ratfor

3 posts

RATFOR & FLECS - Emirp primes

For the final post in this series, let's write a real program in RATFOR and FLECS and see how they compare with the original FORTRAN. We'll be implementing the reverse-primes emirp program we did before.

FLECS version

C     FLECS PROGRAM TO DISPLAY EMIRPS  
C  
C     *** TEST IF A NUMBER IS PRIME ***  
      LOGICAL FUNCTION PRIME(N)
      INTEGER N
C     DEAL WITH NUMBERS <= 3  
      IF (N .LE. 1) GOTO 200
      IF (N .EQ. 2 .OR. N .EQ. 3) GOTO 100
C     CHECK IF DIVISIBLE BY 2 OR 3  
      IF (MOD(N,2) .EQ. 0) GOTO 200
      IF (MOD(N,3) .EQ. 0) GOTO 200
C     SEE IF DIVISIBLE BY 5, 7, ..., UP TO APPROX SQRT(N)  
      DO (I=5,999999,2)
      IF (I*I .GT. N) GOTO 100
      IF (MOD(N,I) .EQ. 0) GOTO 200
      FIN
 100  PRIME = .TRUE.
      RETURN
 200  PRIME = .FALSE.
      RETURN
      END
C  
C     *** REVERSE AN INTEGER'S DIGITS ***  
      INTEGER FUNCTION REVRSE(N)
      INTEGER N
      INTEGER M,R
C     M IS COPY OF N FROM WHICH WE TAKE DIGITS  
C     R IS REVERSED DIGITS  
      M = N
      R = 0
C     LOOP UNTIL NO MORE DIGITS  
      UNTIL (M .LT. 1)
C     TAKE LAST DIGIT FROM M AND APPEND TO R  
      R = R * 10
      R = R + MOD(M, 10)
      M = M / 10
      FIN
      REVRSE = R
      RETURN
      END
C  
C     *** TEST IF AN INTEGER IS AN EMIRP ***  
      LOGICAL FUNCTION EMIRP(N)
      INTEGER N
C     EXTERNAL FUNCTIONS  
      INTEGER REVRSE
      LOGICAL PRIME
C     R CONTAINS REVERSED DIGITS OF N  
      INTEGER R
      R = REVRSE(N)
C     N AND R MUST BOTH BE PRIME AND NOT THE SAME VALUE  
      IF (N .NE. R)
      IF (PRIME(N))
      IF (PRIME(R))
      EMIRP = .TRUE.
      RETURN
      FIN
      FIN
      FIN
      EMIRP = .FALSE.
      RETURN
      END
C  
C     *** DISPLAY AN INTEGER ***  
      SUBROUTINE SHOW(N)
      INTEGER N
      WRITE(6,50) N
 50   FORMAT(I10)
      RETURN
      END
C  
C  
C     *** MAIN ENTRY POINT ***  
C     I IS COUNT OF EMIRPS FOUND  
C     N IS NUMBER TO TEST  
C     EXTERNAL FUNCTION  
      LOGICAL EMIRP
      INTEGER I,N
      TEST-1
      TEST-2
      TEST-3
      STOP
C  
C     *** SHOW FIRST 20 EMIRPS ***  
      TO TEST-1
      N = 0
      I = 0
      WHILE (I .LT. 20)
      N = N + 1
      IF (EMIRP(N))
      CALL SHOW(N)
      I = I + 1
      FIN
      FIN
      FIN
C  
C     *** SHOW EMIRPS BETWEEN 7,700 AND 8,000 ***  
      TO TEST-2
      DO (N=7700,8000)
      IF (EMIRP(N)) CALL SHOW(N)
      FIN
      FIN
C  
C     *** SHOW 10,000TH EMIRP ***  
      TO TEST-3
      N = 0
      DO (I=1,10000)
      REPEAT UNTIL (EMIRP(N)) N = N + 1
      FIN
      CALL SHOW(N)
      FIN
C  
      END

Apart from the FORMAT specification and the PRIME function we've eliminated all line numbers. PRIME could be written without line numbers but with the multiple paths out of the function that would need their own RETURN I think it's better this way.

