.. Objectives
.. ----------
.. By the end of this tutorial, you will be able to
.. 1. Slice strings and get sub-strings out of them
.. #. Reverse strings
.. #. Replace characters in strings.
.. #. Convert strings to upper or lower case
.. #. joining a list of strings
.. Prerequisites
.. -------------
.. 1. getting started with strings
.. #. getting started with lists
.. #. basic datatypes
.. Author : Puneeth
Internal Reviewer : Amit
External Reviewer :
Checklist OK? : <put date stamp here, if OK> [2010-10-05]
Script
------
{{{ Show the slide containing title }}}
Hello Friends. Welcome to this tutorial on manipulating strings.
{{{ show the slide with outline }}}
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 and joining a list of strings.
.. #[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. For now, 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
.. #[Amit: Sequence data type???]
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]``.
Following is an exercise that you must do.
.. #[Amit: I don't know I am not sure about the sentence formation.]
%%1%% Obtain the sub-string excluding the first and last characters
from the string s.
Please, pause the video here. Do the exercise(s) and then continue.
::
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]
.. #[amit: I think using reversed in not required after this]
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.
.. #[amit: I wish we could include this right when s.upper() is used so
.. that it is clear]
Following is an exercise that you must do.
%%2%% Check if ``s`` is a valid name of a day of the week. Change the
solution to this problem, to include forms like, SAT, SATURDAY,
Saturday and Sat.
Please, pause the video here. Do the exercise and then continue.
::
s in week
s.lower()[:3] in week
.. #[amit: May be a sentence or two about what our original problem was and
.. how this helps in solving it. One can loose the flow.]
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
Following is an exercise that you must do.
%%3%% Replace the ``[dot]`` with ``.`` in ``email``
Please, pause the video here. Do the exercise and then continue.
::
email = email.replace("[dot]", ".")
print email
Now, let's look at another interesting problem where we have a list of
e-mail addresses and we wish to obtain one long string of e-mail
addresses separated by commas or semi-colons.
::
email_list = ["info@fossee.in", "enquiries@fossee.in", "help@fossee.in"]
Now, if we wish to obtain one long string, separating each of the
email id by a comma, we use the join operator on ``,``.
::
email_str = ", ".join(email_list)
print email_str
Notice that the email ids are joined by a comma followed by a space.
Following is an exercise that you must do.
%%3%% From the email_str that we generated, change the separator to be
a semicolon instead of a comma.
Please, pause the video here. Do the exercise and then continue.
::
email_str = email_str.replace(",", ";")
That brings us to the end of the tutorial.
{{{ show summary slide }}}
In this tutorial, we have learnt how to get substrings, reverse
strings and a few useful methods, namely upper, lower, replace and
join.
{{{ Show the "sponsored by FOSSEE" slide }}}
This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India
Hope you have enjoyed and found it useful.
Thank you!