minnie.tuhs.org/unixen.tar/unixen/PDP-11/Trees/V7/usr/doc/ratfor/m2

Text File | 1979-01-10 | 27.0 KB | 1,377 lines

.NH
LANGUAGE DESCRIPTION
.SH
Design
.PP
Ratfor
attempts to retain the merits of Fortran
(universality, portability, efficiency)
while hiding the worst Fortran inadequacies.
The language
.ul
is
Fortran except for two aspects.
First,
since control flow is central to any program,
regardless of the specific application,
the primary task of
Ratfor
is to conceal this part of Fortran from the user,
by providing decent control flow structures.
These structures are sufficient and comfortable
for structured programming in the narrow sense of programming without
.UC GOTO 's.
Second, since the preprocessor must examine an entire program
to translate the control structure,
it is possible at the same time to clean up many of the
``cosmetic'' deficiencies of Fortran,
and thus provide a language which is easier
and more pleasant to read and write.
.PP
Beyond these two aspects _ control flow and cosmetics _
Ratfor
does nothing about the host of other weaknesses of Fortran.
Although it would be straightforward to extend
it
to provide
character strings,
for example,
they are not needed by everyone,
and of course
the preprocessor would be harder to implement.
Throughout, the design principle which has determined
what should be in
Ratfor
and what should not has
been
.ul
Ratfor
.ul
doesn't know any Fortran.
Any language feature which would require that
Ratfor
really understand Fortran has been omitted.
We will return to this point in the section
on implementation.
.PP
Even within the confines of control flow and cosmetics,
we have attempted to be selective
in what features to provide.
The intent has been to provide a small set of the most useful
constructs,
rather than to throw in everything that has ever been thought useful
by someone.
.PP
The rest of this section contains an informal description
of the
Ratfor
language.
The control flow aspects will be
quite familiar to readers used to languages like
Algol, PL/I, Pascal, etc.,
and the cosmetic changes are equally straightforward.
We shall concentrate on
showing what the language looks like.
.SH
Statement Grouping
.PP
Fortran provides no way to group statements together,
short of making them into a subroutine.
The standard construction
``if a condition is true,
do this group of things,''
for example,
.P1
if (x > 100)
    { call error("x>100"); err = 1; return }
.P2
cannot be written directly in Fortran.
Instead
a programmer is forced to translate this relatively
clear thought into murky Fortran,
by stating the negative condition
and branching around the group of statements:
.P1
    if (x .le. 100) goto 10
        call error(5hx>100)
        err = 1
        return
10    ...
.P2
When the program doesn't work,
or when it must be modified,
this must be translated back into
a clearer form before one can be sure what it does.
.PP
Ratfor
eliminates this error-prone and confusing back-and-forth translation;
the first form
.ul
is
the way the computation is written in
Ratfor.
A group of statements can be treated as a unit
by enclosing them in the braces { and }.
This is true throughout the language:
wherever a single
Ratfor
statement can be used,
there can be several enclosed in braces.
(Braces seem clearer and less obtrusive than
.UL begin
and
.UL end
or
.UL do
and
.UL end ,
and of course
.UL do
and
.UL end
already have Fortran meanings.)
.PP
Cosmetics
contribute to the readability of code,
and thus to its understandability.
The character ``>'' is clearer than
.UC ``.GT.'' ,
so
Ratfor
translates it appropriately,
along with several other similar shorthands.
Although many Fortran compilers permit character strings in quotes
(like
.UL """x>100""" ),
quotes are
not allowed in
.UC ANSI
Fortran,
so
Ratfor
converts it into the right number of
.UL H 's:
computers count better than people do.
.PP
Ratfor
is a free-form language:
statements may appear anywhere on a line,
and several may appear on one line
if they are separated by semicolons.
The example above could also be written as
.P1
if (x > 100) {
    call error("x>100")
    err = 1
    return
}
.P2
In this case, no semicolon is needed at the end of each line because
Ratfor
assumes there is one statement per line
unless told otherwise.
.PP
Of course,
if the statement that follows the
.UL if
is a single statement
(Ratfor
or otherwise),
no braces are needed:
.P1
if (y <= 0.0 & z <= 0.0)
    write(6, 20) y, z
.P2
No continuation need be indicated
because the statement is clearly not finished on the first line.
