Iteration | Computer for GCSE/IGCSE - Year 11 PDF Download

What is iteration? 

Iteration, an essential programming concept, involves repeating a set of instructions until a certain condition is fulfilled. It's utilized to automate repetitive tasks.
There are three main types of iteration: 

• Count-controlled loops
• Precondition loops
• Postcondition loops

Count-controlled Loops

• Count-controlled loops are utilized when the number of iterations is predetermined beforehand. 
• They are also referred to as definite loops.
• These loops operate based on a counter variable that is either increased or decreased after each iteration, and the loop continues until the counter reaches a specified value.

Example in Pseudocode:
count ← 1
FOR i <= 10
OUTPUT count
NEXT i

Example in Python:
for count in range(1, 11):
print(count)

Example in Java:
for(int count=1; count<=10; count++){
System.out.println(count);
}

Example in Visual Basic:
For count = 1 To 10
Console.WriteLine(count)
Next count

Pre-condition Loops

• A precondition loop is employed when the quantity of iterations isn't predetermined but relies on a condition being true.
• It's alternatively referred to as an indefinite loop.
• This loop persists in execution as long as the condition remains true, ceasing when the condition turns false.

Example in Pseudocode:
INPUT temperature
WHILE temperature > 37 DO
OUTPUT "Patient has a fever"
INPUT temperature
END WHILE

Example in Python:
temperature = float(input("Enter temperature: "))
while temperature > 37:
print("Patient has a fever")
temperature = float(input("Enter temperature: "))

Example in Java:
Scanner input = new Scanner(System.in);
float temperature = input.nextFloat();
while (temperature > 37) {
System.out.println("Patient has a fever");
temperature = input.nextFloat();
}

Example in Visual Basic:
temperature = InputBox("Enter temperature")
Do While temperature > 37
Console.WriteLine( "Patient has a fever")
Console.WriteLine("Enter temperature")
temperature=Console.ReadLine()
Loop

Postcondition Loops

• Postcondition loops are utilized when a loop needs to run at least once, irrespective of the initial condition being false.
• The condition is assessed at the conclusion of the loop's execution.

Example in Pseudocode:
REPEAT
INPUT guess
UNTIL guess = 42

Example in Python:
Postcondition loops don’t exist in Python and would need to be restructured to a precondition loop

Example in Java:
Scanner input = new Scanner(System.in);
int guess = 0;
do {
System.out.print("Enter guess: ");
guess = input.nextInt();
} while (guess != 42);

Example in Visual Basic:
Do
Console.WriteLine("Enter guess")
guess = Console.ReadLine()
Loop Until guess = 42