Handling
of Sequence Control
Mathematical
Operators and Precedence
FORTRAN
provides operators for addition, subtraction, multiplication, division,
and exponentiation. Binary and unary operators have the same precedence.
Using parenthesis can change operator precedence.
Logical Operators
FORTRAN
provides logical operators for not, and, and or. Their representation and
precedence is below. The exclusive or operator, .NEQV. is also provided.
|
(
)
|
|
.NOT.
|
|
.AND.
|
|
.OR.,
.NEQV.
|
Relational Operators
FORTRAN uses two characters surrounded by periods
for its relational operators. FORTRAN 90 also has mathematical symbol
representations.
|
Operation
|
Operator
|
FORTRAN
90
|
|
Equal
|
.EQ.
|
==
|
|
Not
Equal
|
.NE.
|
/=
|
|
Greater
Than
|
.GT.
|
>
|
|
Less
Than
|
.LT.
|
<
|
|
Greater
than or equal
|
.GE.
|
>=
|
|
Less
than or equal
|
.LE.
|
<=
|
Labels and Goto Statement
FORTRAN
makes extensive use of GO TO statements using unsigned integer constants
for labels. Functions and procedures can therefore have multiple
entrances. This is flexible, but detrimental to readability and error
prone.
Short-circuiting of Expressions
When evaluating logical expressions, FORTRAN may employ short-circuiting
of the expression, however if a function is left unevaluated, its value
may be set to undefined.
IF statement
FORTRAN IV began with a simple one-way IF selector:
IF
(Boolean expression) statement
Only
one statement was selectable, and nesting was not an option. In the 1960's
ALGOL had introduced 2-way selection with their else statement. Soon other
imperative languages followed suit. FORTRAN 90's IF selector now allows
nesting and provides a method to represent a series of nested 2-way
selectors that is very readable.
IF
(<logical expression>) THEN
<statements>
ELSEIF
(<logical expression>) THEN
<statements>
...
<statements>
ENDIF
IF 3-way
FORTRAN provides a degenerate multiple selector
called the 3-way arithmetic IF. The statement has the format:
IF
(arithmetic expression) <N1>,
<N2>, <N3>
Where
the N values are labels that direct control when the value of the
expression is greater than, equal to, and less than 0 respectively. The
different cases are terminated with GO TO statements that direct control
to the next part of the program. While this is very flexible, it does not
provide a single entry point, nor insure that only one of the cases will
be selected.
Computed GOTO and Multiple Selection
FORTRAN's original multiple selection statement
takes the form:
GOTO (<label 1>{,< label n>}),
<expression>
The expression is evaluated and the labels serve as
positional identifiers for possible results. The first label is associated
with the value, 1, the second, 2 , etc. If the expression evaluates to a
value greater than n, the entire selection is ignored. FORTRAN also
provides a similar assigned GO TO. As in the 3-way IF, both of these
provide the possibility of multiple entry points and selection of more
than one of the cases.
FORTRAN 90 provides a case statement
similar to Ada. It allows for value ranges and value lists for each case.
It requires that the list be exhaustive, but provides a default case if
the others do not apply.
SELECT
CASE (<expression>)
CASE
(<first list of values>)
<statements>
CASE
(<second list of values>)
<statements>
...
CASE
DEFAULT
<statements>
END
SELECT
The default step size is 1, and the
parameters are all unsigned integers or integer variables with positive
values. Label is the label number assigned to the last statement in the
loop. FORTRAN 90 allows replacement of the label value with a closing
special word, END DO. The FORTRAN 90 loops and closing statements can be
named to avoid ambiguity.
No Logical Loops
Neither FORTRAN 77 or FORTRAN 90 provide posttest
or pretest logical loops officially as part of the language. These
statements must be constructed using DO loops, IF statements and GO TO
exits. However, some compilers allow for both pre-test and post-test
logical loops.