Flow of Control

6.1 INTRODUCTION

The concept of flow of control in programming refers to the order in which individual statements, instructions, or function calls are executed or evaluated. In the initial chapters, students learn about the sequence structure where statements are executed one after another in the order they appear. This is analogous to a bus carrying children to school following a fixed route, moving from one milestone to the next without deviation. The bus driver has no choice but to follow the road to reach the destination. Similarly, in sequence execution, the program runs statements from the beginning to the end without any branching or repetition. Flow of control is fundamental to programming as it determines how a program behaves and responds to different inputs or conditions. To implement complex logic, programming languages provide control structures that alter the flow of execution. Python supports two primary control structures: selection and repetition. Selection allows the program to choose between different paths based on conditions, while repetition enables executing a set of statements multiple times. Proper indentation is crucial in Python as it defines the blocks of code and controls the flow of execution. Unlike many other languages that use curly braces, Python uses indentation as a syntactical element, making code more readable and less prone to errors related to block definition. This chapter introduces these concepts with examples and flowcharts, helping students understand how to control the execution flow in Python programs effectively.

• Flow of control defines the order of execution of statements in a program.
• Sequence is the simplest flow where statements execute one after another.
• Control structures like selection and repetition alter the flow based on conditions or loops.
• Python uses indentation to define blocks of code, which affects flow of control.
• Understanding flow of control is essential for writing complex and efficient programs.
• This chapter covers selection, repetition, break and continue statements, and nested loops.
• 📌 Flow of Control: The order in which individual statements are executed in a program.
• 📌 Sequence: Execution of statements one after another in the order they appear.
• 📌 Control Structures: Constructs that alter the flow of execution in a program.

6.2 SELECTION

Selection, also known as decision making, allows a program to choose between different paths based on conditions. This is essential when a program needs to perform different actions depending on the input or state. The if statement is the fundamental selection construct in Python. The section begins with a real-life example of choosing a pen priced at ₹10 among many options, illustrating the need to select based on conditions. Similarly, digital maps offer multiple routes, and the user selects one based on preferences like shortest distance or least traffic. The syntax of the if statement in Python is: if condition: statement(s) If the condition evaluates to True, the indented statements under the if block are executed. Otherwise, they are skipped. The if..else statement provides two alternative paths: if condition: statement(s) else: statement(s) This allows the program to execute one block if the condition is true and another if it is false. For multiple conditions, Python provides the if..elif..else construct, allowing chaining of conditions. Examples include checking if a number is positive, negative, or zero, or displaying messages based on traffic signal colors. A practical example is modifying the difference program to always output a positive difference by selecting which number to subtract from which. The section also introduces nested if statements, where an if or else block contains another if statement, providing finer control. Proper indentation is critical to define the blocks belonging to each condition. The section concludes with a program to create a simple calculator performing addition, subtraction (positive difference), multiplication, and division with error handling for division by zero and invalid operators.

• Selection allows programs to choose different execution paths based on conditions.
• The if statement executes a block only if its condition is true.
• The if..else statement provides two alternative blocks depending on the condition.
• The if..elif..else construct allows multiple conditions to be checked sequentially.
• Nested if statements enable conditions within conditions for complex decision making.
• Indentation defines the scope of statements under each condition in Python.
• 📌 Selection: Decision making in programming to choose between alternatives.
• 📌 if statement: Executes code block if a condition is true.
• 📌 if..else statement: Executes one of two blocks based on a condition.