accessing-pieces-arrays/script.rst
author Nishanth <nishanth@fossee.in>
Wed, 06 Oct 2010 16:15:42 +0530
changeset 221 7cd975ff5f0d
parent 217 b595f90016c5
child 224 4e0cdb5cca53
permissions -rw-r--r--
Added questions
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
217
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
     1
.. Objectives
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
     2
.. ----------
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
     3
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
     4
.. Clearly state the objectives of the LO (along with RBT level)
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
     5
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
     6
.. Prerequisites
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
     7
.. -------------
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
     8
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
     9
..   1. Name of LO-1
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    10
..   2. Name of LO-2
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    11
..   3. Name of LO-3
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    12
     
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    13
.. Author              : Puneeth
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    14
   Internal Reviewer   : 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    15
   External Reviewer   :
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    16
   Checklist OK?       : <put date stamp here, if OK> [2010-10-05]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    17
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    18
Script
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    19
------
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    20
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    21
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    22
{{{ Screen shows welcome slide }}}
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    23
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    24
Welcome to the tutorial on accessing pieces of arrays
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    25
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    26
{{{ Show the outline for this tutorial }}} 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    27
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    28
In this tutorial we shall learn to access individual elements of
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    29
arrays, get rows and columns and other chunks of arrays using
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    30
slicing and striding. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    31
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    32
{{{ switch back to the terminal }}}
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    33
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    34
As usual, we start IPython, using 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    35
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    36
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    37
  ipython -pylab 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    38
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    39
Let us have two arrays, A and C, as the sample arrays that we will
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    40
use to work through this tutorial. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    41
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    42
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    43
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    44
  A = array([12, 23, 34, 45, 56])
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    45
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    46
  C = array([[11, 12, 13, 14, 15],
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    47
             [21, 22, 23, 24, 25],
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    48
             [31, 32, 33, 34, 35],
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    49
             [41, 42, 43, 44, 45],
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    50
             [51, 52, 53, 54, 55]])
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    51
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    52
Pause the video here and make sure you have the arrays A and C,
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    53
typed in correctly.
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    54
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    55
Let us begin with the most elementary thing, accessing individual
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    56
elements. Also, let us first do it with the one-dimensional array
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    57
A, and then do the same thing with the two-dimensional array. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    58
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    59
To access, the element 34 in A, we say, 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    60
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    61
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    62
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    63
  A[1]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    64
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    65
Like lists, indexing starts from 0 in arrays, too. So, 34, the
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    66
third element has the index 2. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    67
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    68
Now, let us access the element 34 from C. To do this, we say
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    69
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    70
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    71
  C[2, 3]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    72
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    73
34 is in the third row and the fourth column, and since indexing
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    74
begins from zero, the row index is 2 and column index is 3. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    75
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    76
Now, that we have accessed one element of the array, let us change
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    77
it. We shall change the 34 to -34 in both A and C. To do this, we
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    78
simply assign the new value after accessing the element. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    79
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    80
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    81
  A[2] = -34
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    82
  C[2, 3] = -34
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    83
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    84
Now that we have accessed and changed a single element, let us
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    85
access and change more than one element at a time; first rows and
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    86
then columns.
