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2012-11-29T06:19:55Z

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{| border=1
!Visual Cue
!Narration
|-
| Show Slide 1

Containing title, name of the production team along with the logo of MHRD
| Hello Friends and Welcome to this tutorial on 'manipulating strings'.

|-
| Show Slide 2

Learning objectives
| At the end of this tutorial, you will be able to,

# Slice strings and get sub-strings out of them.
# Reverse strings.
# Replace characters in strings.
# Convert strings to upper or lower case.
# Join a list of strings.

|-
| Show Slide 3

Pre-requisite slide
| Before beginning this tutorial,we would suggest you to complete the tutorial on "getting started with strings", "getting started with lists" and "basic datatypes and operators".

|-
| Open the terminal

ipython
| Let us invoke our ipython interpreter

|-
| <nowiki>week = ["sun", "mon", "tue", "wed", "thu", "fri", "sat"]</nowiki>
| Let us consider a simple problem, and learn how to slice strings and get sub-strings.

Let's say the variable <tt>week</tt> has the list of the names of the days of the week.

|-
| s = "saturday"
| Now given a string <tt>s</tt>, we should be able to check if the string is a valid name of a day of the week or not. Let us define our string as,

|-
| Show Slide 4

Forms
| <tt>s</tt> could be in any of the forms --- sat, saturday, Sat, Saturday, SAT, SATURDAY. For now, we shall 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.

|-
| Switch to the terminal

<nowiki>s[0:3]</nowiki>
| So, we need to check if the first three characters of the given string exists in the variable <tt>week</tt>.

As with any of the sequence data-types, strings can be sliced into sub-strings. To get the first three characters of s, we say,

|-
|
| 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 <tt><nowiki>s[-1]</nowiki></tt>.

Pause the video here, try out the following exercise and resume the video.

|-
| Show Slide 5

Assignment 1
| '''Obtain the sub-string excluding the first and last characters'''

from the string s.

|-
| Continue from paused state Switch to the terminal

<nowiki>s[1:-1]</nowiki>
| Switch to the terminal for solution

|-
| <nowiki>s[:3]</nowiki>
<nowiki>s[:3] in week</nowiki>
| We get the substring of s, without the first and the last characters of s.

Now let us check if a particular substring is present in the variable <tt>week</tt>. We shall check for 'sat'.

|-
| s1 = "malayalam"
| We get the result as true. 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 <tt>malayalam</tt>.

|-
| <nowiki>s1[::-1]</nowiki>
| Now, we need to compare this string with it's reverse.

<nowiki>Again, we will use a technique common to all sequence data-types, that is, [::-1]</nowiki>

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

|-
| <nowiki>s1 == s1[::-1]</nowiki>
| Now, to check if the string is <tt>s</tt> is palindromic, we say

|-
| s1 = "Malayalam"
s1.upper()
s1
| As, expected, we get <tt>True</tt>.

Now, if the string we are given is <tt>Malayalam</tt> instead of <tt>malayalam</tt>, the above comparison would return a False. So, we will have to convert the string to all lower case or to all upper case, before comparing. Python provides methods, <tt>s.lower</tt> and <tt>s.upper</tt> to achieve this.

Let's try it out.

|-
| s1.lower()
<nowiki>s1.lower() == s1.lower()[::-1]</nowiki>
| As you can see, s has not changed. It is because, <tt>upper</tt> returns a new string. It doesn't change the original string. Similarly,

|-
|
| Pause the video here, try out the following exercise and resume the video.

|-
| Show Slide 6

Assignment 2
| '''Check if <tt>s</tt> 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.

|-
| Continue from paused state Switch to the terminal

s in week

<nowiki>s.lower()[:3] in week</nowiki>
| Switch to your terminal for solution

|-
| <nowiki>email = "info[at]fossee[dot]in"</nowiki>
| So, as you can see, now we can check for presence of <tt>s</tt> in <tt>week</tt>, in whichever format it is present -- capitalized, or all caps, full name or short form.

We just convert any input string to lower case and then check if it is present in the list <tt>week</tt>.

<nowiki>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.</nowiki>

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

|-
| <nowiki>email = email.replace("[at]", "@")</nowiki>
print email
| Now, we first replace the <tt><nowiki>[at]</nowiki></tt> with the <tt>@</tt>, using the replace method of strings.

|-
|
| Pause the video here, try out the following exercise and resume the video.

|-
| Show Slide 7

Assignment 3
| Replace the <tt><nowiki>[dot]</nowiki></tt> with <tt>.</tt> in <tt>email</tt>

|-
| Continue from paused state Switch to the terminal

<nowiki>email = email.replace("[dot]", ".")</nowiki>
print email
| Switch to the terminal for solution

|-
| <nowiki>email_list = ["info@fossee.in", "enquiries@fossee.in", "help@fossee.in"]</nowiki>
| Now, let us 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_str = ", ".join(email_list)
print email_str
| Now, if we wish to obtain one long string, separating each of the email id by a comma, we use the join operator on <tt>,</tt>.

|-
|
| Notice that the email ids are joined by a comma followed by a space.

Pause the video here, try out the following exercise and resume the video.

|-
| Show Slide 8

Assignment 4
| '''From the email_str that we generated, change the separator to be'''

a semicolon instead of a comma.

|-
| Continue from paused state Switch to the terminal

email_str = email_str.replace(",", ";")
print email_str
| Switch to the terminal for solution

|-
|
| We see that the email ids are joined by a semicolon followed by a space.

|-
| Show Slide 9

Summary slide
| This brings us to the end of this tutorial. In this tutorial, we have learnt to,

# Obtain sub-strings and reverse of strings by using the index numbers
# Use following functions - - <tt>upper()</tt> -- to obtain the upper case of a string - <tt>lower()</tt> -- to obtain the lower case of a string - <tt>replace()</tt> -- to replace a character by another one - <tt>join()</tt> -- to join a list of strings with an operator

|-
| Show Slide 10

Self assessment questions slide
| Here are some self assessment questions for you to solve

# Given a string <tt>s = "this is a string"</tt>, how will you change it to <tt>"this isn't a list"</tt> ?
# Given the string "F.R.I.E.N.D.S" in s, obtain the string "friends".

|-
| Show Slide 11

Solution of self assessment questions on slide
| And the answers,

1. We will use the <tt>replace</tt> function to accomplish this.

s = s.replace("string", "list")
s = s.replace("is", "isn't")
s

We notice that every 'is' in the statement has been replaced by <tt>isn't</tt>.

# In order to change the string to lower case, we use the method <tt>lower()</tt>

Enumerated list ends without a blank line; unexpected unindent.

<nowiki>s[::2].lower()</nowiki>

|-
| Show Slide 12

Acknowledgment slide
| Hope you have enjoyed this tutorial and found it useful. Thank you!

|}

Python/C3/Manipulating-strings /English - Revision history

Chandrika: Created page with '{| border=1 !Visual Cue !Narration |- | Show Slide 1 Containing title, name of the production team along with the logo of MHRD | Hello Friends and Welcome to this tutorial on …'