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#+TITLE:    Accessing parts of arrays

#+AUTHOR:    FOSSEE

#+EMAIL:     

#+DATE:    

#+DESCRIPTION: 

#+KEYWORDS: 

#+LANGUAGE:  en

#+OPTIONS:   H:3 num:nil toc:nil \n:nil @:t ::t |:t ^:t -:t f:t *:t <:t

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* Outline

  - Manipulating one and multi dimensional arrays

  - Access and change individual elements 

  - Access and change rows and columns 

  - Slicing and striding on arrays to access chunks 

  - Read images into arrays and manipulations

* Sample Arrays

  #+begin_src python

    In []: A = array([12, 23, 34, 45, 56])

    In []: C = array([[11, 12, 13, 14, 15],

                      [21, 22, 23, 24, 25],

                      [31, 32, 33, 34, 35],

                      [41, 42, 43, 44, 45],

                      [51, 52, 53, 54, 55]])

  #+end_src

* Question 1

  Change the last column of ~C~ to zeroes. 

* Solution 1

  #+begin_src python

    In []:  C[:, -1] = 0

  #+end_src

* Question 2

  Change ~A~ to ~[11, 12, 13, 14, 15]~. 

* Solution 2

  #+begin_src python

    In []:  A[:] = [11, 12, 13, 14, 15]

  #+end_src

* squares.png

  #+begin_latex

    \begin{center}

      \includegraphics[scale=0.6]{squares}    

    \end{center}

  #+end_latex

* Question 3

  - obtain ~[22, 23]~ from ~C~. 

  - obtain ~[11, 21, 31, 41]~ from ~C~. 

  - obtain ~[21, 31, 41, 0]~.   

* Solution 3

  #+begin_src python

    In []:  C[1, 1:3]

    In []:  C[0:4, 0]

    In []:  C[1:5, 0]

  #+end_src

* Question 4

  Obtain ~[[23, 24], [33, -34]]~ from ~C~

* Solution 4

  #+begin_src python

    In []:  C[1:3, 2:4]

  #+end_src

* Question 5

  Obtain the square in the center of the image

* Solution 5

  #+begin_src python

    In []: imshow(I[75:225, 75:225])

  #+end_src

* Question 6

  Obtain the following

  #+begin_src python

    [[12, 0], [42, 0]]

    [[12, 13, 14], [0, 0, 0]]

  #+end_src

* Solution 6

  #+begin_src python

    In []: C[::3, 1::3]

    In []: C[::4, 1:4]

  #+end_src

* Summary

  You should now be able to --

      - Access and change individual elements 

      - Access and change rows and columns 

      - Slice and stride on arrays

  - Read images into arrays and manipulate them.

* Thank you!

#+begin_latex

  \begin{block}{}

  \begin{center}

  This spoken tutorial has been produced by the

  \textcolor{blue}{FOSSEE} team, which is funded by the 

  \end{center}

  \begin{center}

    \textcolor{blue}{National Mission on Education through \\

      Information \& Communication Technology \\ 

      MHRD, Govt. of India}.

  \end{center}  

  \end{block}

#+end_latex