# Indexing

Lesson 10 Chapter 4

First, let's define and slice an array.

# Import Numpy library import numpy as np # Create a matrix A = np.array([[2,1,3,4],[5,2,9,4],[5,2,10,1],[2,2,11,-1]]) print('A=\n', A) # Extract the first two rows # Remember 0:2 in indexing means {0,1} and does NOT include 2! # Using : merely, means ALL! print('The first two rows= \n', A[0:2,:]) # Extract the first three rows and the last two columns # -2: means the second to the last to the end! # 0:3 mean {0,1,2} print('The first three rows and last two columns= \n', A[0:3,-2:]) # Let's point to one element # The second row (index 1) and third column (index 2) # Remember Python indexing starts from zero!! print('The second row and third column= \n', A[1,2])

A= [[ 2 1 3 4] [ 5 2 9 4] [ 5 2 10 1] [ 2 2 11 -1]] The first two rows= [[2 1 3 4] [5 2 9 4]] The first three rows and last two columns= [[ 3 4] [ 9 4] [10 1]] The second row and third column= 9

Let's review the code above. At **line 11**, I used **sliced indexing** by selecting from a range of indices. At **line 21**, I only used **integer** **indices**. We can simply combine both, but there might be a difference in the output matrix ranking. Check the example below:

# Import Numpy library import numpy as np # Create a matrix A = np.array([[2,1,3,4],[5,2,9,4],[5,2,10,1],[2,2,11,-1]]) print('A=\n', A) # Extract the first row and all colums using two approachs print('The first row with slice indexing= \n', A[0:1,:]) # Slice indexing print('The first row with integer indexing= \n', A[0,:]) # Integer indexing print('The first row shape slice indexing= \n', A[0:1,:].shape) # Slice indexing print('The first row shape integer indexing= \n', A[0,:].shape) # Integer indexing

A= [[ 2 1 3 4] [ 5 2 9 4] [ 5 2 10 1] [ 2 2 11 -1]] The first row with slice indexing= [[2 1 3 4]] The first row with integer indexing= [2 1 3 4] The first row shape slice indexing= (1, 4) The first row shape integer indexing= (4,)

If you see the results, the shape of the matrix would be different. Basically, ** with slice indexing, we have a rank-2 matrix** and with

**. Be careful about this difference when you are dealing with Numpy indexing.**

*integer indexing, we will have a rank-1 matrix*