Difference between revisions of "Python-3.4.3/C3/Manipulating-strings/English"

Objectives

• Slice a string and get substrings out of them
• Reverse a string
• Replace characters in a string
• Convert a string to upper or lowercase and
• Join list elements to form a string

System Specifications

• Ubuntu Linux 16.04 operating system
• Python 3.4.3 and
• IPython 5.1.0

Pre-requisite

• basic datatypes, operators, strings and lists

If not, see the relevant Python tutorials on this website.

string slicing

String slicing allows us to obtain substrings.

The syntax for the string slicing is:

string_name inside square brackets start colon stop or

string_name inside square brackets start colon stop colon step

Open the terminal.

Press Enter

From here onwards, remember to press the Enter key after typing every command on the terminal.

Type, data is equal to inside double quotes python

Highlight the output pyt

We get the sliced substring as pyt.

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

By doing so, the string elements from index 3 are not included.

<<PAUSE>>

Type as shown.

Highlight ‘sat’

We need to check if the first three characters of the given string exists in the list week underscore name.

Highlight the output

To get the first three characters of the string s,

type, s inside square brackets 0 colon 3

As you can see, the character at last index ie 3 is not included in the sliced output.

Highlight True

Type s inside square brackets 0 colon 3 in week underscore name

We get the output as True which indicates that the substring is present in the list.

Try this exercise and then resume the video.

Assignment 1

Highlight -1

As we already know, the last element of the string can be accessed using the index -1.

<<PAUSE>>

A palindromic string is a string that is same as its reverse.

Type, s1 is equal to inside double quotes dad

For reversing s1, we stride the string from the first to last character.

It is done by keeping the start and stop values as empty and step as -1.

That is s1 inside square brackets colon colon -1

Highlight the output

Type, s1 is double equal to s1 inside square brackets colon colon -1

The output True indicates that it is a palindrome.

So, we have to convert the string to all lowercase or uppercase, before comparison.

Let's try it out.

Type, s2 is equal to inside double quotes capital D ad

We get all the characters in uppercase.

Highlight the output

As you can see, s2 has not changed.

It is because, method upper returns a new string.

It doesn't change the original string.

Highlight the output

Type as shown.

As expected the output is True.

Try this exercise and then resume the video.

['SATURDAY', 'python', 'Sunday']

Highlight python False

It returns False for python as it is not there in the list week underscore name.

Highlight at and dot

Type, email is equal to inside double quotes info[at]fossee[dot]in

We often encounter email addresses which have @ symbol and periods replaced with text as shown.

Let us learn how to get back proper email addresses.

print (email_at)

Type as shown.

You can see that the @ symbol is properly replaced now.

Try this exercise and then resume the video.

info@fossee[dot]in

print (email_proper)

Now the variable email underscore proper has the email in the proper form.

<<PAUSE>>

Let us say, we have a list of email addresses in the variable email underscore list as shown.

We will obtain one long string of email addresses, separated by commas or semicolons.

Type as shown.

Comma followed by a space will give the string with the same formatting.

Highlight the output

You can see that all the email addresses joined into a single string.

Try this exercise and then resume the video.

print (email_str)

Highlight the output

We see that the email addresses are joined by a semicolon followed by a space.

<<PAUSE>>

Summary

In this tutorial, we have learnt to,

• Obtain substrings
• Reverse strings by using the index numbers
• Use the following methods
  • upper()
  • lower()
  • replace() and
  • join()

Self assessment questions

s = "this was a string"

1. Given a string s = "this was a string”, how will you change it to "this wasn't a list"?
2. The string s is assigned as shown, change the string to "friends"

Solutions of self assessment questions

s = s.replace("string", "list")

s = s.replace("was", "wasn't")

print(s)

1. We will use the replace method to accomplish this.

Here string and was in the statement will be replaced by list and wasn't respectively.

Solution of self assessment questions

s[::2].lower()

To change the string to lowercase, we use the string method lower().

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