Logical expressions are statements that follow the rules of propositional logic both in their composition and their evaluation. They are used ubiquitously in programming and GIS applications to express the conditions or constraints that must be met prior to conducting certain operations.
Most logical expressions have a simple structure: two operands P and Q, connected with each other through a logical operator, such as AND, OR, XOR (exclusive OR), IMPLIES, and so on. All expressions evaluate to one of only two truth values: true or false. For example, the logical expression
P AND Q
evaluates to true if and only if both P and Q are true and evaluates to false in all other cases.
Both the operands of a logical expression and the expression itself can be made ...