Merge pull request #4 from jetbrains-academy/sofia/array_indexing

Created Array indexing and slicing section;

Author: sofiiako

Numpy/Array Indexing and Slicing/Indexing Basics/__init__.py

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+type: edu
+files:
+- name: task.py
+  visible: true
+  placeholders:
+  - offset: 240
+    length: 7
+    placeholder_text: '# TODO'
+  - offset: 252
+    length: 11
+    placeholder_text: '# TODO'
+- name: tests/test_task.py
+  visible: false
+- name: __init__.py
+  visible: false
+- name: tests/__init__.py
+  visible: false

Numpy/Array Indexing and Slicing/Indexing Basics/task-info.yaml

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+## Indexing Basics
+
+Array indexing refers to any use of the square brackets (`[]`) to index array values.
+
+### Single element indexing
+
+Single element indexing for a 1-D array works exactly like that 
+for other standard Python sequences. It is 0-based, and accepts negative indices for 
+indexing from the end of the array.
+
+Unlike lists and tuples, NumPy arrays support multidimensional indexing for multidimensional 
+arrays. That means that **it is not necessary to separate each dimension’s index into its 
+own set of square brackets**.
+
+```python
+x = np.arange(10).reshape(2, 5) # 2-dimensional
+print(x[1, 3])
+print(x[1, -1])
+print(x[0])
+```
+Output:
+```text
+8
+9
+[0, 1, 2, 3, 4]
+```
+Note that `x[0, 2]` and `x[0][2]` will produce the same result, `2`. However, the second case 
+is less efficient as a new temporary array is created after the first index that is subsequently 
+indexed by 2.
+
+### Slicing
+
+It is possible to slice arrays to extract arrays of the same number of dimensions, 
+but of different sizes than the original. The slicing works exactly the same way it 
+does for lists and tuples except that they can be applied to multiple dimensions as well.
+Basic slicing occurs when obj is a slice object (constructed by `start:stop:step` notation inside 
+of brackets), an integer, or a tuple of slice objects and integers.
+
+1-D array:
+```python
+x = np.arange(35)
+print(x[2:15])
+print(x[1:30:5])  # [start:stop:step]
+```
+Output:
+```text
+[ 2  3  4  5  6  7  8  9 10 11 12 13 14]
+[ 1  6 11 16 21 26]
+```
+2-D array:
+```python
+y = x.reshape(5, 7)
+print(y[1:5:2])
+print(y[1:5:2, ::3])  # First slicing in the first dimension, then in the second.
+```
+Output:
+```text
+[[ 7  8  9 10 11 12 13]
+ [21 22 23 24 25 26 27]]
+[[ 7 10 13]
+ [21 24 27]]
+```
+
+These examples show the use of indexing when referencing data in an array. 
+They work just as well when assigning to an array.
+Read more about this topic [here](https://numpy.org/doc/stable/user/basics.indexing.html#basics-indexing).
+
+### Task
+1. Variable `a`: use indexing on the array `x` to extract number `19` from it. Please
+   **do not** just assign `a` the value `19` :)
+2. Variable `b`: use slicing to extract every second element in every second row. The resulting array 
+should have shape `(5, 2)`.
+   

Numpy/Array Indexing and Slicing/Indexing Basics/task.md

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+import numpy as np
+
+x = np.arange(40).reshape(10, 4)
+# Array x:
+# [[ 0  1  2  3]
+#  [ 4  5  6  7]
+#  [ 8  9 10 11]
+#  [12 13 14 15]
+#  [16 17 18 19]
+#  [20 21 22 23]
+#  [24 25 26 27]
+#  [28 29 30 31]
+#  [32 33 34 35]
+#  [36 37 38 39]]
+
+a = x[4, 3]
+b = x[::2, ::2]
+
+if __name__ == '__main__':
+    print(a)
+    print(b)

Numpy/Array Indexing and Slicing/Indexing Basics/task.py

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+import unittest
+import numpy as np
+
+from task import a, b, x
+
+
+class TestCase(unittest.TestCase):
+    def test_arrays_shape(self):
+        self.assertEqual(b.shape, (5, 2), msg="Wrong shape of array b")
+        self.assertEqual(type(a), np.int64, msg="a has to be a numpy.int64")
+
+    def test_array_content(self):
+        self.assertEqual(a, 19, msg="a has to be equal to 19")
+        np.testing.assert_array_equal(b, x[::2, ::2], err_msg='Something wrong in array b!')
+

Numpy/Array Indexing and Slicing/Indexing Basics/tests/__init__.py

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+content:
+- Indexing Basics

Numpy/Array Indexing and Slicing/Indexing Basics/tests/test_task.py

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 content:
 - Introduction
 - Array Basics
+- Array Indexing and Slicing