: 3B - Boolean Expressions
Boolean expressions are used to compare two values and get a true-or-false
answer:
value1 relational_operator value2
The following relational operators are used:
<
less than
>
greater than
=
equal to
<=
less than or equal to
>=
greater than or equal to
<>
not equal to
More complex Boolean expressions are formed by using the Logical Boolean
operators:
not
negation (~)
and
conjunction (^)
or
disjunction (v)
xor
NOT
exclusive-or
is a unary operator — it is applied to only one value and inverts it:


not true = false
not false = true
if (not found) then
writeln(‘does not exist’);
AND
yields TRUE only if both values are TRUE:

TRUE and FALSE = FALSE
if (mark >= 90) and (mark <=100)
then

OR
TRUE and TRUE = TRUE
grade:= ‘A’
yields TRUE if at least one value is TRUE:




TRUE or TRUE = TRUE
TRUE or FALSE = TRUE
FALSE or TRUE = TRUE
FALSE or FALSE = FALSE
if (rank >=5) or ( age >50) then
writeln(‘Car Park allocated ‘);
When combining two Boolean expressions using relational and Boolean
operators, be careful to use parentheses.
(a>b) or (x<=1)
This is because the Boolean operators are higher on the order of operations than
the relational operators:
1.
2.
3.
4.
not
* / div mod and
+ - or
< > <= >= = <>
Selection: The If statement
The general format for the IF statement is as follows : IF (Boolean expression)
then
Statement;
In the case where more than one statement needs to be executed the format is
as follows;
if (Boolean expression) then
Begin
Statement_1;
Statement_2;
Statement_3
End
Else
Begin
Statement_1;
Statement_2;
End;