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

1. Odd/Even Check number = int(input("Enter a number: ")) if number % 2 == 0: print("Even") else: print("Odd")
2. 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")
3. Multiple of Seven number = int(input("Enter a number: ")) if number % 7 == 0: print("Multiple of 7") else: print("Not a multiple of 7")