f77

unix v7是最后一个广泛发布的研究型UNIX版本

.IP
In a
.B subroutine
or subroutine
.B entry
statement,
some of the arguments may be noted by an asterisk, as in
.DS
subroutine s(a, \(**, b, \(**)
.DE
The meaning of the ``alternate returns'' is described in section 5.2 of the Appendix.
.NH 1
Declarations
.NH 2
CHARACTER Data Type
.IP
One of the biggest improvements to the language is the addition of a character-string data type.
Local and
common character variables must have a length denoted by a constant expression:
.DS
character\(**17 a, b(3,4)
character\(**(6+3) c
.DE
If the length is omitted entirely, it is assumed equal to 1.
A character string argument may have a constant length,
or the length may be declared to be the same as that of the corresponding actual argument at run time
by a statement like
.DS
character\(**(\(**) a
.DE
(There is an intrinsic function
.B len
that returns the actual length of a character string).
Character arrays and common blocks containing character variables must be packed:
in an array of character variables, the first character of one element must follow the last character of
the preceding element, without holes.
.NH 2
IMPLICIT Statement
.IP
The traditional implied declaration rules still hold:
a variable whose name begins with
\fBi, j, k, l, m,\fR or \fBn\fR is of type
.B integer,
other variables are of type
.B real,
unless otherwise declared.
This general rule may be overridden with an
.B implicit
statement:
.DS
implicit real(a-c,g), complex(w-z), character\(**(17) (s)
.DE
declares that variables whose name begins with an
\fBa ,b, c,\fR
or
\fBg\fR
are
.B real,
those beginning with
\fBw, x, y,\fR
or
\fBz\fR
are assumed
.B complex,
and so on.
It is still poor practice to depend on implicit typing, but this statement is an industry standard.
.NH 2
PARAMETER Statement
.IP
It is now possible to give a constant a symbolic name, as in
.DS
parameter (x=17, y=x/3, pi=3.14159d0, s=\(fmhello\(fm)
.DE
The type of each parameter name is governed by the same implicit and explicit rules as for a variable.
The right side of each equal sign must be a constant expression
(an expression made up of constants, operators, and already defined parameters).
.NH 2
Array Declarations
.IP
Arrays may now have as many as seven dimensions.
(Only three were permitted in 1966).
The lower bound of each dimension may be declared
to be other than 1 by
using a colon.
Furthermore, an adjustable array bound may be an integer expression involving constants,
arguments, and variables in
.B common.
.DS
real a(\(mi5:3, 7, m:n), b(n+1:2\(**n)
.DE
The upper bound on the last dimension of an array argument may be denoted by an asterisk
to indicate that the upper bound is not specified:
.DS
integer a(5, \(**),  b(\(**), c(0:1, \(mi2:\(**)
.DE
.NH 2
SAVE Statement
.IP
A poorly known rule of Fortran 66 is that local variables in a procedure do not necessarily retain their values between
invocations of that procedure.
At any instant in the execution of a program,
if a common block is declared neither in the currently executing procedure
nor in any of the procedures in the chain of callers,
all of the variables in that common block also become undefined.
(The only exceptions are variables that have been defined in a
.B data
statement and never changed).
These rules permit overlay and stack implementations for the affected variables.
Fortran 77 permits one to specify that certain variables and common blocks are to retain their
values between invocations.
The declaration
.DS
save a, /b/, c
.DE
leaves the values of the variables
.B a
and
.B c
and all of the contents of common block
.B b
unaffected by a return.
The simple declaration
.DS
save
.DE
has this effect on all variables and common blocks in the procedure.
A common block must be \fBsave\fRd in every procedure in which it is declared if the desired effect is to occur.
.NH 2
INTRINSIC Statement
.IP
All of the functions specified in the Standard are in a single category,
``intrinsic functions'', rather than being divided into ``intrinsic'' and ``basic external'' functions.
If an intrinsic function is to be passed to another procedure, it must be declared
.B intrinsic.
Declaring it
.B external
(as in Fortran 66) causes a function other than the built-in one to be passed.
.NH 1
Expressions
.NH 2
Character Constants
.IP
Character string constants are marked by strings surrounded by apostrophes.
