Learning Python Part-13: Python Strings

c = ”’A string in triple quotes, 

a = “Hello world”

print(a[4])          ### Output will be ‘o’ i.e. 4th position only
print(a[6:11])     ### Output will be ‘world’. Starting from 6 to 11th position

• Concatenation of Two or More Strings
• Joining of two or more strings into a single one is called concatenation.
• The + operator does this in Python. 
• Simply writing two string literals together also concatenates them.
• The * operator can be used to repeat the string for a given number of times.

str1 = ‘Hello’
str2 =’World!’

• Writing two string literals together also concatenates them like + operator.

str = ‘Hello ”World!’
print(str)

• Iterating Through String:

• Using for loop we can iterate through a string. Here is an example to count the number of ‘l’ in a string.

count = 0
for letter in ‘Hello World’:
     if(letter == ‘l’):
     count += 1
     print(count,’letter found’)

• String Membership Test:

• We can test if a sub string exists within a string or not, using the keyword in.

>>> ‘a’ in ‘program’
True
>>> ‘a’ not in ‘program’
False

• Various built-in functions that work with sequence, works with string as well.
• Some of the commonly used ones are enumerate() and len(). 
• The enumerate() function returns an enumerate object. 
• It contains the index and value of all the items in the string as pairs. 
• This can be useful for iteration.
• Similarly, len() returns the length (number of characters) of the string.

str = ‘cold’

• Escape Sequence
• Raw String to ignore escape sequence
• The format() Method for Formatting Strings
• Old style formatting
• Common Python String Methods

Escape Sequence:

• If we want to print a text like -He said, “What’s there?”- we can neither use single quote or double quotes.
• This will result into SyntaxError as the text itself contains both single and double quotes.
• One way to get around this problem is to use triple quotes. 

print(”’He said, “What’s there?””’)

• Alternatively, we can use escape sequences.
• An escape sequence starts with a backslash and is interpreted differently. 
• If we use single quote to represent a string, all the single quotes inside the string must be escaped. Similar is the case with double quotes. 

print(‘He said, “What\’s there?”‘)

• Sometimes we may wish to ignore the escape sequences inside a string. 
• To do this we can place r or R in front of the string. 
• This will imply that it is a raw string and any escape sequence inside it will be ignored.

# Without raw string

• The format() method that is available with the string object is very versatile and powerful in formatting strings. 
• Format strings contains curly braces {} as placeholders or replacement fields which gets replaced.

Old style formatting:

• We can even format strings like the old sprintf() style used in C programming language. 
• We use the % operator to accomplish this.

x = 12.3456789
print(‘The value of x is %3.2f’ %x)
print(‘The value of x is %3.4f’ %x) 

• There are numerous methods available with the string object. 
• The format() method that we mentioned above is one of them. 
• Some of the commonly used methods are
• lower()
• upper()
• join()
• split()
• find()
• replace()

Examples:

a = “PytHONic”.lower()