Iteration

Iteration

What is iteration?

• Iteration is the process of doing something more than once (repeat), otherwise known as a loop
• A loop can be count controlled which means the code is repeated a fixed number of times
• A loop can also be condition controlled which means the code is repeated until a condition is met
• Three common loops are:
  • **for loops **(count controlled)
  • while loops (condition controlled)
  • do while loops (condition controlled)

For Loops

• A for loop is a count controlled loop that will repeat a fixed number of times. It provides a concise and structured way to perform repetitive tasks.

Syntax of a for loop

The syntax of a for loop consists of three main parts:

01 for i = x to y
02  // Code to be executed in each iteration
03 next i

1. Initialisation: The initialisation is executed only once at the beginning of the loop. It is used to initialise a counter variable that controls the loop’s execution which is i in this example
2. Range: The range that the count variable will increment through
3. Increment/Decrement: The default is to increment by 1 each time unless specified

Pseudocode example

for loop example pseudocode

Python example

01 for i in range(0,6):
02   print i

In Python, the range specifies the numbers used in the loop. The final number (6 in this case) is 1 higher than the number we want to run the loop with.

Java example

01 for (int i = 0; i < 6; i++) {
02   System.out.println(i);
03 }

Iterating over an array

Pseudocode example

for loop iterating over an array and outputting each item

Python example

01 fruits = ["apple", "banana", "orange"]
02 for i in range(len(fruits)):
03    num = int(input("Enter a number: "))
04    numbers.append(num)
05 # Assume the first number is the largest
06 largest_number = numbers[0]
07 # Identifying the largest number
08 for num in numbers:
09    if num > largest_number:
10       largest_number = num
11 print("The largest number is:", largest_number)

Java example

01 String[] fruits = {"apple", "banana", "orange"};
02 for (int i = 0; i < fruits.length; i++) {
03    System.out.println(fruits[i]);
04 }

While Loops

• A while loop is a condition controlled loop that will repeat until a condition is met
• While loops provide a flexible and powerful way to handle situations where the number of iterations is unknown in advance

Syntax of a while loop

• The syntax of a while loop consists of a condition and a code block:

01 while condition
02   // Code to be executed as long as the condition is true
03 endwhile

• The condition is evaluated **before **each iteration. If the condition evaluates to true, the code block is executed
• If the condition evaluates to false, the loop terminates.

Pseudocode example

while loop example pseudocode

Python example

01 i = 0
02 while i <= 5:
03    print(i)
04    i = i + 1

Java example

01 int i = 0;
02 while (i <= 5) {
03    System.out.println(i);
04    i = i + 1;
05 }

Checking if the password is ‘secret’

Pseudocode example

while loop checking if the password is correct

Python example

01 password = ""
02 while password != "secret":
03    password = input("What is the password? ")

Java example

01 import java.util.Scanner;
02 public class Main {
03    public static void main(String[] args) {
04       Scanner scanner = new Scanner(System.in);
05       String password = "";
06       while (!password.equals("secret")) {
07          System.out.print("What is the password? ");
08          password = scanner.nextLine();
09       }
10       scanner.close();
11    }
12 }

Examiner Tips and Tricks

• Incrementing a variable can be done in different ways (depending on the language)
• For example:
  • i= i + 1
  • i + = 1
  • i++

Worked Example

A simple Python program is shown below.

01 //Program to calculate the number of times a number goes into 100
02
03 count = 0
04 num = int(input("Enter a number"))
05 while (count*num)<=100
06    count=count+1
07 endwhile
08 count=count-1 //Take one off as gone over
09 print(str(num) + " goes into 100 " + str(count) + " times.")

State the output of the program when the number 30 is entered.

[1]

How to answer this question:

• If 30 is entered this is saved in the variable num
• The while loop will run while count*num is less than or equal to 100
• count is 0, so 30 * 0 = 0
• As there is a loop to iterate through it would be useful to produce a trace table to help us keep track of the value of the different variables

count	num	count * num
0	30
1		30
2		60
3		90
4		120
3

• The loop will repeat and when count is 4, count*num is 120 which causes the condition to be false and the loop to stop
• Count is decremented
• The statement which is printed is 30 goes into 100 3 times

Answer:

30 goes into 100 3 times.

Do While Loops

• A do while loop is another example of a condition controlled loop that will repeat until a condition is met.
• Do while loops provide a variation of the while loop with a slightly different behaviour
• The code within a do while loop will always run at least once, unlike a while loop which may not run at all if the condition is already met

Syntax of a do while loop

• The syntax of a do while loop consists of a code block and a condition:

01 do
02   // Code to be executed at least once
03 until condition

• Code Block: The code block is executed first before evaluating the condition
• Condition: The condition is evaluated after executing the code block. If the condition evaluates to false, the loop continues executing. If the condition evaluates to true, the loop terminates

Pseudocode example

Do while example pseudocode

Python example

• It isn’t possible to use a do while loop in Python so the code would need to be written to use a while loop

01 import random
02 desired_number = 6
03 roll = 0

04 while roll != desired_number:
05    roll = random.randint(1, 6)
06    print("Rolled:", roll)

Java example

01 import java.util.Random;
02 public class Main {
03    public static void main(String[] args) {
04       Random random = new Random();
05       int desiredNumber = 6;
06       int roll;

07       do {
08          roll = random.nextInt(6) + 1;
09          System.out.println("Rolled: " + roll);
10       } while (roll != desiredNumber);
11    }
12 }

Examiner Tips and Tricks

You can use either a while loop or a do while loop but don’t forget that a do while loop will always run once before checking if the condition is true