Singapore-Cambridge A-Level Computing (H2)

## Algorithms An **algorithm** is a finite, ordered set of unambiguous steps that solves a problem or carries out a task. Given the same input it gives the same output, it ends after a finite number of steps, and each step is precise enough to be carried out without judgement. Writing the algorithm down before any code is written lets its logic be checked on its own, separately from the details of a programming language. Every algorithm, however large, is built from three control structures. - **Sequence**: statements run one after another, in the order written. - **Selection**: a condition decides which statements run, using `if`, `elif` and `else`. - **Iteration**: statements repeat, either while a condition holds (`while`) or for each item in a range or collection (`for`). A program that finds the larger of two numbers uses sequence to read the numbers in, selection to compare them and sequence again to show the answer. A program that adds up the marks in a list uses iteration to visit each mark. ## Pseudocode **Pseudocode** describes an algorithm in structured, plain language, close enough to code to be translated easily but free of the syntax rules of any one language. There is no required syntax for it. What matters is that the logic is correct, that every step is unambiguous and that the layout shows which statements belong inside a selection or a loop, usually by indentation. ```text INPUT mark IF mark >= 50 THEN OUTPUT "Pass" ELSE OUTPUT "Fail" ENDIF ``` The same…