In general
Ratfor
continues lines when it seems obvious that they are not yet done.
(The continuation convention is discussed in detail later.)
.PP
Although a free-form language permits wide latitude in formatting styles,
it is wise to pick one that is readable, then stick to it.
In particular, proper indentation is vital,
to make the logical structure of the program obvious to the reader.
.sp
.SH
The ``else'' Clause
.PP
Ratfor
provides an
.UL "else"
statement to handle the construction
``if a condition is true,
do
this
thing,
.ul
otherwise
do that thing.''
.P1
if (a <= b)
    { sw = 0; write(6, 1) a, b }
else
    { sw = 1; write(6, 1) b, a }
.P2
This writes out the smaller of
.UL a
and
.UL b ,
then the larger, and sets
.UL sw
appropriately.
.PP
The Fortran equivalent of this code is circuitous indeed:
.P1
    if (a .gt. b) goto 10
        sw = 0
        write(6, 1) a, b
        goto 20
10    sw = 1
    write(6, 1) b, a
20    ...
.P2
This is a mechanical translation;
shorter forms exist,
as they do for many similar situations.
But all translations suffer from the same problem:
since they are translations,
they are less clear and understandable than code
that is not a translation.
To understand the Fortran version,
one must scan the entire program to make
sure that no other statement branches
to statements 10 or 20
before one knows that indeed this is an
.UL if-else
construction.
With the
Ratfor
version,
there is no question about how one gets to the parts of the statement.
The
.UL if-else
is a single unit,
which can be read, understood, and ignored if not relevant.
The program says what it means.
.PP
As before, if the statement following an
.UL if
or an
.UL else
is a single statement, no braces are needed:
.P1
if (a <= b)
    sw = 0
else
    sw = 1
.P2
.PP
The syntax of the
.UL if
statement is
.P1
if (\fIlegal Fortran condition\fP)
    \fIRatfor statement\fP
else
    \fIRatfor statement\fP
.P2
where the
.UL else
part is optional.
The
.ul
legal Fortran condition
is
anything that can legally go into a Fortran Logical
.UC IF .
Ratfor
does not check this clause,
since it does not know enough Fortran
to know what is permitted.
The
.ul
Ratfor
.ul
statement
is any Ratfor or Fortran statement, or any collection of them
in braces.
.SH
Nested if's
.PP
Since the statement that follows an
.UL if
or
an
.UL else
can be any Ratfor statement, this leads immediately
to the possibility of another
.UL if
or
.UL else .
As a useful example, consider this problem:
the variable
.UL f
is to be set to
\-1 if
.UL x
is less than zero,
to
+1
if
.UL x
is greater than 100,
and to 0 otherwise.
Then in Ratfor, we write
.P1
if (x < 0)
    f = -1
else if (x > 100)
    f = +1
else
    f = 0
.P2
Here the statement after the first
.UL else
is another
.UL if-else .
Logically it is just a single statement,
although it is rather complicated.
.PP
This code says what it means.
Any version written in straight Fortran
will necessarily be indirect
because Fortran does not let you say what you mean.
And as always, clever shortcuts may turn out
to be too clever to understand a year from now.
.PP
Following an
.UL else
with an
.UL if
is one way to write a multi-way branch in Ratfor.
In general the structure
.P1
if (...)
    - - -
else if (...)
    - - -
else if (...)
    - - -
 ...
else
    - - -
.P2
provides a way to specify the choice of exactly one of several alternatives.
(Ratfor also provides a
.UL switch
statement which does the same job
in certain special cases;
in more general situations, we have to make do
with spare parts.)
The tests are laid out in sequence, and each one
is followed by the code associated with it.
Read down the list
of decisions until one is found that is satisfied.
The code associated with this condition is executed,
and then the entire structure is finished.
The trailing
.UL else
part handles the ``default'' case,
where none of the other conditions apply.
