This document provides an overview of strings in Python, defining them as sequences of characters and explaining their characteristics, such as being immutable and indexed. It covers string operations like concatenation, slicing, and comparison, as well as built-in functions such as len(), upper(), lower(), and replace(). Additionally, it highlights the use of membership operators to check for substrings within strings.

1. Strings in Python

2. What are Strings?
• A consecutive sequence of characters
enclosed by single/double quotes.
• Triple quotes are for strings that span multiple
lines

3. What are Strings?

4. Empty string
• A string without any characters inside
• Length is zero

5. Traversing a String
• To access all the elements of the string one by
one
• Each character has an index value
• Starts from 0 from left
• Starts from -1 from right

6. Traversing a String
str H e l l o W o r l d !
Pos
index
0 1 2 3 4 5 6 7 8 9 10 11
Neg
index
-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1

7. Traversing a String
str H e l l o W o r l d !
Pos
index
0 1 2 3 4 5 6 7 8 9 10 11
Neg
index
-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1
How to get output in one line?

8. Special String Operations
• Concatenate: string1 + string2
• Replicate: 3*str1

9. • Comparison operators:
• Can use relational operators(>,>=,<,<=,==,!=) to compare 2
strings
• It starts with the first elements in both strings, then keeps
comparing the next element in both strings
Special String Operations

10. • Check out the functions: ascii() , ord() and chr()
• Find out what type of values they work on.
• What kind of outputs do you get?
• Some examples to get you started:
Special String Operations
• Can you find something interesting to share with the
class?

11. • String Slicing
• Slicing is done to extract a substring/character from the
string
• The slice operator is two indices in square brackets
separated by a colon
• Syntax: string_name[start:end] with end index not
included
Special String Operations
str H e l l o W o r l d !
Pos
index
0 1 2 3 4 5 6 7 8 9 10 11
Neg
index
-12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1

12. Strings are immutable
• Contents of a string once created can not be
changed

13. String methods & built-in functions
str1=‘ALL is well’
• str1.upper()
• str1.lower()
• str1.casefold()

14. String methods & built-in functions
• len( ) – returns the length of the string
• capitalize( ) – returns the exact copy but with the first
letter capitalized

15. String methods & built-in functions
• split() – breaks a string at specified separator and returns a
list of substrings. The split() takes 2 parameters-separator
and maxsplit
str.split(separator,maxsplit)

16. String methods & built-in functions
• replace() – This function replaces all occurrences of the old
string with the new string.
Syntax: str.replace(old,new)
• find()- This function finds the substring and gives the
lowest index
• index()- This function finds the substring and gives the
lowest index, just as find(), but raises ValueError when
substring not found

17. • Membership operators:
• ‘in’ operator : Returns true if character/substring exists in
string
• ‘not in’ operator : Returns true if character/substring does
not exist in string
Special String Operations
Note
The find() method should be used only if you need to know the position of sub.
To check if sub is a substring or not, use the in operator:

18. String methods & built-in functions
Try out the following functions and write what they do:
a=‘ABC1’
b=‘ ’

19. Ch4_uq7a

20. Ch4_uq7b

21. Ch4_uq7b