Understand the function of and be able to identify the structural…

The idea

Programs are built from recognisable structural components, and you must both explain what each does and spot it in a given program. A constant holds a value fixed at design time, such as a maximum size, while a variable holds a value that can change while the program runs. Initialisation gives a variable its first value; assignment later replaces that value. Command sequence means statements carried out in order, one after another. Selection chooses between paths, usually with if or case. Repetition and iteration both describe looping, where a block runs more than once, either a set number of times or until a condition changes. Data structures organise related values, for example a list or record. Subprograms are named blocks that perform a task and can be called; parameters pass values into them. Input brings data in and output sends results out.

What earns the marks

• Constant: a value fixed when the program is written and not changed while it runs.
• Variable: a named store whose value can change during execution.
• Initialisation: giving a variable its first value; assignment: replacing the stored value later.
• Command sequence: statements executed in the written order.
• Selection: choosing between alternative paths, for example with if or case.
• Repetition and iteration: repeating a block of statements, either a set number of times or until a condition is met.
• Data structures: organised collections of related values such as lists or records.
• Subprograms and parameters: named reusable blocks and the values passed into them.
• Input/output: data entering the program and results being sent out.

Common mistakes and how to put them right

• Treating a constant as a variable.

  Put it right: A constant is fixed at design time, whereas a variable can be reassigned while the program runs.

• Confusing initialisation with assignment.

  Put it right: Initialisation sets the first value, while assignment can replace a value at any later point.

• Thinking repetition and iteration are unrelated ideas.

  Put it right: Both refer to looping, where a block of statements runs more than once.

• Mixing up parameters with input.

  Put it right: Parameters pass values into a subprogram, while input brings data into the program from outside.

Exam tips

• Read the code first and name the component you can actually see before checking the options.
• Eliminate options that describe a different component, such as a definition of selection when the code shows a loop.
• Check whether a value is ever changed after it is set; if not, it is likely a constant.