SIMSCRIPT II.5 Programming Language

SIMSCRIPT II.5 Basic Concepts
and or. (In this context, a comma cannot be substituted for the word.) If E1 and E2 are logical
expressions, then:
E1 and E2 is true if both E1 and E2 are true
E1 or E2 is true if either or both of E1 and E2 are true
Compound logical expressions may contain more than two simple logical expressions, as in the logical
expression:
E1 and E2 or E3 and E4
When more than two simple logical expressions appear in an unparenthesized compound logical expression
with the operators and or or, the operator and is evaluated first. Parentheses can be used,
however, to indicate a specific order of evaluation. In the absence of parentheses, the above expression
is, by convention, evaluated, as though it had been written:
(1) (E1 and E2) or (E3 and E4)
If a program requires some other logic, the statement can be written as:
(2) E1 and (E2 or E3) and E4
which means something quite different. Version (1) is true either if both E1 and E2 are true or if
both E3 and E4 are true. Version (2) is true if E1 is true and E4 is true, and either E2 or E3 is true.
Compound logical expressions can be used with is false and is true phrases. An is false
or is true phrase always applies to the logical expression preceding it. If this logical expression
is compound, it must be enclosed in parentheses, as shown in the logical expression:
E1 is false and (E2 or E3) is true
A few simple rules that govern the writing and evaluation of logical expressions are given below.
1. A logical expression enclosed in parentheses remains a logical expression.
2. In the absence of parentheses, and is evaluated before or. Successive logical expressions
are used as operands of and operators, and these evaluated expressions are then used as operands
of or operators. Parentheses can always be used to indicate specific operator hierarchies.
3. Is true and is false phrases apply to logical expressions preceding them. If such a
logical expression is compound, it must be enclosed in parentheses. Otherwise, the phrase
only applies to the expression adjacent to it.
Some examples that illustrate the writing and evaluation of complex logical expressions are given
below. In these examples, the variables I, J, K, M, and N are positive numbers; the variables Q,
R, S, and T are negative numbers; and the variable Z is zero.
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