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NumPy Array Slicing


Slicing arrays

Slicing in python means taking elements from one given index to another given index.

We pass slice instead of index like this: [start:end].

We can also define the step, like this: [start:end:step].

If we don't pass start its considered 0

If we don't pass end its considered length of array in that dimension

If we don't pass step its considered 1

Example

Slice elements from index 1 to index 5 from the following array:

import numpy as np

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

print(arr[1:5])
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Note: The result includes the start index, but excludes the end index.

Example

Slice elements from index 4 to the end of the array:

import numpy as np

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

print(arr[4:])
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Example

Slice elements from the beginning to index 4 (not included):

import numpy as np

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

print(arr[:4])
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Negative Slicing

Use the minus operator to refer to an index from the end:

Example

Slice from the index 3 from the end to index 1 from the end:

import numpy as np

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

print(arr[-3:-1])
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STEP

Use the step value to determine the step of the slicing:

Example

Return every other element from index 1 to index 5:

import numpy as np

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

print(arr[1:5:2])
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Example

Return every other element from the entire array:

import numpy as np

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

print(arr[::2])
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Slicing 2-D Arrays

Example

From the second element, slice elements from index 1 to index 4 (not included):

import numpy as np

arr = np.array([[1, 2, 3, 4, 5], [6, 7, 8, 9, 10]])

print(arr[1, 1:4])
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Note: Remember that second element has index 1.

Example

From both elements, return index 2:

import numpy as np

arr = np.array([[1, 2, 3, 4, 5], [6, 7, 8, 9, 10]])

print(arr[0:2, 2])
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Example

From both elements, slice index 1 to index 4 (not included), this will return a 2-D array:

import numpy as np

arr = np.array([[1, 2, 3, 4, 5], [6, 7, 8, 9, 10]])

print(arr[0:2, 1:4])
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Test Yourself With Exercises

Exercise:

Insert the correct slicing syntax to print the following selection of the array:

Everything from (including) the second item to (not including) the fifth item.

arr = np.array([10, 15, 20, 25, 30, 35, 40])

print(arr)

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