112 lines
1.5 KiB
Markdown
112 lines
1.5 KiB
Markdown
# Advanced Indexing
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{:.no_toc}
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<nav markdown="1" class="toc-class">
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* TOC
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{:toc}
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</nav>
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## The goal
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Beside slicing there is something called advanced indexing
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Questions to [David Rotermund](mailto:davrot@uni-bremen.de)
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## Boolean Array
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We can use Boolean arrays for more complicate indexing:
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```python
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import numpy as np
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a = np.arange(1,10).reshape(3,3)
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b = np.zeros_like(a)
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b[a.sum(axis=1) > 6, :] = 1
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print(a)
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print()
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print(b)
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```
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Output:
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```python
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[[1 2 3]
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[4 5 6]
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[7 8 9]]
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[[0 0 0]
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[1 1 1]
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[1 1 1]]
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```
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Behind the curtains more or less this happens:
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```python
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import numpy as np
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a = np.arange(1, 10).reshape(3, 3)
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b = np.zeros_like(a)
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temp_0 = a.sum(axis=1)
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temp_1 = temp_0 > 6
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temp_2 = np.nonzero(temp_1)
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b[temp_2] = 1
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print(temp_0)
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print()
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print(temp_1)
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print()
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print(temp_2)
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print()
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print(b)
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```
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Output:
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```python
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[ 6 15 24]
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[False True True]
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(array([1, 2]),)
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[[0 0 0]
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[1 1 1]
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[1 1 1]]
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```
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## Index vs Slices / Views
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This procedure is called indexing:
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```python
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import numpy as np
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a = np.arange(0, 10)
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idx = np.arange(2,5)
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b = a[idx]
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print(idx) # -> [2 3 4]
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print()
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print(b) # -> [2 3 4]
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print()
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print(np.may_share_memory(a,b)) # -> False
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```
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While this is called slicing:
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```python
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import numpy as np
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a = np.arange(0, 10)
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b = a[2:5]
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print(b) # -> [2 3 4]
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print()
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print(np.may_share_memory(a, b)) # -> True
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```
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As you can see lies the biggest different in the creation of a view when we use slicing. Indexing creates a new object instead.
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