The three combining principles (sequence, iteration/repetition and…

The idea

High-level imperative programs are built from three combining principles. Sequence means statements execute one after another in the order written. Selection (choice) means the program takes one path or another depending on a condition, for example an if statement choosing between two branches. Iteration (repetition) means a block repeats, either a set number of times or while a condition holds. These principles are described as basic to all high-level imperative languages because they let you express any algorithm by combining them. Recognise each principle in code, and be ready to say which one a given snippet uses.

What earns the marks

• Sequence: statements are executed one after another in the order they are written, with no branching or repetition.
• Selection (choice): a condition determines which of two or more paths the program follows, for example an if statement.
• Iteration (repetition): a block of statements repeats, either a fixed number of times or while a condition remains true.
• The three principles are described as basic to all high-level imperative programming languages, so they apply across languages, not just one.
• Recognising which principle a given piece of code demonstrates, and explaining how combining them expresses an algorithm.

Common mistakes and how to put them right

• Confusing selection with iteration.

  Put it right: Selection chooses a path once based on a condition; iteration repeats a block, possibly many times.

• Thinking sequence means statements can run in any order.

  Put it right: Sequence means they run in the written order, top to bottom.

• Assuming iteration must always run a fixed number of times.

  Put it right: Iteration can be count-controlled or condition-controlled.

Exam tips

• For each snippet, name the principle and point to the exact line or keyword that shows it.
• Use the specification's own terms, sequence, iteration/repetition and selection/choice, so your answer matches the wording.
• When a question asks which principle is used, check whether the code branches (selection) or loops (iteration) before deciding.