The internal procedures come in handy, eliminating the need for subroutines for TEST1-3, though this does make N and I global which makes me a little uneasy if this was a larger program.

We use the block structure often, with UNTIL, WHILE and REPEAT ... UNTIL; this simplifies code, though without indentation it's a little hard to follow; the output of the preprocesor is useful here to show what it thinks the indentation should be, for example:

  86           TO TEST-1
  87           .  N = 0
  88           .  I = 0
  89           .  WHILE (I .LT. 20)
  90           .  .  N = N + 1
  91           .  .  IF (EMIRP(N))
  92           .  .  .  CALL SHOW(N)
  93           .  .  .  I = I + 1
  94           .  .  ...FIN
  95           .  ...FIN
  96           ...FIN

The compiler diagnostics also helped a lot with catching errors with missing FINs.

RATFOR

Now let's try writing the RATFOR version.

######################################################################
# Ratfor program to display emirps
######################################################################

######### Test if a number is prime #########
logical function prime(n)  
    integer n  # Number to test

    # Deal with numbers <= 3
    if (n < 1) goto 200
    if (n == 2 | n == 3) goto 100

    # Check if divisible by 2 or 3
    if (mod(n,2) == 0) goto 200
    if (mod(n,3) == 0) goto 200

    # See if divisible by 5, 7, ..., up to approx sqrt(n)
    for (i = 5; i < 1000000; i = i + 2) {
        if (I*I > n) goto 100
        if (mod(n,i) == 0) goto 200
    }

 100  prime = .true.
      return
 200  prime = .false.
      return
end

######### Reverse an integer's digits #########
integer function revrse(n)  
    integer n  # Number to reverse
    integer m  # Copy of n from which we take digits
    integer r  # Reversed digits
    m = n
    r = 0
    while (m >= 1) {
        # Take last digit from m and append to r
        r = r * 10
        r = r + mod(m, 10)
        m = m / 10
    }
    revrse = r
    return
end

######### Test if an integer is an emirp #########
logical function emirp(n)  
    integer n       # Number to test
    integer revrse  # External function
    logical prime   # External function
    integer r       # Reversed digits of n
    r = revrse(n)
    emirp = .false.
    # n and r must both be prime and not the same value
    if (n .ne. r & prime(n) & prime(r)) {
        emirp = .true.
    }
    return
end

######### Display an integer #########
subroutine show(n)  
    integer n
    write(6,50) n
50  format(i10)  
    return
end

######### Show first 20 emirps #########
subroutine test1  
    logical emirp   # External function
    integer i       # Count of emirps found
    integer n       # Number to test
    n = 0
    for (i = 1; i <= 20; i = i + 1) {
        repeat {
            n = n + 1
        } until (emirp(n))
        call show(n)
    }
    return
end

######### Show emirps between 7,700 and 8,000 #########
subroutine test2  
    logical emirp   # External function
    integer n       # Number to test
    for (n = 7700; n <= 8000; n = n + 1) {
        if (emirp(n)) {
            call show(n)
        }
    }
    return
end

######### Show 10,000th emirp #########
subroutine test3  
    logical emirp   # External function
    integer i       # Count of emirps found
    integer n       # Number to test
    n = 0
    for (i = 1; i <= 10000; i = i + 1) {
        repeat {
            n = n + 1
        } until (emirp(n))
    }
    call show(n)
    return
end

######### Main entry point #########
call test1  
call test2  
call test3  
stop  
end  

I feel right at home with the braces and the C style for loops, though I miss the increment operator ++. prime would be much better if I could just return (.true.) but that does not work on the version of RATFOR on MTS so we keep the line numbers and gotos.

