Explain simple algorithms in terms of their inputs, processing and outputs.

The idea

An algorithm is a finite sequence of steps that solves a problem. To explain one, split it into three roles. Inputs are the data the algorithm receives before or during the run, such as two numbers typed by a user or a list of marks read from a file. Processing is the work done on that data: arithmetic, comparison, selection, iteration, sorting or searching. Outputs are the results returned, displayed or stored, such as a total, a message or a sorted list. A useful method is to trace a short algorithm line by line, labelling each step I, P or O, then state the overall purpose. For example, an algorithm that reads a temperature, decides whether it is above a threshold and prints a warning has one input, a comparison as processing and a printed message as output. Some steps both consume and produce data, so decide the role from what the step does at that point.

What earns the marks

• Identifies inputs as the data supplied to the algorithm, for example values entered by a user or read from a file.
• Identifies processing as the operations performed on data, such as calculation, comparison, selection or iteration.
• Identifies outputs as the results produced, for example a displayed value, a stored value or a printed message.
• Explains the algorithm as a whole by linking its input, its processing steps and its output in a logical order.
• Correctly classifies a given step from a short algorithm as input, processing or output with a reason.

Common mistakes and how to put them right

• Treating a displayed result as an input.

  Put it right by noting that a value shown to the user is an output, while a value the user supplies is an input.

• Assuming every algorithm must have an input.

  Put it right by recognising that some algorithms produce output from fixed data with no user input.

• Describing processing as 'the middle part' without naming the operation.

  Put it right by stating the actual operation, such as adding two values or comparing them.

Exam tips

• Annotate each line of a short algorithm with I, P or O before writing your answer.
• Name the specific operation in processing rather than saying 'it processes the data'.
• Check that your stated output matches what the algorithm actually produces, not what you assume it should.