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Indexing in NumPy

Explore the fundamentals of indexing in NumPy arrays to efficiently access and manipulate multi-dimensional data. Learn how to retrieve single values, rows, columns, subsets, and specific elements from grids, and understand how to arrange and extract values using advanced indexing techniques.

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Indexing means to refer to any value in an array. Each item in a numpy array is stored at a specific index. To access value at a specific index write:

Z=np.arrange(9)
Z[0] #get the value at index 0

Get the First Value

To get the first value of a matrix, write: Z[0,0].

   ┏━━━┓───┬───┐   ┏━━━┓
   ┃ 0 ┃ 1 │ 2 │ → ┃ 0 ┃ (scalar)
   ┗━━━┛───┼───┤   ┗━━━┛
Z  │ 3 │ 4 │ 5 │  
   ├───┼───┼───┤
   │ 6 │ 7 │ 8 │
   └───┴───┴───┘
Python 3.5
import numpy as np
Z = np.arange(9).reshape(3,3)
print(Z[0,0])

Get the Last Value

To get the last value of a matrix, write: Z[-1,-1].

   ┌───┬───┬───┐
   │ 0 │ 1 │ 2 │
   ├───┼───┼───┤
 Z │ 3 │ 4 │ 5 │
   ├───┼───┏━━━┓   ┏━━━┓
   │ 6 │ 7 ┃ 8 ┃ → ┃ 8 ┃ (scalar)
   └───┴───┗━━━┛   ┗━━━┛
Python 3.5
import numpy as np
Z = np.arange(9).reshape(3,3)
print(Z[-1,-1])

Get a row from a Grid

To get a row from a grid, write: Z[row_index].

To get the first row from a grid, write: Z[1].

   ┌───┬───┬───┐   
   │ 0 │ 1 │ 2 │ 
   ┏━━━┳━━━┳━━━┓   ┏━━━┳━━━┳━━━┓
Z  ┃ 3 ┃ 4 ┃ 5 ┃ → ┃ 3 ┃ 4 ┃ 5 ┃
   ┗━━━┻━━━┻━━━┛   ┗━━━┻━━━┻━━━┛
   │ 6 │ 7 │ 8 │      (view)
   └───┴───┴───┘
Python 3.5
import numpy as np
Z = np.arange(9).reshape(3,3)
print(Z[1])

Get a Column from a Grid

To get the column from a grid, use Z[:,column_index]

To get the second column from a grid, use ...