With the above, plus the free form input (which was supported on MTS FORTRAN anyway) and the operators like < it was easy to write. However, I got precisely zero diagnostics from the RATFOR preprocessor, with all my typos caught by the FORTRAN compiler, from which I'd have to find the problem in the original source. Easy enough in a small program but would be painful in larger ones.

Final thoughts

RATFOR and FLECS both make writing FORTRAN easier and more pleasant at the cost of an extra step in the development process, and I found both succeed at that. RATFOR is clearer and easier to get started with (especially coming from a C background today); the implementation is almost aggressively simple, as the authors admit in their paper, and I wonder how well it would scale for writing larger programs. FLECS has a more robust implementation but a more diffuse design, such as two versions of switch; features like printing a neatly indented output would certainly help on MTS or its contemporaries but the language lacks the cosmetic features that make RATFOR easier to read.

Neither are much used today; FORTRAN 77 and beyond took some of these ideas and built them into the core language. The idea of translating a richer language into a widely used but less expressive language is still alive though: think of Coffeescript or Typescript producing Javascript.

Further information

Full source code for these programs can be found on github.

RATFOR & FLECS - Language Features

FORTRAN meeting From the UM Computing Center Newsletter, Volume 5 Number 14, 24 September 1975, via Google Books. Proposal 2) seems to indicate a different preprocessor was being considered for UM as well as FLECS, I wonder if this was RATFOR or something else?

Hi and welcome back. Today let's continue our exploration of RATFOR and FLECS by comparing the language features they add to vanilla FORTRAN. The quotes below are from the RATFOR paper and FLECS manual, links to which are provided at the end of this post. Code samples for FORTRAN and FLECS are shown in upper case, RATFOR in lower case.

Design

RATFOR attempts to retain the merits of FORTRAN (universality, portability, efficiency) while hiding the worst FORTRAN inadequacies. The language is FORTRAN except for two aspects - [control flow and syntactic sugar] ... Throughout, the design principle which has determined what should be in RATFOR and what should not has been RATFOR doesn’t know any FORTRAN.

RATFOR focuses on control flow - if statements, blocks, looping - and cosmetics such as free form input, comments and other features that make FORTRAN more pleasant to write. By not knowing any FORTRAN, the design limits what features can be made available but also keeps it simple to implement and reduces the temptation to change FORTRAN into a different language altogether.

FLECS is a language extension of FORTRAN which has additional control mechanisms . These mechanisms make it easier to write FORTRAN by eliminating much of the clerical detail associated with constructing FORTRAN programs. FLECS is also easier to read and comprehend than FORTRAN.

FLECS also tries ti improve FORTRAN's control statements, taking ideas from several different languages including Pascal and Lisp. It has less cosmetic additions than RATFOR but adds the concept of internal procedures and includes features in the translator that help the programmer see the structure of their program.

Structure

RATFOR allows blocks of statements to be introduced within braces where FORTRAN would only allow a single statement. The fixed column format in classic FORTRAN is relaxed so any indentation is allowed. Multiple statements can appear on the same line if they are separated by semicolons.

if (x > 100) {  
   call error(x)
   err = 1; return
}

FLECS also has blocks which extend from the start of a control statement to the keyword FIN. It retains the fixed formatting of FORTRAN but prints a nicely indented view of the program when translating. So the example above would be entered as this in FLECS:

      IF (X .GT. 100)
      CALL ERROR(X)
      ERR = 1
      FIN

and the translator would print

IF (X .GT. 100)  
.  CALL ERROR(X)
.  ERR = 1
...FIN

This is useful when entering programs via cards where it is difficult to get indentation right.

It's possible to have a single statement after a control structure in which case the FIN is not needed:

IF (X .GT. 100) CALL ERROR(X)  

RATFOR comments are introduced with # and apply from that point to the end of the line, less restrictive than C in FORTRAN and FLECS which must be in the first column.

% will stop RATFOR processing the rest of the line, passing it through to FORTRAN directly. FLECS will look for a FLECS statement in column 7 and if found will translate the line; if not found it will pass through the whole line to FORTRAN.

