Cambridge O Level Computer Science · Syllabus 2210 · Boolean Logic
Logic Expression
What is Logic Expression?
A written form of a logic circuit in which the inputs are letters, the operations are written as capitalised words such as NOT, AND, OR, NAND, NOR and XOR, and brackets show exactly which parts are grouped together; the output is named on the left of an equals sign.
This definition is part of the Boolean Logic chapter in Cambridge O Level Computer Science.
Logic Expression in context
One logic problem can be written in four ways: as a problem statement in words, as a logic expression in letters and word operators, as a logic circuit of gate symbols and wires, and as a truth table listing the output for every possible input combination. Topic 10 is, almost entirely, the skill of moving between them. The syllabus lists nine such moves, and each one is a question that can be asked.
A logic expression writes a circuit as text. Inputs are single letters, the operators are written as whole words in capitals — NOT, AND, OR, NAND, NOR, XOR — and brackets show exactly what is grouped with what. The output is given its own letter on the left of an equals sign.
A problem statement describes a system in words. Turning it into a logic expression is a six-step routine, and the steps are worth following even when the answer looks obvious — because the statements that look obvious are the ones with a hidden “not” in them.