If there is no default action, this final
.UL else
part
is omitted:
.P1
if (x < 0)
    x = 0
else if (x > 100)
    x = 100
.P2
.SH
if-else ambiguity
.PP
There is one thing to notice about complicated structures
involving nested
.UL if 's
and
.UL else 's.
Consider
.P1
if (x > 0)
    if (y > 0)
        write(6, 1) x, y
    else
        write(6, 2) y
.P2
There are two
.UL if 's
and
only one
.UL else .
Which
.UL if
does the
.UL else
go with?
.PP
This is a genuine ambiguity in Ratfor, as it is in many other programming
languages.
The ambiguity is resolved in Ratfor
(as elsewhere) by saying that in such cases the
.UL else
goes with the closest previous
.UL else 'ed un-
.UL if .
Thus in this case, the
.UL else
goes with the inner
.UL if ,
as we have indicated by the indentation.
.PP
It is a wise practice to resolve such cases by explicit braces,
just to make your intent clear.
In the case above, we would write
.P1
if (x > 0) {
    if (y > 0)
        write(6, 1) x, y
    else
        write(6, 2) y
}
.P2
which does not change the meaning, but leaves
no doubt in the reader's mind.
If we want the other association, we
.ul
must
write
.P1
if (x > 0) {
    if (y > 0)
        write(6, 1) x, y
}
else
    write(6, 2) y
.P2
.SH
The ``switch'' Statement
.PP
The
.UL switch
statement
provides a clean way to express multi-way branches
which branch on the value of some integer-valued expression.
The syntax is
.P1
\f3switch (\fIexpression\|\f3) {

    case \fIexpr1\f3 :
        \f2statements\f3
    case \fIexpr2, expr3\f3 :
        \f2statements\f3
    ...
    default:
        \f2statements\f3
}
.P2
.PP
Each
.UL case
is followed by a
list of comma-separated integer expressions.
The
.ul
expression
inside
.UL switch
is compared against the case expressions
.ul
expr1,
.ul
expr2,
and so on in turn
until one matches,
at which time the statements following that
.UL case
are executed.
If no cases match
.ul
expression,
and there is a
.UL default
section,
the statements with it are done;
if there is no
.UL default,
nothing is done.
In all situations,
as soon as some block of statements is executed,
the entire
.UL switch
is exited immediately.
(Readers familiar with C[4] should beware that this
behavior is not the same as the C
.UL switch .)
.SH
The ``do'' Statement
.PP
The
.UL do
statement in
Ratfor
is quite similar to the
.UC DO
statement in Fortran,
except that it uses no statement number.
The statement number, after all, serves only to mark the end
of the
.UC DO ,
and this can be done just as easily with braces.
Thus
.P1
    do i = 1, n {
        x(i) = 0.0
        y(i) = 0.0
        z(i) = 0.0
    }
.P2
is the same as
.P1
    do 10 i = 1, n
        x(i) = 0.0
        y(i) = 0.0
        z(i) = 0.0
10    continue
.P2
The syntax is:
.P1
do \fIlegal\(hyFortran\(hyDO\(hytext\fP
    \fIRatfor statement\fP
.P2
The part that follows
the keyword
.UL do
has to be something that can legally go into a Fortran
.UC DO
statement.
Thus if a local version of Fortran allows
.UC DO
limits to be expressions
(which is not currently permitted in
.UC ANSI
Fortran),
they can be used in a
Ratfor
.UL do.
.PP
The
.ul
Ratfor statement
part will often be enclosed in braces, but
as with the
.UL if ,
a single statement need not have braces around it.
This code sets an array to zero:
.P1
do i = 1, n
    x(i) = 0.0
.P2
Slightly more complicated,
.P1
do i = 1, n
    do j = 1, n
        m(i, j) = 0
.P2
sets the entire array
.UL m
to zero, and
.P1
do i = 1, n
    do j = 1, n
        if (i < j)
            m(i, j) = -1
        else if (i == j)
            m(i, j) = 0
        else
            m(i, j) = +1
.P2
sets the upper triangle of
.UL m
to \-1, the diagonal to zero, and the lower triangle to +1.
(The operator == is ``equals'', that is, ``.EQ.''.)
In each case, the statement that follows the
.UL do
is logically a
.ul
single
statement, even though complicated,
and thus needs no braces.
