B2.3.3 Construct programs that utilize looping structures to perform repeated actions.

The Big Idea

In programming, looping structures allow us to execute a block of code multiple times, often with changing conditions or counters. Instead of writing the same instructions repeatedly, loops enable efficient, concise, and flexible repetition of actions—a core concept in all computational processes.

There are two broad categories of loops:

• Counted loops: repeat a specific number of times.
• Conditional loops: repeat while a condition remains true.

Effective use of loops—especially with conditional logic—allows programs to process data sets, interact with users, and perform algorithmic operations in a scalable way.

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1. Types of Loops

A. Counted Loops (Definite iteration)

Counted loops execute a fixed number of times, often using a loop control variable.

Python Example:

for i in range(5):
    print("Iteration:", i)

Java Example:

for (int i = 0; i < 5; i++) {
    System.out.println("Iteration: " + i);
}

• Commonly used when the number of iterations is known ahead of time.
• The loop variable (i) controls how many times the loop runs.
• These are known as for loops in most languages.

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B. Conditional Loops (Indefinite iteration)

Conditional loops repeat until a condition becomes false. These are used when the number of repetitions is not known in advance.

Python Example:

password = ""
while password != "open123":
    password = input("Enter password: ")

Java Example:

Scanner sc = new Scanner(System.in);
String password = "";
while (!password.equals("open123")) {
    System.out.print("Enter password: ");
    password = sc.nextLine();
}

• These are typically while loops.
• Loop continuation depends on Boolean or relational expressions.

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2. Loop Variants and Their Use Cases

Loop Type	Syntax Form	Best Use Case
for loop	Count-controlled	When the number of iterations is known
while loop	Condition-controlled	When looping until a condition changes
do-while loop (Java only)	Executes at least once	When the body must run before the condition is checked

Java do-while Example:

int num;
do {
    num = scanner.nextInt();
} while (num <= 0);

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3. Using Conditional Statements Within Loops

Inside a loop, if, elif, else (Python) or if, else if, <strong>else</strong> (Java) can be used to modify control flow, such as filtering values, breaking loops, or skipping iterations.

Example: Filtering Odd Numbers (Python)

for i in range(10):
    if i % 2 == 0:
        continue  # Skip even numbers
    print(i)

Example: Early Exit on Match (Java)

for (int i = 0; i < numbers.length; i++) {
    if (numbers[i] == target) {
        System.out.println("Found!");
        break;
    }
}

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4. Boolean and Relational Conditions in Loops

Loops often rely on relational and Boolean logic for continuation or branching.

Python Example:

while age < 18 or not has_permission:
    print("Access denied.")
    # request age or permission

Java Example:

while (score >= 0 && score <= 100) {
    System.out.println("Valid score");
    // get new input
}

Operator Type	Examples	Use
Relational	<, <=, >, >=, ==, !=	Compare values
Boolean	and, or, not (Python) / &&, ||, ! (Java)	Combine or invert relational expressions to form compound conditions

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5. Common Errors and How to Avoid Them

Mistake	Description
Infinite loops	Forgetting to update loop control variables or incorrect condition (e.g. while True: with no break)
Off-by-one errors	Misunderstanding range limits (e.g. i < n vs i <= n)
Incorrect condition logic	Using or instead of and (or vice versa), leading to loops that always run or never run
Missing initialization	Using a loop variable that was not initialized before the loop
Altering loop counter inside loop body	Can cause unexpected behavior or missed iterations

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6. Example: Accumulate Input Until Sentinel Value

Python:

total = 0
num = int(input("Enter number (-1 to quit): "))
while num != -1:
    total += num
    num = int(input("Enter number (-1 to quit): "))
print("Total:", total)

Java:

Scanner scanner = new Scanner(System.in);
int total = 0;
int num = scanner.nextInt();
while (num != -1) {
    total += num;
    num = scanner.nextInt();
}
System.out.println("Total: " + total);

This pattern is called a sentinel-controlled loop.

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Summary

Feature	Description
Counted loop	Loop that runs a fixed number of times (e.g. for loop)
Conditional loop	Loop that runs based on a condition (while, do-while)
Control variables	Used to limit and manage iteration
Conditional statements	Guide decisions within the loop (if, else)
Boolean logic	Governs when to continue, break, or skip
Common errors	Infinite loops, off-by-one, improper condition updates

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Final Thoughts

Looping structures allow for structured repetition, which is a foundational concept in procedural and algorithmic programming. Whether you're iterating over datasets, accumulating results, validating user input, or searching for a condition, loops provide the mechanism for efficient repetition and control. Choosing the correct type of loop—and controlling it with properly designed Boolean logic—is essential for writing robust, correct, and performant software.