Programming Concepts: Nested Selection and Iteration

Introduction

This section explores the use of nested selection statements (if-else within if-else) and various iteration statements (loops) in programming. Understanding these concepts is crucial for creating programs that can handle complex decision-making and repetitive tasks.

Nested Selection

Nested selection allows you to create more intricate decision-making structures. This involves placing an `if` statement inside another `if` or `else` block. This is useful when you need to evaluate multiple conditions.

Example: Determining if a number is within a specific range and also positive.

Consider a scenario where you want to check if a student's score is above a certain threshold and also if they have attended at least 80% of the classes. This requires nesting selection statements.

Syntax:

        if (condition1) {
            // Code to execute if condition1 is true
            if (condition2) {
                // Code to execute if condition1 AND condition2 are true
            } else {
                // Code to execute if condition1 is true but condition2 is false
            }
        } else {
            // Code to execute if condition1 is false
        }
    

Example Program (Python):

The following Python code demonstrates nested selection to determine if a number is within a range and positive.

        number = 25
        if number > 10:
            print("The number is greater than 10.")
            if number > 20:
                print("The number is also greater than 20.")
            else:
                print("The number is between 10 and 20.")
        else:
            print("The number is not greater than 10.")
    

Iteration Statements

Iteration statements allow you to repeat a block of code multiple times. The main types of iteration statements are for and while loops.

For Loop

A for loop is used to iterate over a sequence (e.g., a list, a string, or a range of numbers). It executes the code block for each item in the sequence.

Syntax:

        for (variable in sequence) {
            // Code to execute for each item in the sequence
        }
    

Example Program (Python) - For Loop:

This Python code uses a for loop to print numbers from 1 to 5.

        for i in range(1, 6):
            print(i)
    

While Loop

A while loop executes a block of code repeatedly as long as a specified condition is true.

Syntax:

        while (condition) {
            // Code to execute as long as the condition is true
        }
    

Example Program (Python) - While Loop:

This Python code uses a while loop to print numbers from 1 to 5.

        i = 1
        while i <= 5:
            print(i)
            i = i + 1
    

Table: Comparison of Selection and Iteration

Feature	Selection	Iteration
Purpose	Execute different blocks of code based on conditions.	Repeat a block of code multiple times.
Types	If-else, Nested If-else	For, While
Use Cases	Decision-making, branching in programs.	Repetitive tasks, processing collections of data.

Combining Selection and Iteration

It is common to combine selection and iteration statements to create more powerful and flexible programs. For example, you might use a for loop to iterate over a list of items and then use an if statement to perform a different action for each item based on a certain condition.