.sp
.SH
``break'' and ``next''
.PP
Ratfor
provides a statement for leaving a loop early,
and one for beginning the next iteration.
.UL "break"
causes an immediate exit from the
.UL do ;
in effect it is a branch to the statement
.ul
after
the
.UL do .
.UL next
is a branch to the bottom of the loop,
so it causes the next iteration to be done.
For example, this code skips over negative values in an array:
.P1
do i = 1, n {
    if (x(i) < 0.0)
        next
    \fIprocess positive element\fP
}
.P2
.UL break
and
.UL next
also work in the other Ratfor looping constructions
that we will talk about in the next few sections.
.PP
.UL break
and
.UL next
can be followed by an integer to indicate breaking or iterating
that level of enclosing loop; thus
.P1
break 2
.P2
exits from two levels of enclosing loops,
and
.UL break\ 1
is equivalent to
.UL break .
.UL next\ 2
iterates the second enclosing loop.
(Realistically,
multi-level
.UL break 's
and
.UL next 's
are
not likely to be much used
because they lead to code that is hard to understand
and somewhat risky to change.)
.sp
.SH
The ``while'' Statement
.PP
One of the problems with the Fortran
.UC DO
statement
is that it generally insists upon being done once,
regardless of its limits.
If a loop begins
.P1
DO I = 2, 1
.P2
this will typically be done once with
.UL I
set to 2,
even though common sense would suggest that perhaps it shouldn't be.
Of course a
Ratfor
.UL do
can easily be preceded by a test
.P1
if (j <= k)
    do i = j, k  {
        _ _ _
    }
.P2
but this has to be a conscious act,
and is often overlooked by programmers.
.PP
A more serious problem with the
.UC DO
statement
is that it encourages that a program be written
in terms of an arithmetic progression
with small positive steps,
even though that may not be the best way to write it.
If code has to be contorted to fit the requirements
imposed by the Fortran
.UC DO ,
it is that much harder to write and understand.
.PP
To overcome these difficulties,
Ratfor
provides a
.UL while
statement,
which is simply a loop:
``while some condition is true,
repeat this group of statements''.
It has
no preconceptions about why one is looping.
For example, this routine to compute sin(x)
by the Maclaurin series
combines two termination criteria.
.P1 1
.ta .3i .6i .9i 1.2i 1.5i 1.8i
real function sin(x, e)
    # returns sin(x) to accuracy e, by
    # sin(x) = x - x**3/3! + x**5/5! - ...

    sin = x
    term = x

    i = 3
    while (abs(term)>e & i<100) {
        term = -term * x**2 / float(i*(i-1))
        sin = sin + term
        i = i + 2
    }

    return
    end
.P2
.PP
Notice that
if the routine is entered with
.UL term
already smaller than
.UL e ,
the
loop will be done
.ul
zero times,
that is, no attempt will be made to compute
.UL x**3
and thus a potential underflow is avoided.
Since the test is made at the top of a
.UL while
loop
instead of the bottom,
a special case disappears _
the code works at one of its boundaries.
(The test
.UL i<100
is the other boundary _
making sure the routine stops after
some maximum number of iterations.)
.PP
As an aside, a sharp character ``#'' in a line
marks the beginning of a comment;
the rest of the line is comment.
Comments and code can co-exist on the same line _
one can make marginal remarks,
which is not possible with Fortran's ``C in column 1'' convention.
Blank lines are also permitted anywhere
(they are not in Fortran);
they should be used to emphasize the natural divisions
of a program.
.PP
The syntax of the
.UL while
statement is
.P1
while (\fIlegal Fortran condition\fP)
    \fIRatfor statement\fP
.P2
As with the
.UL if ,
.ul
legal Fortran condition
is something that can go into
a Fortran Logical
.UC IF ,
and
.ul
Ratfor statement
is a single statement,
which may be multiple statements in braces.
.PP
The
.UL while
encourages a style of coding not normally
practiced by Fortran programmers.
For example, suppose
.UL nextch
is a function which returns the next input character
both as a function value and in its argument.
