Iteration (for, while, do-while loops)

Iteration means repeating a block of code. Loops are one of the three fundamental programming constructs and are tested extensively in GCSE Computer Science (AQA 3.2 / OCR J277 2.2). This lesson covers the three types of loop you need to know.

---

Why Do We Need Iteration?

Without loops, repeating an action 100 times would require writing the same code 100 times. Iteration lets you write the code once and repeat it as many times as needed — making programs shorter, more efficient, and easier to maintain.

---

Count-Controlled Loops (FOR Loops)

A FOR loop repeats a block of code a known number of times. You use it when you know in advance how many repetitions are needed.

Pseudocode

FOR i ← 1 TO 10
    OUTPUT i
ENDFOR

This outputs the numbers 1 through 10.

Python

for i in range(1, 11):
    print(i)

Important: In Python, range(1, 11) generates numbers from 1 to 10 (the upper bound is exclusive). In AQA pseudocode, FOR i ← 1 TO 10 is inclusive of 10.

Using a Step Value