:]))print(repr(arr[1:]))print(repr(arr[2:4]))print(repr(arr[:-1]))print(repr(arr[-2:]))
For multi-dimensional arrays, we can use a comma to separate slices across each dimension.
The code below shows example slices of a 2-D NumPy array.
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arr = np.array([[1, 2, 3],[4, 5, 6],[7, 8, 9]])print(repr(arr[:]))print(repr(arr[1:]))print(repr(arr[:, -1]))print(repr(arr[:, 1:]))print(repr(arr[0:1, 1:]))print(repr(arr[0, 1:]))
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