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    87
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    88
Let us access one row of C, say the third row. We do it by saying, 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    89
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    90
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    91
  C[2] 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    92
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    93
How do we access the last row of C? We could say,
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    94
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    95
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    96
  C[4] 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    97
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    98
for the fifth row, or as with lists, use negative indexing and say
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
    99
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   100
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   101
  C[-1]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   102
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   103
Now, we could change the last row into all zeros, using either 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   104
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   105
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   106
  C[-1] = [0, 0, 0, 0, 0]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   107
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   108
or 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   109
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   110
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   111
  
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   112
  C[-1] = 0
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   113
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   114
Now, how do we access one column of C? As with accessing
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   115
individual elements, the column is the second parameter to be
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   116
specified (after the comma). The first parameter, is now replaced
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   117
with a ``:`` to say, that we want all the elements of that
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   118
dimension, instead of one particular element. We access the third
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   119
column by
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   120
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   121
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   122
  
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   123
  C[:, 2]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   124
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   125
Following is an exercise that you must do. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   126
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   127
%%1%% Change the last column of C to zeroes. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   128
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   129
Please, pause the video here. Do the exercises and then continue. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   130
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   131
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   132
  
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   133
  C[:, -1] = 0
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   134
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   135
Since A is one dimensional, rows and columns of A don't make much
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   136
sense. It has just one row and 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   137
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   138
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   139
  A[:] 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   140
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   141
gives the whole of A. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   142
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   143
Following is an exercise that you must do. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   144
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   145
%%2%% Change ``A`` to ``[11, 12, 13, 14, 15]``. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   146
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   147
Please, pause the video here. Do the exercises and then continue. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   148
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   149
To change A, we say
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   150
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   151
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   152
  A[:] = [11, 12, 13, 14, 15]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   153
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   154
Now, that we know how to access, rows and columns of an array, we
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   155
shall learn how to access other pieces of an array. For this
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   156
purpose, we will be using image arrays. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   157
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   158
To read an image into an array, we use the ``imread`` command. We
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   159
shall use the image ``squares.png`` present in ``/home/fossee``. We
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   160
shall first navigate to that path in the OS and see what the image
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   161
contains. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   162
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   163
{{{ switch to the browser and show the image }}}
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   164
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   165
{{{ switch back to the ipython terminal }}}
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   166
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   167
Let us now read the data in ``squares.png`` into the array ``I``. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   168
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   169
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   170
  I = imread('/home/fossee/squares.png')
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   171
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   172
We can see the contents of the image, using the command
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   173
``imshow``. We say, 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   174
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   175
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   176
  imshow(I) 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   177
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   178
to see what has been read into ``I``. We do not see white and black
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   179
because, ``pylab`` has mapped white and black to different
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   180
colors. This can be changed by using a different colormap. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   181
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   182
To see that ``I`` is really, just an array, we say, 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   183
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   184
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   185
  I 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   186
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   187
at the prompt, and see that an array is displayed. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   188
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   189
To check the dimensions of any array, we can use the method
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   190
shape. We say
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   191
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   192
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   193
  I.shape 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   194
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   195
to get the dimensions of the image. As we can see, ``squares.png``
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   196
has the dimensions of 300x300. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   197
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   198
Our goal for this part of the tutorial would be to get the
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   199
top-left quadrant of the image. To do this, we need to access, a
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   200
few of the rows and a few of the columns of the array. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   201
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   202
To access, the third column of C, we said, ``C[:, 2]``. Essentially,
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   203
we are accessing all the rows in column three of C. Now, let us
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   204
modify this to access only the first three rows, of column three
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   205
of C. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   206
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   207
We say, 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   208
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   209
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   210
  C[0:3, 2]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   211
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   212
to get the elements of rows indexed from 0 to 3, 3 not included
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   213
and column indexed 2. Note that, the index before the colon is
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   214
included and the index after it is not included, in the slice that
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   215
we have obtained. This is very similar to the ``range`` function,
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   216
where ``range`` returns a list, in which the upper limit or stop
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   217
value is not included.