Then a loop to find the first non-blank character is just
.P1
while (nextch(ich) == iblank)
    ;
.P2
A semicolon by itself is a null statement,
which is necessary here to mark the end of the
.UL while ;
if it were not present, the
.UL while
would control the next statement.
When the loop is broken,
.UL ich
contains the first non-blank.
Of course the same code can be written in Fortran as
.P1 1
100    if (nextch(ich) .eq. iblank) goto 100
.P2
but many Fortran programmers (and a few compilers) believe this line is illegal.
The language at one's disposal
strongly influences how one thinks about a problem.
.sp
.SH
The ``for'' Statement
.PP
The
.UL for
statement
is another Ratfor loop, which
attempts to carry the separation of
loop-body from reason-for-looping
a step further
than the
.UL while.
A
.UL for
statement allows explicit initialization
and increment steps as part of the statement.
For example,
a
.UC DO
loop is just
.P1
for (i = 1; i <= n; i = i + 1) ...
.P2
This is equivalent to
.P1
i = 1
while (i <= n) {
    ...
    i = i + 1
}
.P2
The initialization and increment of
.UL i
have been moved into the
.UL for
statement,
making it easier to see at a glance
what controls the loop.
.PP
The
.UL for
and
.UL while
versions have the advantage that they will be done zero times
if
.UL n
is less than 1; this is not true of the
.UL do .
.PP
The loop of the sine routine in the previous section
can be re-written
with a
.UL for
as
.P1 3
for (i=3; abs(term) > e & i < 100; i=i+2) {
    term = -term * x**2 / float(i*(i-1))
    sin = sin + term
}
.P2
.PP
The syntax of the
.UL for
statement is
.P1
for ( \fIinit\fP ; \fIcondition\fP ; \fIincrement\fP )
    \fIRatfor statement\fP
.P2
.ul
init
is any single Fortran statement, which gets done once
before the loop begins.
.ul
increment
is any single Fortran statement,
which gets done at the end of each pass through the loop,
before the test.
.ul
condition
is again anything that is legal in a logical
.UC IF.
Any of
.ul
init,
.ul
condition,
and
.ul
increment
may be omitted,
although the semicolons
.ul
must
always be present.
A non-existent
.ul
condition
is treated as always true,
so
.UL "for(;;)"
is an indefinite repeat.
(But see the
.UL repeat-until
in the next section.)
.PP
The
.UL for
statement is particularly
useful for
backward loops, chaining along lists,
loops that might be done zero times,
and similar things which are hard to express with a
.UC DO
statement,
and obscure to write out
with
.UC IF 's
and
.UC GOTO 's.
For example,
here is a
backwards
.UC DO
loop
to find the last non-blank character on a card:
.P1
for (i = 80; i > 0; i = i - 1)
    if (card(i) != blank)
        break
.P2
(``!='' is the same as
.UC ``.NE.'' ).
The code scans the columns from 80 through to 1.
If a non-blank is found, the loop
is immediately broken.
.UL break \& (
and
.UL next
work in
.UL for 's
and
.UL while  's
just as in
.UL do 's).
If
.UL i
reaches zero,
the card is all blank.
.PP
This code is rather nasty to write with a regular Fortran
.UC DO ,
since the loop must go forward,
and we must explicitly set up proper conditions
when we fall out of the loop.
(Forgetting this is a common error.)
Thus:
.P1 1
.ta .3i .6i .9i 1.2i 1.5i 1.8i
    DO 10 J = 1, 80
        I = 81 - J
        IF (CARD(I) .NE. BLANK) GO TO 11
10    CONTINUE
    I = 0
11    ...
.P2
The version that uses the
.UL for
handles the termination condition properly for free;
.UL i
.ul
is
zero when we fall out of the
.UL for
loop.
.PP
The increment
in a
.UL for
need not be an arithmetic progression;
the following program walks along a list
(stored in an integer array
.UL ptr )
until a zero pointer is found,
adding up elements from a parallel array of values:
.P1
sum = 0.0
for (i = first; i > 0; i = ptr(i))
    sum = sum + value(i)
.P2
Notice that the code works correctly if the list is empty.
Again, placing the test at the top of a loop
instead of the bottom eliminates a potential boundary error.
