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{{{ show the welcome slide }}}

Welcome to this tutorial on manipulating strings. 

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In this tutorial we shall learn to manipulate strings, specifically

slicing and reversing them, or replacing characters, converting from

upper to lower case and vice-versa 

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

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

through out this session. 

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

get sub-strings. 

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

of the week. 

::

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

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

valid name of a day of the week or not. 

::

    s = saturday

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

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

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

the tutorial. 

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

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

exists in the variable ``week``. 

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

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

::

    s[0:3]

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

not included. 

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

using ``s[-1]``.  

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

first and last characters from the string. 

::

    s[1:-1]

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

characters. 

::

    s = saturday

    s[:3]

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

``week``. 

::

    s[:3] in week          

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

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

that remains same even when it has been reversed.

Let the string given be ``malayalam``.

::

    s = "malayalam"

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

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

[::-1]

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

::

    s[::-1]

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

::

    s == s[::-1]

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

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

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

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

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

achieve this. 

Let's try it out. 

::

   s = "Malayalam"

   s.upper()

   s

   s.lower()

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

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

a new string.

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

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

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

SAT, SATURDAY, Saturday and Sat. 

::

    s.lower()[:3] in week

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

present in the list ``week``. 

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

which have @ and periods replaced with text, something like

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

addresses.  

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

::

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

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

method of strings. 

::

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

   print email

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

resume the video. 

::

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

   print email

That brings us to the end of the tutorial. 

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

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

Thank You!