Be able to write programs that use relational operators (equal to, less…

The idea

This criterion requires you to write program code that compares values using relational operators, which produce a Boolean result of true or false. In a language such as Python, equal to is written with two equals signs (score == 10), not equal to uses an exclamation mark before the equals sign (score != 10), less than and greater than use the angle brackets (score < 10 and score > 10), and the inclusive comparisons combine an angle bracket with an equals sign (score <= 10 and score >= 10). These results commonly control selection or iteration, for example if temperature >= 100: then sound an alarm, or while count < limit: to repeat a loop. Choose the operator that matches the boundary described, place the values in the correct order, and test with values just below, exactly on and just above the boundary.

What earns the marks

• Selects the relational operator that matches a stated comparison, for example greater than or equal to when a value on the boundary must also be accepted.
• Writes a complete comparison with a value on each side of the operator, such as score >= 50 or name == "Ada".
• Uses two equals signs for equality testing and reserves a single equals sign for assignment.
• Uses the not-equal operator when a condition must be false for the tested value.
• Places the operands in the correct order so the comparison means what is intended, for example age < 18 rather than 18 < age when testing for a minor.
• Uses the Boolean result correctly in a selection or loop, for example as the condition of an if statement.

Common mistakes and how to put them right

• Using a single equals sign where a comparison is required.

  Put it right: A single equals sign assigns a value, so use two equals signs to test equality.

• Reversing the operands so the comparison means the opposite of what is intended.

  Put it right: Read the comparison aloud in words and check that the subject and the limit are in the order described.

• Treating less than as including the boundary value.

  Put it right: Less than excludes the boundary, so use less than or equal to when the boundary must be accepted.

• Assuming a comparison returns a number rather than a Boolean.

  Put it right: Relational operators produce true or false, which is then used by the surrounding selection or loop.

Exam tips

• Underline the boundary words in the question, such as 'at least' or 'fewer than', because they decide whether the inclusive form is needed.
• Test each candidate comparison with three values: one below the boundary, the boundary value itself and one above it.
• Check the direction of the comparison by substituting a simple number and reading the result as a sentence.
• Eliminate options that use a single equals sign for a comparison, since that is an assignment rather than a test.