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   218
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   219
Now, if we wish to access the elements of row with index 2, and in
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   220
columns indexed 0 to 2 (included), we say, 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   221
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   222
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   223
  C[2, 0:3]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   224
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   225
Following is an exercise that you must do. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   226
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   227
%%3%% First, obtain the elements [22, 23] from C. Then, obtain the
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   228
elements [11, 21, 31, 41] from C. Finally, obtain the elements [21,
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   229
31, 41, 0]. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   230
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   231
Please, pause the video here. Do the exercises and then continue. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   232
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   233
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   234
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   235
  C[1, 1:3] 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   236
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   237
gives the elements [22, 23]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   238
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   239
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   240
  C[0:4, 0]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   241
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   242
gives the elements [11, 21, 31, 41]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   243
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   244
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   245
  C[1:5, 0]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   246
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   247
gives the elements [21, 31, 41, 0]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   248
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   249
Note that when specifying ranges, if you are starting from or
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   250
going up-to the end, the corresponding element may be dropped. So,
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   251
in the previous example to obtain [11, 21, 31, 41], we could have
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   252
simply said, 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   253
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   254
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   255
  C[:4, 0]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   256
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   257
and 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   258
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   259
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   260
  C[1:, 0]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   261
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   262
gives the elements [21, 31, 41, 0]. If we skip both the indexes,
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   263
we get the slice from end to end, as we already know. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   264
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   265
Following is an exercise that you must do. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   266
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   267
%%4%% Obtain the elements [[23, 24], [33, -34]] from C. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   268
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   269
Please, pause the video here. Do the exercises and then continue. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   270
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   271
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   272
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   273
  C[1:3, 2:4] 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   274
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   275
gives us the elements, [[23, 24], [33, -34]]. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   276
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   277
Now, we wish to obtain the top left quarter of the image. How do
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   278
we go about doing it? Since, we know the shape of the image to be
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   279
300, we know that we need to get the first 150 rows and first 150
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   280
columns. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   281
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   282
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   283
  I[:150, :150]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   284
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   285
gives us the top-left corner of the image. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   286
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   287
We use the ``imshow`` command to see the slice we obtained in the
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   288
form of an image and confirm. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   289
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   290
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   291
  imshow(I[:150, :150])
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   292
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   293
Following is an exercise that you must do. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   294
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   295
%%5%% Pause the video here, and obtain the square in the center
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   296
of the image. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   297
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   298
Following is an exercise that you must do. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   299
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   300
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   301
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   302
  imshow(I[75:225, 75:225])
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   303
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   304
Our next goal is to compress the image, using a very simple
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   305
technique to reduce the space that the image takes on disk while
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   306
not compromising too heavily on the image quality. The idea is to
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   307
drop alternate rows and columns of the image and save it. This way
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   308
we will be reducing the data to a fourth of the original data but
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   309
losing only so much of visual information. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   310
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   311
We shall first learn the idea of striding using the smaller array
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   312
C. Suppose we wish to access only the odd rows and columns (first,
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   313
third, fifth). We do this by, 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   314
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   315
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   316
  C[0:5:2, 0:5:2]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   317
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   318
if we wish to be explicit, or simply, 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   319
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   320
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   321
  C[::2, ::2]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   322
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   323
This is very similar to the step specified to the ``range``
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   324
function. It specifies, the jump or step in which to move, while
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   325
accessing the elements. If no step is specified, a default value
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   326
of 1 is assumed. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   327
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   328
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   329
  C[1::2, ::2] 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   330
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   331
gives the elements, [[21, 23, 0], [41, 43, 0]]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   332
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   333
Following is an exercise that you must do. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   334
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   335
%%6%% Obtain the following. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   336
[[12, 0], [42, 0]]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   337
[[12, 13, 14], [0, 0, 0]]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   338
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   339
Please, pause the video here. Do the exercises and then continue. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   340
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   341
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   342
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   343
  C[::3, 1::3]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   344
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   345
gives the elements [[12, 0], [42, 0]]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   346
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   347
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   348
  C[::4, 1:4]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   349
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   350
gives the elements [[12, 13, 14], [0, 0, 0]]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   351
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   352
Now, that we know how to stride over an image, we can drop
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   353
alternate rows and columns out of the image in I. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   354
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   355
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   356
  I[::2, ::2]
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   357
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   358
To see this image, we say, 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   359
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   360
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   361
  imshow(I[::2, ::2])
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   362
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   363
This does not have much data to notice any real difference, but
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   364
notice that the scale has reduced to show that we have dropped
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   365
alternate rows and columns. If you notice carefully, you will be
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   366
able to observe some blurring near the edges. To notice this
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   367
effect more clearly, increase the step to 4. 
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   368
::
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   369
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   370
  imshow(I[::4, ::4])
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   371
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   372
{{{ show summary slide }}}
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   373
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   374
That brings us to the end of this tutorial. In this tutorial, we
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   375
have learnt to access parts of arrays, specifically individual
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   376
elements, rows and columns and larger pieces of arrays. We have
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   377
also learnt how to modify arrays, element wise or in larger
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   378
pieces.
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   379
b595f90016c5 Changed structure of my scripts.
Puneeth Chaganti <punchagan@fossee.in>
parents:
diff changeset
   380
Thank You!