Textual substitution

RATFOR allows constants to be set with define SYMBOL VALUE; any use of SYMBOL in the RATFOR program will be replaced with VALUE in the generated FORTRAN program.

include FILE will insert a copy of FILE at that point in the program, just like C's #include.

Operators

RATFOR allows the now-familiar symbols <, <=, !=, | etc to be used instead of .LT., .LE., .NE., .OR. etc. FLECS retains the FORTRAN operators.

Strings

Text in RATFOR programs in single or double quotes is converted to FORTRAN nH strings. Backslash escapes the next character. FLECS keeps FORTRAN strings.

Conditionals

FORTRAN has a simple iF statement where only one statement can be executed if the condition is true. RATFOR extends this by allowing else and nested ifs. An else clause is attached to the nearest if.

if (x > 0) {  
  if (x > 10)
    write(6, 1) x
  else
    write(6, 2) x
else  
  weite(6, 3)

FLECS has IF and for negative tests UNLESS. It also has WHEN ... ELSE for a single positive and negative test.

The switch statement added in RATFOR looks like C but does not have break; the switch is exited after each case or default is executed. FLECS's equivalent is SELECT, so comparing the two:

switch (x) {  
  case 1: y=3
  case 2, 3: y=5
  default y=0
}
      SELECT (X)
      (1) Y=3
      (2) Y=5
      (3) Y=5
      (OTHERWISE) Y=0
      FIN

FLECS has CONDITIONAL which looks a lot like LISP's cond:

      CONDITIONAL
      (X.LT.-5.0)  U = U+W
      (X.LE.1.0)   U = U+W+Z
      (X.LE.10.5)  U = U-Z
      (OTHERWISE)  U = 0
      FIN

Looping

The FORTRAN DO loop has to have a line number marking the point where the loop will restart:

      DO 10 i = 1, n
      x(i) = 0.0
      y(i) = 0.0
      z(i) = 0.0
 10   CONTINUE

RATFOR replaces this with a block:

do i = 1, n {  
  x(i) = 0.0
  y(i) = 0.0
  z(i) = 0.0
}

It also allows break to exit a loop early and next to restart the loop like C's continue. It can be followed by an integer to say how many levels to apply, so break 2 would move out of a two level do statement immediately.

RATFOR also adds a while and for statement that look like C's - these allow immediate exit from the statement if the condition is true on entry, unlike in FORTRAN DO where the statement is always executed at least once (in the IBM implementation at least) and the conditional is tested at the end of the statement. A version of C's do ... while is provided as repeat ... until.

The FLECS equivalent for the above do loop would be:

      DO (I = 1, N)
      X(I) = 0.0
      Y(I) = 0.0
      Z(I) = 0.0
      FIN

FLEC's WHILE construct is similar to RATFOR's, with the conditional tested before the loop starts. By using REPEAT WHILE the body of the loop is executed at least once and the test made at the end of the loop. UNTIL can be used instead of WHILE in both cases to indicate that the loop ends when the conditional becomes true

      X = 0
      UNTIL (X.EQ.5)
      X = X + 1
      FIN

Return

To return a value from a function in FORTRAN and FLECS you must assign a value to the name of the function:

INTEGER FUNCTION DECREMENT(I)  
INTEGER I  
DECREMENT = I - 1  
RETURN  
END  

In the RATFOR paper it sayd you can give return a value:

integer function decrement(i)  
integer i  
return (i-1)  
end  

However, note this is not supported in the version supplied with MTS - it will just pass through such a return statement causing an error from the FORTRAN compiler.

Internal procedures

FLECS allows a group of statements to be defined as a procedure with TO which can then be called by giving its name. No parameters are passed - it uses global variables to communicate. The below example will print 5.

      INTEGER X
      X = 1
      INCREMENT-IT
      DOUBLE-AND-INCREMENT
      WRITE(6,50) X
      STOP
 50   FORMAT(I10) 
      TO INCREMENT-IT X = X + 1
      TO DOUBLE-AND-INCREMENT
      X = X * 2
      INCREMENT
      FIN
      END

Procedure names must include at least one hyphen and recursion is not allowed.

