Complete Python Language Series: Chapter 2

Introduction

Lecture Focus: Python Strings and Conditional Statements
Resources: Notes available in the playlist for download in the description box

Python Strings

Data Type

Definition: Sequence of characters (single character, word, sentence, paragraph)
Creation: Strings can be created using:
- Double quotes: "This is a string"
- Single quotes: 'single word'
- Triple quotes: '''This is a string'''

String Creation Examples

Example using VS Code lecture2.py file:

string1 = "This is a string"
string2 = 'single word'
string3 = '''This is a string'''

Preferred Method: Double quotes
Reason for Different Quotes: To handle apostrophes conveniently:
```
string = "This is Amit's book"
```
Ensures no confusion by the interpreter.

Multi-line Strings

Escape Sequence Characters: For formatting (e.g., new lines \n, tabs \t)

Example:

string = "This is a string\nWe are creating it in Python"
print(string)

Output:

This is a string
We are creating it in Python

String Operations

Concatenation

Definition: Joining two strings using + operator

Example:

string1 = "Amit"
string2 = "College"
final_string = string1 + " " + string2
print(final_string) # Output: "Amit College"

Length of String

Function: len() to find string length

Example:

length = len(string1)
print(length) # Output: 4

String Indexing & Slicing

Indexing

Zero-based Index: First character index is 0
Access Character: Using [index]

Example:

string = "Python"
print(string[0]) # Output: 'P'
print(string[1]) # Output: 'y'

Negative Indexing: To access characters from the end, starting with -1
```
print(string[-1]) # Output: 'n'
print(string[-2]) # Output: 'o'
```

Slicing

Definition: Accessing parts of a string using [start:end]
```
part = string[1:4] # Output: 'yth'
```
Note: start index included, end index excluded

Omitting Indices: Implies start from beginning/end

part = string[:4] # Output: 'Pyth'
part = string[3:] # Output: 'hon'

String Functions

Common Functions: Endswith, Capitalize, Replace, Find, and Count
Example Usage:
- endswith: Checks if a string ends with certain characters
```
string.endswith("ing") # Output: True
```
- capitalize: Capitalizes the first character
```
string.capitalize()
```
- replace: Replaces old substring with new
```
string.replace("Python", "Java")
```
- find: Finds first occurrence of substring
```
string.find("thon") # Output: 2
```
- count: Counts occurrences of substring
```
string.count("a")
```

Conditional Statements

Introduction

Purpose: Control the flow of code execution based on conditions
Keywords: if, elif, else

Syntax: Writing Rules

Example:

if condition:
    # Execute this block
elif other_condition:
    # Execute this block
else:
    # Execute this block

Note: Indentation (4 spaces/tab) is crucial

Practical Examples

Example: Voter Eligibility

Code: Check if age allows voting

age = 21
if age >= 18:
    print("Can vote and apply for license")
else:
    print("Cannot vote")

Example: Traffic Lights

Code: Actions based on traffic light color

light = "green"
if light == "red":
    print("Stop")
elif light == "green":
    print("Go")
else:
    print("Look")

Nested Conditional Statements

Definition: if statement inside another if
Usage: For complex conditions

**Example:

age = 95
if age >= 18:
    if age >= 80:
        print("Cannot drive")
    else:
        print("Can drive")
else:
    print("Cannot drive")

Practice Problems

Odd/Even Check

number = int(input("Enter a number: "))
if number % 2 == 0:
    print("Even")
else:
    print("Odd")

Greatest of Three Numbers

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
c = int(input("Enter third number: "))
if a >= b and a >= c:
    print("First is largest")
elif b >= a and b >= c:
    print("Second is largest")
else:
    print("Third is largest")

Multiple of Seven

number = int(input("Enter a number: "))
if number % 7 == 0:
    print("Multiple of 7")
else:
    print("Not a multiple of 7")