.SH
The ``repeat-until'' statement
.PP
In spite of the dire warnings,
there are times when one really needs a loop that tests at the bottom
after one pass through.
This service is provided by the
.UL repeat-until :
.P1
repeat
    \fIRatfor statement\fP
until (\fIlegal Fortran condition\fP)
.P2
The
.ul
Ratfor statement
part is done once,
then the condition is evaluated.
If it is true, the loop is exited;
if it is false, another pass is made.
.PP
The
.UL until
part is optional, so a bare
.UL repeat
is the cleanest way to specify an infinite loop.
Of course such a loop must ultimately be broken by some
transfer of control such as
.UL stop ,
.UL return ,
or
.UL break ,
or an implicit stop such as running out of input with
a
.UC READ
statement.
.PP
As a matter of observed fact[8], the
.UL repeat-until
statement is
.ul
much
less used than the other looping constructions;
in particular, it is typically outnumbered ten to one by
.UL for
and
.UL while .
Be cautious about using it, for loops that test only at the
bottom often don't handle null cases well.
.SH
More on break and next
.PP
.UL break
exits immediately from
.UL do ,
.UL while ,
.UL for ,
and
.UL repeat-until .
.UL next
goes to the test part of
.UL do ,
.UL while
and
.UL repeat-until ,
and to the increment step of a
.UL for .
.SH
``return'' Statement
.PP
The standard Fortran mechanism for returning a value from a function uses the name of the function as a variable which can be assigned to;
the last value stored in it
is the function value upon return.
For example, here is a routine
.UL equal
which returns 1 if two arrays are identical,
and zero if they differ.
The array ends are marked by the special value \-1.
.P1 1
.ta .3i .6i .9i 1.2i 1.5i 1.8i
# equal _ compare str1 to str2;
#    return 1 if equal, 0 if not
    integer function equal(str1, str2)
    integer str1(100), str2(100)
    integer i

    for (i = 1; str1(i) == str2(i); i = i + 1)
        if (str1(i) == -1) {
            equal = 1
            return
        }
    equal = 0
    return
    end
.P2
.PP
In many languages (e.g., PL/I)
one instead says
.P1
return (\fIexpression\fP)
.P2
to return a value from a function.
Since this is often clearer, Ratfor provides such a
.UL return
statement _
in a function
.UL F ,
.UL return (expression)
is equivalent to
.P1
{ F = expression; return }
.P2
For example, here is
.UL equal
again:
.P1 1
.ta .3i .6i .9i 1.2i 1.5i 1.8i
# equal _ compare str1 to str2;
#    return 1 if equal, 0 if not
    integer function equal(str1, str2)
    integer str1(100), str2(100)
    integer i

    for (i = 1; str1(i) == str2(i); i = i + 1)
        if (str1(i) == -1)
            return(1)
    return(0)
    end
.P2
If there is no parenthesized expression after
.UL return ,
a normal
.UC RETURN
is made.
(Another version of
.UL equal
is presented shortly.)
.sp
.SH
Cosmetics
.PP
As we said above,
the visual appearance of a language
has a substantial effect
on how easy it is to read and understand
programs.
Accordingly, Ratfor provides a number of cosmetic facilities
which may be used to make programs more readable.
.SH
Free-form Input
.PP
Statements can be placed anywhere on a line;
long statements are continued automatically,
as are long conditions in
.UL if ,
.UL while ,
.UL for ,
and
.UL until .
Blank lines are ignored.
Multiple statements may appear on one line,
if they are separated by semicolons.
No semicolon is needed at the end of a line,
if
Ratfor
can make some reasonable guess about whether the statement
ends there.
Lines ending with any of the characters
.P1
=    +    -    *    ,    |    &    (    \(ru
.P2
are assumed to be continued on the next line.
Underscores are discarded wherever they occur;
all others remain as part of the statement.
.PP
Any statement that begins with an all-numeric field is
assumed to be a Fortran label,
and placed in columns 1-5 upon output.
