1 .. Objectives

     2 .. ----------

     3 

     4 .. At the end of this tutorial, you should know --

     5 

     6 ..   1. How to define strings

     7 ..   #. Different ways of defining a string

     8 ..   #. How to concatenate strings 

     9 ..   #. How to print a string repeatedly 

    10 ..   #. Accessing individual elements of the string

    11 ..   #. Immutability of strings

    12 

    13 .. Prerequisites

    14 .. -------------

    15 

    16 .. 1. getting started with ipython

    17      

    18 .. Author              : Madhu

    19    Internal Reviewer   : Punch

    20    External Reviewer   :

    21    Language Reviewer   : Bhanukiran

    22    Checklist OK?       : <15-11-2010, Anand, OK> [2010-10-05]

    23 

    24 Script

    25 ------

    26 

    27 {{{ Show the slide containing the title }}}

    28 

    29 Hello friends. Welcome to this spoken tutorial on Getting started with

    30 strings.

    31 

    32 {{{ Show the slide containing the outline }}}

    33 

    34 In this tutorial, we will look at what we really mean by strings, how

    35 Python supports the use of strings and some of the operations that can

    36 be performed on strings. 

    37 

    38 {{{ Shift to terminal and start ipython }}}

    39 

    40 To begin with let us start ipython, by typing::

    41 

    42   ipython

    43 

    44 on the terminal

    45 

    46 So, what are strings? In Python anything within either single quotes

    47 or double quotes or triple single quotes or triple double quotes are

    48 strings. 

    49 

    50 {{{ Type in ipython the following and read them as you type }}}::

    51 

    52   'This is a string'

    53   "This is a string too'

    54   '''This is a string as well'''

    55   """This is also a string"""

    56   'p'

    57   ""

    58 

    59 Note that it really doesn't matter how many characters are present in

    60 the string. The last example is a null string or an empty string. 

    61 

    62 Having more than one control character to define strings is handy when

    63 one of the control characters itself is part of the string. For

    64 example::

    65 

    66   "Python's string manipulation functions are very useful"

    67 

    68 By having multiple control characters, we avoid the need for

    69 escaping characters -- in this case the apostrophe. 

    70 

    71 The triple quoted strings let us define multi-line strings without

    72 using any escaping. Everything within the triple quotes is a single

    73 string no matter how many lines it extends::

    74 

    75    """Having more than one control character to define

    76    strings come as very handy when one of the control

    77    characters itself is part of the string."""

    78 

    79 We can assign this string to any variable::

    80 

    81   a = 'Hello, World!'

    82 

    83 Now 'a' is a string variable. String is a collection of characters. In

    84 addition string is an immutable collection. So all the operations that

    85 are applicable to any other immutable collection in Python works on

    86 string as well. So we can add two strings::

    87 

    88   a = 'Hello'

    89   b = 'World'

    90   c = a + ', ' + b + '!'

    91 

    92 We can add string variables as well as the strings themselves all in

    93 the same statement. The addition operation performs the concatenation

    94 of two strings.

    95 

    96 Similarly we can multiply a string with an integer::

    97 

    98   a = 'Hello'

    99   a * 5

   100 

   101 gives another string in which the original string 'Hello' is repeated

   102 5 times.

   103 

   104 Following is an exercise that you must do. 

   105 

   106 %% %% Obtain the string ``%% -------------------- %%`` (20 hyphens)

   107       without typing out all the twenty hyphens. 

   108 

   109 Please, pause the video here. Do the exercise and then continue. 

   110 

   111 ::

   112 

   113   s = "%% " + "-"*20 + " %%"

   114 

   115 Let's now look at accessing individual elements of strings. Since,

   116 strings are collections we can access individual items in the string

   117 using the subscripts::

   118 

   119   a[0]

   120 

   121 gives us the first character in the string. The indexing starts from 0

   122 for the first character and goes up to n-1 for the last character. We

   123 can access the strings from the end using negative indices::

   124 

   125   a[-1]

   126 

   127 gives us the last element of the string and 

   128 ::

   129 

   130     a[-2]

   131 

   132 gives us second element from the end of the string

   133 

   134 Following is an exercise that you must do. 

   135 

   136 %% %% Given a string, ``s = "Hello World"``, what is the output of::

   137 

   138       s[-5] 

   139       s[-10]

   140       s[-15]

   141 

   142 Please, pause the video here. Do the exercise and then continue. 

   143 

   144 ::

   145 

   146   s[-5] 

   147 

   148 gives us 'W'

   149 ::

   150 

   151   s[-10] 

   152 

   153 gives us 'e' and 

   154 ::

   155 

   156   s[-15] 

   157 

   158 gives us an ``IndexError``, as should be expected, since the string

   159 given to us is only 11 characters long. 

   160 

   161 Let us attempt to change one of the characters in a string::

   162 

   163   a = 'hello'

   164   a[0] = 'H'

   165 

   166 As said earlier, strings are immutable. We cannot manipulate a

   167 string. Although there are some methods which let us manipulate

   168 strings, we will look at them in the advanced session on strings. In

   169 addition to the methods that let us manipulate the strings we have

   170 methods like split which lets us break the string on the specified

   171 separator, the join method which lets us combine the list of strings

   172 into a single string based on the specified separator.

   173 

   174 {{{ Show summary slide }}}

   175 

   176 This brings us to the end of another session. In this tutorial session

   177 we learnt

   178 

   179   * How to define strings

   180   * Different ways of defining a string

   181   * String concatenation and repetition

   182   * Accessing individual elements of the string

   183   * Immutability of strings

   184 

   185 {{{ Show the "sponsored by FOSSEE" slide }}}

   186 

   187 This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India

   188 

   189 Hope you have enjoyed and found it useful.

   190 Thank you!

   191