Boolean operations in a programming language: OR.

The idea

The Boolean operator OR combines two conditions and yields True when at least one of them is True; it is False only when both are False. In a program, an expression such as isWeekend OR isHoliday evaluates each side and combines them, so the result is True on a Saturday even if it is not a holiday. A truth table for A OR B has three True rows and a single False row at A = False, B = False. OR is used when any one of several acceptable conditions should trigger an action, for example flagging a message if a score is below ten OR a deadline has passed. Contrast this with AND, which needs both True. In a multiple-choice question, evaluate each operand, apply the OR rule, and reject options that describe AND behaviour.

What earns the marks

• OR returns True when at least one operand is True.
• OR returns False only when both operands are False.
• A truth table for A OR B has exactly one False outcome, at A = False and B = False.
• OR is used when any one of several conditions should trigger an action.
• Evaluating an expression means testing each operand and then combining with the OR rule.

Common mistakes and how to put them right

• Assuming OR returns False when one operand is True.

  Put it right by stressing only both False gives False.

• Confusing OR with AND; correct by comparing their truth tables.

• Treating OR as exclusive so that both True gives False.

  Put it right by noting OR is inclusive here.

Exam tips

• Identify the single False row of the OR truth table to check your answer.
• Test each operand separately before combining with OR.
• Reject options that describe AND behaviour instead of OR.