Thus
.P1
write(6, 100); 100 format("hello")
.P2
is converted into
.P1
    write(6, 100)
100    format(5hhello)
.P2
.SH
Translation Services
.PP
Text enclosed in matching single or double quotes
is converted to
.UL nH...
but is otherwise unaltered
(except for formatting _ it may get split across card boundaries
during the reformatting process).
Within quoted strings, the backslash `\e' serves as an escape character:
the next character is taken literally.
This provides a way to get quotes (and of course the backslash itself) into
quoted strings:
.P1
"\e\e\e\(fm"
.P2
is a string containing a backslash and an apostrophe.
(This is
.ul
not
the standard convention of doubled quotes,
but it is easier to use and more general.)
.PP
Any line that begins with the character `%'
is left absolutely unaltered
except for stripping off the `%'
and moving the line one position to the left.
This is useful for inserting control cards,
and other things that should not be transmogrified
(like an existing Fortran program).
Use `%' only for ordinary statements,
not for the condition parts of
.UL if ,
.UL while ,
etc., or the output may come out in an unexpected place.
.PP
The following character translations are made,
except within single or double quotes
or on a line beginning with a `%'.
.P1
.ta .5i 1.5i 2i
==    .eq.    !=    .ne.
>    .gt.    >=    .ge.
<    .lt.    <=    .le.
&    .and.    |    .or.
!    .not.    ^    .not.
.P2
In addition, the following translations are provided
for input devices with restricted character sets.
.P1
.ta .5i 1.5i 2i
[    {    ]    }
$(    {    $)    }
.P2
.SH
``define'' Statement
.PP
Any string of alphanumeric characters can be defined as a name;
thereafter, whenever that name occurs in the input
(delimited by non-alphanumerics)
it is replaced by the rest of the definition line.
(Comments and trailing white spaces are stripped off).
A defined name can be arbitrarily long,
and must begin with a letter.
.PP
.UL define
is typically used to create symbolic parameters:
.P1
define    ROWS    100
define    COLS    50
.if t .sp 5p
dimension a(ROWS), b(ROWS, COLS)
.if t .sp 5p
    if (i > ROWS  \(or  j > COLS) ...
.P2
Alternately, definitions may be written as
.P1
define(ROWS, 100)
.P2
In this case, the defining text is everything after the comma up to the balancing
right parenthesis;
this allows multi-line definitions.
.PP
It is generally a wise practice to use symbolic parameters
for most constants, to help make clear the function of what
would otherwise be mysterious numbers.
As an example, here is the routine
.UL equal
again, this time with symbolic constants.
.P1 3
.ta .3i .6i .9i 1.2i 1.5i 1.8i
define    YES        1
define    NO        0
define    EOS        -1
define    ARB        100

# equal _ compare str1 to str2;
#    return YES if equal, NO if not
    integer function equal(str1, str2)
    integer str1(ARB), str2(ARB)
    integer i

    for (i = 1; str1(i) == str2(i); i = i + 1)
        if (str1(i) == EOS)
            return(YES)
    return(NO)
    end
.P2
.SH
``include'' Statement
.PP
The statement
.P1
    include file
.P2
inserts the file
found on input stream
.ul
file
into the
Ratfor
input in place of the
.UL include
statement.
The standard usage is to place
.UC COMMON
blocks on a file,
and
.UL include
that file whenever a copy is needed:
.P1
subroutine x
    include commonblocks
    ...
    end

suroutine y
    include commonblocks
    ...
    end
.P2
This ensures that all copies of the
.UC COMMON
blocks are identical
.SH
Pitfalls, Botches, Blemishes and other Failings
.PP
Ratfor catches certain syntax errors, such as missing braces,
.UL else
clauses without an
.UL if ,
and most errors involving missing parentheses in statements.
Beyond that, since Ratfor knows no Fortran,
any errors you make will be reported by the Fortran compiler,
so you will from time to time have to relate a Fortran diagnostic back
to the Ratfor source.
.PP
Keywords are reserved _
using
.UL if ,
.UL else ,
etc., as variable names will typically wreak havoc.
Don't leave spaces in keywords.
Don't use the Arithmetic
.UC IF .
.PP
The Fortran
.UL nH
convention is not recognized anywhere by Ratfor;
use quotes instead.