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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