If an apostrophe is to be included in a constant, it is repeated:
.DS
 \(fmabc\(fm
 \(fmain\(fm\(fmt\(fm
.DE
There are no null (zero-length) character strings in Fortran 77.
Our compiler has two different quotation marks, `` \(fm ''' and `` " ''.
(See Section 2.9 in the main text.)
.NH 2
Concatenation
.IP
One new operator has been added, character string concatenation, marked by a double slash
(``//'').
The result of a concatenation is the string containing the characters of the left operand followed by the characters of
the right operand.
The strings
.DS
 \(fmab\(fm // \(fmcd\(fm
 \(fmabcd\(fm
.DE
are equal.
The strings being concatenated must be of constant length in all concatenations
that are not the right sides of assignments.
(The only concatenation expressions in which a
character string declared adjustable with a ``\(**(\(**)'' modifier
or a substring denotation with nonconstant position values may appear
are the right sides of assignments).
.NH 2
Character String Assignment
.IP
The left and right sides of a character assignment may not share storage.
(The assumed implementation of character assignment is to copy characters from the right to the left side.)
If the left side is longer than the right, it is padded with blanks.
If the left side is shorter than the right, trailing characters are discarded.
.NH 2
Substrings
.IP
It is possible to extract a substring of a character variable or character array element, using the colon notation:
.DS
a(i,\|j) (m:n)
.DE
is the string of $(n-m+1)$ characters beginning at the
$m sup th$ character of the character array element $a sub ij$.
Results are undefined unless $m<=n$.
Substrings may be used on the left sides of assignments and as procedure actual arguments.
.NH 2
Exponentiation
.IP
It is now permissible to raise real quantities to complex powers,
or complex quantities to real or complex powers.
(The principal part of the logarithm is used).
Also, multiple exponentiation is now defined:
.DS
a\(**\(**b\(**\(**c = a \(**\(** (b\(**\(**c)
.DE
.NH 2
Relaxation of Restrictions
.IP
Mixed mode expressions are now permitted.
(For instance,
it is permissible to combine integer and complex quantities in an expression.)
.IP
Constant expressions are permitted where a constant is allowed,
except in
.B data
statements.
(A constant expression is made up of explicit constants and
\fBparameter\fRs
and the Fortran operators,
except for exponentiation to a floating-point power).
An adjustable dimension may now be an integer expression involving constants,
arguments, and variables in
B common..
.IP
Subscripts may now be general integer expressions;
the old
$c v +- c'$
rules have been removed.
.B do
loop bounds may be general integer, real, or double precision expressions.
Computed
.B goto
expressions and I/O unit numbers may be general integer expressions.
.NH 1
Executable Statements
.NH 2
IF-THEN-ELSE
.IP
At last, the
if-then-else
branching structure has been added to Fortran.
It is called a ``Block If''.
A Block If begins with a statement of the form
.DS
if ( . . . ) then
.DE
and ends with an
.DS
end if
.DE
statement.
Two other new statements may appear in a Block If.
There may be several
.DS
else if(. . .) then
.DE
statements,
followed by at most one
.DS
else
.DE
statement.
If the logical expression in the Block If statement is true, the statements following it up to the
next
.B elseif,
.B else,
or
.B endif
are executed.
Otherwise, the next
.B elseif
statement in the group is executed.
If none of the
.B elseif
conditions are true, control passes to the statements following the
.B else
statement, if any.
(The
.B else
must follow all \fBelseif\fRs in a Block If.
Of course, there may be Block Ifs embedded inside of other Block If structures).
A
case
construct may be rendered
.DS
if (s .eq. \(fmab\(fm) then
 . . .
else if (s .eq. \(fmcd\(fm) then
 . . .
else
 . . .
end if
.DE
.NH 2
Alternate Returns
.IP
Some of the arguments of a subroutine call may be statement labels preceded by an asterisk, as in
.DS
call joe(j, \(**10, m, \(**2)
.DE
A
.B return
statement may have an integer expression, such as
.DS
return k
.DE
If the entry point has
$n$
alternate return (asterisk) arguments
and if $1<=k<=n$, the return is followed by a branch to the corresponding statement label;
otherwise the usual return to the statement following the
.B call
is executed.
