Unit II: Flow of Control and Strings

Table of Contents

1. Introduction to Flow of Control
2. Conditional Statements
3. Iterative Statements (Loops)
4. Break and Continue Statements
5. Strings: Operations and Methods

1. Introduction to Flow of Control

Flow of Control refers to the order in which statements are executed in a program. By default, Python executes statements sequentially, from top to bottom.

Indentation: Python uses indentation to define blocks of code. Consistent use of whitespace is mandatory for determining the structure of conditional and iterative statements.
Sequential Flow: Execution of statements one after another.
Conditional Flow: Execution based on whether a condition is true or false.
Iterative Flow: Execution of a set of statements multiple times.

2. Conditional Statements

Conditional statements allow the program to make decisions and branch to different sections of code.

if Statement: Executes a block if the condition is true.
if-else Statement: Provides an alternative block if the condition is false.
if-elif-else Statement: Used to check multiple conditions sequentially.

Practical Examples:

Calculating the absolute value of a number.
Sorting 3 numbers.
Checking the divisibility of a number.

3. Iterative Statements (Loops)

Loops are used to repeat a block of code multiple times.

The for Loop

The for loop is used for iterating over a sequence (like a list, tuple, or string).

range() function: Generates a sequence of numbers, commonly used to control the number of iterations in a for loop.

The while Loop

The while loop repeats a block as long as a specified condition remains true.

[Image of while loop flowchart in Python]

Suggested Programs:

Generating Patterns: Using nested loops to print geometric shapes.
Summation of Series: Calculating the sum of a mathematical progression.
Factorial: Finding the factorial of a positive number.

4. Break and Continue Statements

These statements provide finer control over loop execution.

break: Terminates the current loop and moves execution to the next statement outside the loop.
continue: Skips the remaining code in the current iteration and jumps to the next iteration of the loop.
Nested Loops: Loops placed inside other loops.

5. Strings: Operations and Methods

A String is a sequence of characters.

Basic Operations:

Concatenation: Joining two or more strings together using the + operator.
Repetition: Repeating a string multiple times using the * operator.
Membership: Checking if a substring exists in a string using in or not in.
Slicing: Extracting a specific portion of a string using index ranges (e.g., string[start:stop]).

String Traversal and Methods:

Traversing: Using loops to access each character in a string one by one.
Built-in functions/methods: Python provides numerous methods like .upper(), .lower(), .find(), and .replace() to manipulate strings.

Common Mistakes

Indentation Errors: Mixing tabs and spaces or inconsistent levels of indentation.
Infinite Loops: Forgetting to update the loop variable in a while loop.
String Immutability: Attempting to change a character in a string directly (strings are immutable).

Frequently Asked Questions

Q: What happens if I forget the colon (:) after an 'if' statement?
Python will raise a SyntaxError because the colon is required to signal the start of a code block.

Q: How does range(5) differ from range(1, 5)?
range(5) produces 0, 1, 2, 3, 4. range(1, 5) produces 1, 2, 3, 4 (it stops before the end value).