Operation

RATFOR runs as a simple translator, taking a RATFOR input file and producing a FORTRAN output file that must then be fed to the FORTRAN compiler. FLECS, as modified at UM, will both translate and call the FORTRAN compiler, producing machine code output that can be run directly.

Error handling

RATFOR will catch some errors, such as missing closing braces, but will otherwise delegate problems with the program to the FORTRAN compiler to catch, as it does not understand FORTRAN syntax. This could be difficult to trace back to the source of the error as the FORTRAN compiler would show the error in the generated FORTRAN, not the RATFOR original.

FLECS will find syntax errors and remove them from the program, allowing translation to continue at the cost of possibly causing further errors; it will not move on to compilation in this case.

Implementation

Not surprisingly given its authors' roots, RATFOR was originally written in around 1000 lines of C using yacc. The authors say it took less than a week to implement. As C was not widely available in the mid 70's, a version of RATFOR in RATFOR was produced that would generate around 2500 lines of basic FORTRAN so it could be used anywhere.

The FLECS implementation comes in at around 2200 lines of FLECS and took around six months to develop according to comments in the source code.

Further information

See Kernighan's RATFOR paper or the FLECSUser's Manual (in component 673/22; I've uploaded a copy here) for more information on the languages.

RATFOR & FLECS - Introduction

Most programmers will agree that FORTRAN is an unpleasant language to program in, yet there are many occasions when they are forced to use it.

From the introduction to 'RATFOR — A Preprocessor for a Rational FORTRAN' by Brian W. Kernighan

FORTRAN was the lingua franca for mainframe programmers in the 1960s and 1970s, but as Kernighan states it's not always easy to program in - the main reasons are lack of good control structures and the fixed line format. As a result, a number of preprocessors were developed that translated enhanced code down to plain FORTRAN that could then be compiled anywhere a compiler was available.

In this series of posts, we'll look at two preprocessors available on MTS: RATFOR and FLECS. MTS also had OVERDRIVE, but this is not available on D6.0 due to copyright reasons.

Prerequisites

No special installation instructions to get these preprocessors running - just do the standard D6.0 setup as described in this guide and then sign on as a regular user such as ST01.

RATFOR

RATFOR was developed by Brian Kernighan at Bell Telephone Labs in 1974; its syntax was (not surprisingly) inspired by the C programming language, with keywords like for, while and until. It was used as the language for examples in Software Tools and became one of the most popular preprocessors in use. Versions are still available today that run on Unix systems.

Preprocessing using *RATFOR

The version on MTS is called *RATFOR and takes a RATFOR program as input on scards and writes FORTRAN source to spunch. The generated file can then be compiled with *FTN.

Hello world

Here's a terminal log of how to compile and run a simple hello world program in RATFOR. This assumes the source code is in file hello.r.

# $list hello.r

      1     # *** Simple hello world program ***
      2     #
      3     integer i
      4     for (i = 0; i < 5; i = i + 1)
      5     {
      6        write(6, 200)
      7     }
      8     stop
      9     200 format("Hello, world!")
     10     end

# $run *ratfor scards=hello.r spunch=-hello.f
Execution begins   21:56:08  
Execution terminated   21:56:08  T=0.004

# #list -hello.f

      1           INTEGERI
      2           CONTINUE
      3           I=0
      4     23000 IF(.NOT.(I.LT.5))GOTO 23002
      5           WRITE(6,200)
      6     23001 I=I+1
      7           GOTO 23000
      8     23002 CONTINUE
      9           STOP
     10     200   FORMAT(13HHello, world!)
     11           END

# $run *ftn scards=-hello.f spunch=-load
Execution begins   21:56:36  
 No errors in MAIN
Execution terminated   21:56:36  T=0.008

# #run -load
Execution begins   21:56:39  
 Hello, world!
 Hello, world!
 Hello, world!
 Hello, world!
 Hello, world!
Execution terminated   21:56:39  T=0.001  

FLECS

FLECS was written in the early 1970s by Terry Beyer at the University of Oregon. It provides a smaller set of control structures that RATFOR but the syntax is closer to FORTRAN. Keywords include IF...THEN...ELSE and CONDITIONAL and multi-line statements are supported. It does not appear to have been used much past the introduction of FORTRAN77, but a a version is still available today for HPUX.

