Iteration: for Loops, while Loops, do-until Loops

This lesson covers iteration (loops) as required by OCR J277 Section 2.3. Iteration means repeating a block of code. OCR requires you to know three types of loop: for loops (count-controlled), while loops (condition-controlled), and do...until loops (condition-controlled, post-test).

---

What Is Iteration?

Iteration is the process of repeating a set of instructions either a fixed number of times or until a condition is met. Without iteration, you would have to write the same code many times over.

There are two main categories:

Type	Description	OCR Loop Types
Count-controlled	Repeats a fixed number of times	for loop
Condition-controlled	Repeats until a condition changes	while loop, do...until loop

The following flowchart shows the logic of a while loop (condition checked before each iteration):

flowchart TD
    A([Start]) --> B{Condition\ntrue?}
    B -- No --> E([End loop])
    B -- Yes --> C["Execute loop\nbody"]
    C --> D["Update\nvariable"]
    D --> B

---

for Loops (Count-Controlled)

A for loop repeats a block of code a specific number of times. You know in advance how many iterations are needed.

OCR Pseudocode:

for i = 0 to 4
    print("Hello " + str(i))
next i

This prints:

Hello 0
Hello 1
Hello 2
Hello 3
Hello 4

Python:

for i in range(5):  # range(5) gives 0, 1, 2, 3, 4
    print("Hello", i)

OCR for Loop Syntax

for variable = start to end
    // code to repeat
next variable

• The loop runs from start to end inclusive.
• for i = 1 to 10 runs 10 times (i = 1, 2, 3, ..., 10).
• for i = 0 to 9 also runs 10 times (i = 0, 1, 2, ..., 9).

OCR Exam Tip: In OCR pseudocode, for i = 0 to 4 iterates 5 times (0, 1, 2, 3, 4) because both endpoints are inclusive. In Python, range(5) also gives 5 values (0 to 4) but the upper bound is exclusive. Be careful with this difference.

---

while Loops (Pre-Test Condition-Controlled)

A while loop repeats as long as a condition is true. The condition is checked before each iteration, so the loop body might not execute at all.

OCR Pseudocode:

password = ""
while password != "secret"
    password = input("Enter password: ")
endwhile
print("Access granted")

Python:

password = ""
while password != "secret":
    password = input("Enter password: ")
print("Access granted")

Key Points About while Loops

• The condition is checked before each iteration.
• If the condition is false initially, the loop body never executes.
• You must ensure the condition eventually becomes false, or you create an infinite loop.
• Use a while loop when you do not know how many iterations are needed.

---

do...until Loops (Post-Test Condition-Controlled)

A do...until loop is similar to a while loop, but the condition is checked after each iteration. This means the loop body always executes at least once.

OCR Pseudocode: