add section on selecting one element from each row of an array

Author: jcjohnson

python-numpy-tutorial.md

@@ -559,7 +559,6 @@ print a[[0, 1, 2], [0, 1, 0]]  # Prints "[1 4 5]"
 # The above example of integer array indexing is equivalent to this:
 print np.array([a[0, 0], a[1, 1], a[2, 0]])  # Prints "[1 4 5]"
 
-
 # When using integer array indexing, you can reuse the same
 # element from the source array:
 print a[[0, 0], [1, 1]]  # Prints "[2 2]"
@@ -568,6 +567,35 @@ print a[[0, 0], [1, 1]]  # Prints "[2 2]"
 print np.array([a[0, 1], a[0, 1]])  # Prints "[2 2]"
 ```
 
+One useful trick with integer array indexing is selecting or mutating one
+element from each row of a matrix:
+
+```python
+import numpy as np
+
+# Create a new array from which we will select elements
+a = np.array([[1,2,3], [4,5,6], [7,8,9], [10, 11, 12]])
+
+print a  # prints "array([[ 1,  2,  3],
+         #                [ 4,  5,  6],
+         #                [ 7,  8,  9],
+         #                [10, 11, 12]])"
+
+# Create an array of indices
+b = np.array([0, 2, 0, 1])
+
+# Select one element from each row of a using the indices in b
+print a[np.arange(4), b]  # Prints "[ 1  6  7 11]"
+
+# Mutate one element from each row of a using the indices in b
+a[np.arange(4), b] += 10
+
+print a  # prints "array([[11,  2,  3],
+         #                [ 4,  5, 16],
+         #                [17,  8,  9],
+         #                [10, 21, 12]])
+```
+
 **Boolean array indexing:**
 Boolean array indexing lets you pick out arbitrary elements of an array.
 Frequently this type of indexing is used to select the elements of an array