Compiling using UNSP:FLX

At the time D6.0 was released, FLECS was unsupported at UM so is available as the file FLX in UNSP:. The preprocessor does not used scards and spunch; instead, all parameters need to be passed in to par. Unlike RATFOR, FLECS can call the FORTRAN compiler directly to generate object code. In the listing below, PAR=SOURCE=hello.fl,P=*SINK*,FTNSOURCE,LOAD=-load would read source from hello.fl, print diagnostics to *SINK including the FORTRAN source generated, and write compiled output to -load.

Hello world

Here's a terminal log of how to compile and run a simple hello world program in FLECS. This assumes the source code is in file hello.fl.

# $list hello.fl

      1     C *** SIMPLE HELLO WORLD PROGRAM ***
      2     C
      3           DO (I = 1,5)
      4           WRITE (6,20)
      5           FIN
      6           STOP
      7        20 FORMAT(13H HELLO, WORLD)
      8           END

# $run UNSP:FLX PAR=SOURCE=hello.fl,P=*SINK*,FTNSOURCE,LOAD=-load
Execution begins   21:47:21  
 FFI(CT206)


 (FLECS VERSION 22.38)  MTS Version CT155 21:47:21    JAN 21, 1916    Page   1

 MTS Line#        Indented Source Listing...

      1     C *** SIMPLE HELLO WORLD PROGRAM ***
      2     C
      3           DO (I = 1,5)
      4           .  WRITE (6,20)
      5           ...FIN
      6           STOP
      7        20 FORMAT(13H HELLO, WORLD)
      8           END

   0.001 seconds CPU time used.  Translation rate is 480000 lines per CPU minute.

 There were   NO MAJOR ERRORS and   NO MINOR ERRORS in the above module.
 No preprocessor errors in module  1.


  MICHIGAN TERMINAL SYSTEM FORTRAN G(21.8) MAIN 01-21-16 21:47:21 PAGE P001

    0001              DO 99998 I = 1,5             3.000
    0002              WRITE (6,20)                 4.000
    0003        99998 CONTINUE                     5.000
    0004              STOP                         6.000
    0005           20 FORMAT(13H HELLO, WORLD)     7.000
    0006              END                          8.000
     *OPTIONS IN EFFECT*  ID,EBCDIC,SOURCE,NOLIST,NODECK,LOAD,NOMAP
     *OPTIONS IN EFFECT*  NAME = MAIN    , LINECNT =       57
     *STATISTICS*    SOURCE STATEMENTS =        6,PROGRAM SIZE =      344
     *STATISTICS*  NO DIAGNOSTICS GENERATED
 No errors in MAIN

 NO STATEMENTS FLAGGED IN THE ABOVE COMPILATIONS.
Execution terminated   21:47:21  T=0.018

# $run -load
Execution begins   21:47:31  
 HELLO, WORLD
 HELLO, WORLD
 HELLO, WORLD
 HELLO, WORLD
 HELLO, WORLD
Execution terminated   21:47:31  T=0.001  

Further information

The Wikipedia article on RATFOR has a basic introduction to language features and the history of its development. Kernighan's paper on RATFOR goes into more detail on the language.

Not much appears to exist on the Internet describing FLECS, but the D6.0 MTS tapes does include the complete User's Manual (in component 673/22) and the interface to MTS.

MTS Volume 6 describes the FORTRAN compilers on MTS, which are needed to compile the RATFOR preprocessor's output.