feat(numpy): update exercise 6

- revert question 2 to its original version
- add question 3 to practice broadcasting

subjects/ai/numpy/README.md

@@ -156,7 +156,7 @@ The goal of this exercise is to learn to concatenate and reshape arrays.
 The goal of this exercise is to learn to access values of n-dimensional arrays efficiently.
 
 1. Create an 2-dimensional array size 9,9 of 1s. Each value has to be an `int8`.
-2. Using a combination of **slicing** and **broadcasting**, output this array:
+2. Using **slicing**, output this array:
 
    ```python
    array([[1, 1, 1, 1, 1, 1, 1, 1, 1],
@@ -170,6 +170,20 @@ The goal of this exercise is to learn to access values of n-dimensional arrays e
        [1, 1, 1, 1, 1, 1, 1, 1, 1]], dtype=int8)
    ```
 
+3. Using **broadcasting** create the ouptu matrix starting from these two arrays:
+
+   ```python
+   array_1 = np.array([1,2,3,4,5], dtype=int8)
+   array_2 = np.array([1,2,3], dtype=int8)
+   ...
+   # output matrix
+   array([[ 1,  2,  3],
+          [ 2,  4,  6],
+          [ 3,  6,  9],
+          [ 4,  8, 12],
+          [ 5, 10, 15]], dtype=int8)
+   ```
+
 https://jakevdp.github.io/PythonDataScienceHandbook/ (section: Computation on Arrays: Broadcasting)
 
 ---

subjects/ai/numpy/audit/README.md

@@ -186,17 +186,33 @@ https://jakevdp.github.io/PythonDataScienceHandbook/ (section: The Basics of Num
         [1, 1, 1, 1, 1, 1, 1, 1, 1]], dtype=int8)
 ```
 
-##### The solution of question 2 is not accepted if the values of the array have been changed one by one manually.
+##### The solution of question 2 is not accepted if the values of the array have been changed one by one manually. The usage of the for loop is not allowed neither.
 
 Here is an example of a possible solution:
 
 ```python
-for step in range(1, 5):
-    array_len = 9 - step * 2
-    if step % 2 == 1:
-        x[step:-step, step:-step] -= np.ones(array_len, dtype='int8')
-    else:
-        x[step:-step, step:-step] += np.ones(array_len, dtype='int8')
+        x[1:8,1:8] = 0
+        x[2:7,2:7] = 1
+        x[3:6,3:6] = 0
+        x[4,4] = 1
+```
+
+###### For question 3, is the output the following?
+
+```console
+   array([[ 1,  2,  3],
+          [ 2,  4,  6],
+          [ 3,  6,  9],
+          [ 4,  8, 12],
+          [ 5, 10, 15]], dtype=int8)
+```
+
+##### The solution of question 3 is not accepted if the values of the array have been changed one by one manually. The usage of the for loop is not allowed neither.
+
+Here is an example of a possible solution:
+
+```python
+    np.reshape(arr_1, (5, 1)) * arr_2
 ```
 
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