.NH 1
Input/Output
.NH 2
Format Variables
.IP
A format may be the value of a character expression (constant or otherwise),
or be stored in a character array, as in
.DS
write(6, \(fm(i5)\(fm) x
.DE
.NH 2
END=, ERR=, and IOSTAT= Clauses
.IP
A
.B read
or
.B write
statement may contain
.B end=,
.B err=,
and
.B iostat=
clauses, as in
.DS
write(6, 101, err=20, iostat=a(4))
read(5, 101, err=20, end=30, iostat=x)
.DE
Here 5 and 6 are the
.I units
on which the I/O is done,
101 is the statement number of the associated format,
20 and 30 are statement numbers,
and
.B a
and
.B x
are integers.
If an error occurs during I/O,
control returns to the program at statement 20.
If the end of the file is reached,
control returns to the program at statement 30.
In any case, the variable referred to in
the
.B iostat=
clause is given a value when
the I/O statement finishes.
(Yes, the value is assigned to the name on the right side of the equal sign.)
This value is zero if all went well,
negative for end of file,
and some positive value for errors.
.NH 2
Formatted I/O
.NH 3
Character Constants
.IP
Character constants in formats are copied literally to the output.
Character constants cannot be read into.
.DS
write(6,\(fm(i2,\(fm\(fm isn\(fm\(fm\(fm\(fmt \(fm\(fm,i1)\(fm) 7, 4
.DE
produces
.DS
 7 isn\(fmt 4
.DE
Here the format is the character constant
.DS
(i2,\(fm isn\(fm\(fmt \(fm,i1)
.DE
and the character constant
.DS
 isn\(fmt
.DE
is copied into the output.
.NH 3
Positional Editing Codes
.IP
.B t,
.B tl,
.B tr,
and
.B x
codes
control where the
next character is in the record.
\fBtr\fIn\fR
or
\fIn\fBx\fR
specifies that the next character is
$n$ to the right of the current position.
\fBtl\fIn\fR
specifies that the next character is
$n$ to the left of the current position,
allowing parts of the record to be reconsidered.
\fBt\fIn\fR
says that the next character is to be character
number $n$ in the record.
(See section 3.4 in the main text.)
.NH 3
Colon
.IP
A colon in the format terminates the I/O operation
if there are no more data items in the I/O list,
otherwise it has no effect.
In the fragment
.DS
x=\(fm("hello", :, " there", i4)\(fm
write(6, x) 12
write(6, x)
.DE
the first
.B write
statement prints
\fBhello there 12\fR,
while the second only prints
\fBhello\fR.
.NH 3
Optional Plus Signs
.IP
According to the Standard,
each implementation has the option of putting
plus signs in front of non-negative
numeric output.
The
.B sp
format code may be used to make the optional plus
signs actually appear for all subsequent items
while the format is active.
The
.B ss
format code guarantees that the I/O system will not
insert the optional plus signs,
and the
.B s
format code restores the default behavior of
the I/O system.
(Since we never put out optional plus signs,
.B ss
and
.B s
codes have the same effect in our implementation.)
.NH 3
Blanks on Input
.IP
Blanks in numeric input fields,
other than leading blanks
will be ignored following a
.B bn
code in a format
statement,
and will be treated as zeros following a
.B bz
code in a format statement.
The default for a unit may be changed by using
the
.B open
statement.
(Blanks are ignored by default.)
.NH 3
Unrepresentable Values
.IP
The Standard requires that if a numeric item
cannot be represented in the form required by a format code,
the output field must be filled with asterisks.
(We think this should have been an option.)
.NH 3
Iw.m
.IP
There is a new integer output code,
\fBi\fIw.m.\fR
It is the same as
\fBi\fIw\fR,
except that there will be at least $m$
digits in the output field,
including,
if necessary,
leading zeros.
The case \fBi\fR$w.0$ is special,
in that if the value being printed is 0,
the output field is
entirely blank.
\fBi\fIw\fB.1\fR
is the same as
\fBi\fIw\fR.
.NH 3
Floating Point
.IP
On input, exponents may start with the letter
\fBE, D, e, \fRor \fBd.\fR
All have the same meaning.
On output we always use \fBe\fR.
The
.B e
and
.B d
format codes also have identical meanings.
A leading zero before the decimal point in