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1. Looping Structures in Algorithms

2. What is a Loop?
A loop is a structure that repeats a block of code until a
condition is met.
• Avoids repetition : write once, execute multiple times.

3. Why Use Loops?
• Code Reusability – write once, execute multiple times
• Efficiency – reduces code length and improves
readability
• Automation – processes large data automatically
• Flexibility – handles dynamic number of iterations

4. Types of Looping Structures
1. Entry Controlled Loop (Pre test Loop): Check first,
then execute
• While Loop
• For Loop
2. Exit Controlled Loop (Post test Loop): Execute first,
then check
• Do-While Loop

5. While Loop
Syntax:
WHILE (condition) DO
statements
END WHILE
Example:
Print numbers 1 to 5
i = 1
WHILE (i <= 5)
PRINT i
i = i + 1
END WHILE

6. While Loop
Characteristics:
• Condition checked first
• Executes zero or more times
• Good for unknown iterations

7. For Loop
Syntax:
FOR (initialization; condition; increment/decrement)
DO
statements
END FOR
Example:
Print 1 to 5
FOR (i = 1 to 5) DO PRINT i END FOR

8. For Loop
Characteristics:
• Fixed number of iterations
• Counter automatically increments
• Simple for known iterations

9. Do-While Loop
Syntax:
DO
statements
WHILE (condition)
Example:
Print 1 to 5
i = 1
DO
PRINT i
i = i + 1
WHILE (i <= 5)

10. Do-While Loop
Characteristics:
• Executes at least once
• Condition checked at end
• Useful for menu-driven programs

11. Key Differences Between Loop Types
• Feature | While Loop | Do-While Loop
• Condition Check | Before execution | After execution
• Minimum Executions | 0 | 1
• Use Case | When iterations may be zero | When at
least one needed

12. Loop Control Statements
1. Break Statement – exits the loop immediately
2. Continue Statement – skips to next iteration
Example (Break): Stops when i == 5
Example (Continue): Skips when i == 3

13. Practical Examples
Example 1: Sum of N Natural Numbers (While Loop)
READ n
sum = 0
i = 1
WHILE (i <= n) DO
sum = sum + i
i = i + 1
END WHILE
PRINT sum

14. Practical Examples
Example 2: Input Validation (Do-While Loop)
DO
PRINT "Enter positive number: "
READ number
WHILE (number <= 0)

15. Common Loop Patterns
• 1. Counter-Controlled Loop – fixed iterations
• 2. Sentinel-Controlled Loop – ends at special value
• 3. Flag-Controlled Loop – uses a boolean flag

16. Best Practices
• Initialize variables before loops
• Ensure termination (avoid infinite loops)
• Use meaningful counter names
• Choose right loop type
• Keep loop bodies simple

17. Infinite Loops (Avoid These!)
Example:
i = 1
WHILE (i > 0) DO
PRINT 'This runs forever!'
END WHILE
Tips to Avoid:
• Update loop variables properly
• Ensure condition becomes false eventually

18. Summary
• Loops simplify repetitive tasks
• Use While/For for entry control, Do-While for exit
control
• Break and Continue manage flow
• Apply best practices to write efficient and safe loops