1 ========

     2  Script

     3 ========

     4 

     5 {{{ show the welcome slide }}}

     6 

     7 Welcome to this tutorial on manipulating strings. 

     8 

     9 {{{ show the slide with outline }}} 

    10 

    11 In this tutorial we shall learn to manipulate strings, specifically

    12 slicing and reversing them, or replacing characters, converting from

    13 upper to lower case and vice-versa 

    14 

    15 #[punch: reversed returns an iterator. should we still teach it?]

    16 

    17 We have an ``ipython`` shell open, in which we are going to work,

    18 through out this session. 

    19 

    20 Let us consider a simple problem, and learn how to slice strings and

    21 get sub-strings. 

    22 

    23 Let's say the variable ``week`` has the list of the names of the days

    24 of the week. 

    25 

    26 ::

    27 

    28     week = ["sun", "mon", "tue", "wed", "thu", "fri", "sat"]

    29 

    30 

    31 Now given a string ``s``, we should be able to check if the string is a

    32 valid name of a day of the week or not. 

    33 

    34 ::

    35 

    36     s = saturday

    37 

    38 

    39 ``s`` could be in any of the forms --- sat, saturday, Sat, Saturday,

    40 SAT, SATURDAY. We shall now be solving the problem only for the forms,

    41 sat and saturday. We shall solve it for the other forms, at the end of

    42 the tutorial. 

    43 

    44 {{{ show these forms in a slide }}}

    45 

    46 So, we need to check if the first three characters of the given string

    47 exists in the variable ``week``. 

    48 

    49 As, with any of the string data-types, strings can be sliced into

    50 sub-strings. To get the first three characters of s, we say, 

    51 

    52 ::

    53 

    54     s[0:3]

    55 

    56 Note that, we are slicing the string from the index 0 to index 3, 3

    57 not included. 

    58 

    59 As we already know, the last element of the string can be accessed

    60 using ``s[-1]``.  

    61 

    62 %%1%% Pause the video here and obtain the sub-string excluding the

    63 first and last characters from the string. 

    64 

    65 ::

    66 

    67     s[1:-1]

    68 

    69 gives the a substring of s, without the first and the last

    70 characters. 

    71 

    72 ::

    73 

    74     s = saturday

    75     s[:3]

    76 

    77 Now, we just check if that substring is present in the variable

    78 ``week``. 

    79 

    80 ::

    81 

    82     s[:3] in week          

    83 

    84 Let us now consider the problem of finding out if a given string is

    85 palindromic or not. First of all, a palindromic string is a string

    86 that remains same even when it has been reversed.

    87 

    88 Let the string given be ``malayalam``.

    89 

    90 ::

    91 

    92     s = "malayalam"

    93 

    94 Now, we need to compare this string with it's reverse. 

    95 

    96 Again, we will use a technique common to all sequence data-types,

    97 [::-1]

    98 

    99 So, we obtain the reverse of s, by simply saying, 

   100 

   101 ::

   102 

   103     s[::-1]

   104 

   105 Now, to check if the string is ``s`` is palindromic, we say

   106 ::

   107 

   108     s == s[::-1]

   109 

   110 As, expected, we get ``True``. 

   111 

   112 Now, if the string we are given is ``Malayalam`` instead of

   113 ``malayalam``, the above comparison would return a False. So, we will

   114 have to convert the string to all lower case or all upper case, before

   115 comparing. Python provides methods, ``s.lower`` and ``s.upper`` to

   116 achieve this. 

   117 

   118 Let's try it out. 

   119 ::

   120 

   121    s = "Malayalam"

   122 

   123    s.upper()

   124 

   125    s

   126 

   127    s.lower()

   128 

   129    s.lower() == s.lower()[::-1]

   130    

   131 Note that these methods, do not change the original string, but return

   132 a new string.

   133 

   134 a%% %% Pause the video here, and finish the problem of checking if

   135 ``s`` is a valid name of a day of the week and then resume the

   136 video. Change the solution to this problem, to include forms like,

   137 SAT, SATURDAY, Saturday and Sat. 

   138 

   139 ::

   140 

   141     s.lower()[:3] in week

   142 

   143 We just convert any input string to lower case and then check if it is

   144 present in the list ``week``. 

   145 

   146 Now, let us consider another problem. We often encounter e-mail id's

   147 which have @ and periods replaced with text, something like

   148 info[at]fossee[dot]in. We now wish to get back proper e-mail

   149 addresses.  

   150 

   151 Let's say the variable email has the email address. 

   152 ::

   153 

   154    email = "info[at]fossee[dot]in"

   155 

   156 Now, we first replace the ``[at]`` with the ``@``, using the replace

   157 method of strings. 

   158 ::

   159 

   160    email = email.replace("[at]", "@")

   161    print email

   162 

   163 %%1%% Pause the video here and replace the ``[dot]`` with ``.`` and then

   164 resume the video. 

   165 

   166 ::

   167 

   168    email = email.replace("[dot]", ".")        

   169    print email

   170 

   171 

   172 That brings us to the end of the tutorial. 

   173 

   174 {{{ show summary slide }}}

   175 

   176 In this tutorial, we have learnt how to get substrings, reverse

   177 strings and a few useful methods, namely upper, lower and replace. 

   178 

   179 Thank You!