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The Boolean operators not, and, or, and xor take operands of any Boolean type and return a value of type Boolean.

 

Operator        Operation                Operand types        Result type        Example

not                negation                Boolean                Boolean                not (C in MySet)

and                conjunction                Boolean                Boolean                Done and (Total > 0)

or                disjunction                Boolean                Boolean                A or B

xor                exclusive disjunction        Boolean                Boolean                A xor B

 

These operations are governed by standard rules of Boolean logic. For example, an expression of the form x and y is True if and only if both x and y are True.

 

Complete versus short-circuit Boolean evaluation

 

The compiler supports two modes of evaluation for the and and or operators: complete evaluation and short-circuit (partial) evaluation. Complete evaluation means that each conjunct or disjunct is evaluated, even when the result of the entire expression is already determined. Short-circuit evaluation means strict left-to-right evaluation that stops as soon as the result of the entire expression is determined. For example, if the expression A and B is evaluated under short-circuit mode when A is False, the compiler won’t evaluate B; it knows that the entire expression is False as soon as it evaluates A.

 

Short-circuit evaluation is usually preferable because it guarantees minimum execution time and, in most cases, minimum code size. Complete evaluation is sometimes convenient when one operand is a function with side effects that alter the execution of the program.

Short-circuit evaluation also allows the use of constructions that might otherwise result in illegal runtime operations. For example, the following code iterates through the string S, up to the first comma.

 

while (I <= Length(S)) and (S[I] <> ',') do

 

begin

 ...

 Inc(I);

end;

 

In a case where S has no commas, the last iteration increments I to a value which is greater than the length of S. When the while condition is next tested, complete evaluation results in an attempt to read S[I], which could cause a runtime error. Under short-circuit evaluation, in contrast, the second part of the while condition — (S[I] <> ',') — is not evaluated after the first part fails.