String Operations

A string is a sequence, which means it is an ordered collection of other values. Strings are immutable.

Creation:

Strings are created by enclosing characters in quotes. Python treats single quotes the same as double quotes. Creating strings is as simple as assigning a value to a variable Example:

fruit = 'banana'

Indexing string:

Each item in a string corresponds to an index number, which is an integer value, starting with the index number 0.

• Any integer expression can be used as an index.
• If you try to read or write an element that does not exist, you get an IndexError.
• If an index has a negative value, it counts backward from the end of the list.

fruit='banana'

Accessing Items in String:

A string is a sequence of characters. You can access the characters one at a time with the bracket operator.

>>> fruit = 'banana'
>>> letter = fruit[1]

• The second statement selects character number 1 from fruit and assigns it to letter .
• The expression in brackets is called an index. The index indicates which character in the sequence you want

>>> letter
'a' 

So b is the 0th letter (“zero-eth”) of 'banana' , a is the 1th letter (“one-eth”), and n is the 2th letter (“two-eth”).

As an index you can use an expression that contains variables and operators:

>>> i = 1
>>> fruit[i]
'a'
>>> fruit[i+1]
'n'

Modifying Items in String:

Strings are immutable, you can’t modify the elements. But you can replace one string with another

s='west'
s='ea'+s[2:]

Modifying string with Operators:

The + operator concatenates lists.

>>> s='east'
>>> s2='west'
>>> c=s+s2
>>> c
eastwest

The * operator repeats a list a given number of times.

>>> print(c*2)
eastwesteastwest

##Deletion: Removing individual character is not possible. It's possible to delete entire string by using del statement.

>>> s='east'
>>> del s

##Traversal: processing a string one character at a time. Often they start at the beginning, select each character in turn, do something to it, and continue until the end. This pattern of processing is called a traversal.

Function that takes a string as an argument and displays the letters one per line.

index = 0
while index < len(fruit):
    letter = fruit[index]
    print(letter)
    index = index + 1

OR

for i in fruit:
    print(i)

Function that takes a string as an argument and displays the letters backwards one per line.

index = len(fruit) - 1
while index >= 0:
    letter = fruit[index]
    print(letter)
    index = index - 1

len

len is a built-in function that returns the number of characters in a string:

>>> fruit = 'banana'
>>> len(fruit)
6