Software Engineers Handbook/Language Dictionary/FORTRAN

FORTRAN

Here is the wikipedia entry. The following briefly describes Fortran77. Fortran 77 was a refinement of Fortran 66 and Fortran IV, but added some desirable new features. Most notably CHARACTERtypes.

Type

Fortran is a full procedural language.

Execution Entry Point

The main entry point to a program is at a PROGRAM statement.

       PROGRAM XYZ

General Syntax

Fortran was designed with punch card in mind, hence the actual Fortran code start on the 7th character offset. Characters are prefix by 6 characters. Characters 1 to 5 indicate a label, character 6 indicates a line continuation.

For example:

12345 PROGRAM 
     +HLO WLD
67890 PRINT *,
     +'HELLO WORLD'
99999 END

N.B. inserting an extra character at the wrong place may cause unexpected behaviour. c.f. wikiquote:Special:Search=C._A._R._Hoare regarding the Mariner space rocket.

A PROGRAM and the <END> statement are optional in this example.

Note also:

The characters from position 76 onward are ignored. So be careful when you insert a character into a long line. (IBM FORT/VS compiler used to insert line numbers in this column for some odd reason)
The space character is ignored, except in quotes, most notably these two lines are interpreted the same, hence the suspicion relating to the Mariner crash.

       DO 19,I=1,99
       DO19I=1,99

Comments

A 'C', '*' and sometimes a 'c' character in the first column indicates a comment. For example:

* This is a hello world program
12345 PROGRAM 
     +HLO WLD
C Output Hello World
67890 PRINT *,
     +'HELLO WORLD'
c all done, I'm out of here...
99999 END

Variable Declarations

In Fortran the type of a variable is implicitly defined by the first letter of the variable name. Variables beginning with the letters I through N were automatically considered to be integers, while A through H and O through Z were considered to be real numbers.

This behavior can be modified with an IMPLICIT INTEGER declaration (setting the letter range of integers) or an explicit declaration of a variable giving its type.

Hence "GOD is REAL (unless declared INTEGER)."

Example:

      PROGRAM EG DEC
* Net line is not needed as this would be IMPLICIT INTEGER
      INTEGER I,J,K
* long INTEGERs
      INTEGER II*8,JJ*8,KK*8
* define some arrays
      INTEGER M(2),N(3),O(4)
* Some array initialisations
      INTEGER MM(2)/1,2/,NNN(3)/1,2,3/,OOOO(4)/1,2,3,4/
* Some REAL initialisations
      REAL R/1.0/,S/2.0/,T/3.0/
* Some DOUBLE PRECISION (long) initialisations
      REAL*8 RR/1.0D/
      DOUBLE PRECISION SS/2.0D/
      END

Method Declaration/Implementation

Fortran has several kind of FUNCTIONS/SUBROUTINES. Example:

 
       PROGRAM P
         PRINT *,'The cube of 2 is',CUBE(2.0)
         PRINT *,'The square of 2 is',SQUARE(2.0)
       END

       FUNCTION CUBE(X)
* FILSQ is an inline Function returning the square of a value
         FILSQ(X)=X*X
         CUBE=FILSQ(X)*X
       RETURN
         ENTRY SQUARE(X)
         CUBE=FILSQ(X)
       END

Produces:

 The cube of 2 is  8.
 The square of 2 is  4.

Note that in the output the first character is normally a blank character. By inserting other characters in the first column you can force the printer to do form feeds, and perform other interesting behaviors.

Scope

The scope of variable was strictly limited to the current PROGRAM/SUBROUTINE or DATA BLOCK. DATA BLOCK can be shared between PROGRAM/SUBROUTINEs, allowed the programmer to extend the scope of a variable.

Conditional Statements

* A one liner
       IF(1 .EQ. 2)PRINT *,'Dreaming'
* A block structured
       IF(1 .EQ. 1)THEN
         PRINT *,'A distinct possibility'
       ELIF(2 .EQ. 2)THEN
         PRINT *,'Too late'
       ELSE
         PRINT *,'Way too late'
       ENDIF
*

Conditional statements also include GOTO statements.

       I=3
       GOTO(111,112,113,114)I
         PRINT *,'I was not found'
       GOTO 119
111    CONTINUE
         print *,'I was 1'
       GOTO 119
112    CONTINUE
         print *,'I was 2'
       GOTO 119
113    CONTINUE
         print *,'I was 3'
       GOTO 119
114    CONTINUE
         print *,'I was 4'
       GOTO 119
119    CONTINUE

FORTRAN also has a 3 way condition.

       I=1
       IF(I)121,122,123 
121    CONTINUE
         print *,'I is less then zero'
       GOTO 129
122    CONTINUE
         print *,'I is zero'
       GOTO 129
123    CONTINUE
         print *,'I is greater then zero'
       GOTO 129
129    CONTINUE

Looping Statements

FORTRAN 77 > DO / WHILE LOOP

       A = 1
       DO WHILE (A .LT. 10)
          PRINT *, 'COUNTING: ', A
          A = A + 1
       END DO

In older FORTRANS labels and GOTOs would be used to the same effect. For example:

       INTEGER A
       IF(.NOT.(A .LT. 10))GO TO 139
         PRINT *,'COUNTING: ',A
         A = A + 1
* A convention is to terminate the label number with a 9
*   in cases where the GOTO breaks out of a loop.
139    CONTINUE

Output Statements

Example one hello world:

       PRINT *,'Hello World'

Or:

       WRITE (*,*)'Hello World'

Or (The stand output channel is 6):

       WRITE (6,*)'Hello World'

Or:

       WRITE (6,'( A)')'Hello World'

Or:

       WRITE (UNIT=6,FMT='( A)')'Hello World'

Or:

       WRITE (UNIT=6,FMT=141)'Hello World'
141    FORMAT(A)

Fortran includes OPEN, CLOSE, READ, WRITE and PRINT statements. The READ and WRITE commands could access recorded in a file by record number, hence Fortran contains a semblence of random file access. Unfortunately on File IO the size of a "word" is implementation dependent, and ranges from 6 bits on CDC, 8bits on IBM and 32 bits on VAXVMS.

Containers

FORTRAN 77 has no structure or class definition. The closest example was the COMMON statement.

       CHARACTER NAME*16, ADDRESS*64
       FLOAT BALANCE*4
       COMMOM /CSTREC/NAME,ADDRESS,BALANCE

An EQUIVALENCE statement allows such COMMON block statement be be overlayed, and allow direct assignment. But this is problematic.

       CHARACTER ALLREC*(16+64+4)
       EQUIVALENCE(ALLREC,CUSREC)

An EQUIVALENCE also have a similar effect as the union from Algol68 (and then C) language union statement. The net effect is two variables ALLREC,CUSREC share the same location in memory.

Algorithms

Access to other languages was only available if the language supported FORTRANs SUBROUTINE calling conventions (Not all FORTRANs used the stack!). And the actual conventions varied from machine to machine.

Garbage collection

Garbage collection is not part of the language, but has been implemented separately using datablock. In Fortran the memory at program startup was all that would be available to the program until it terminated. (The exception would be if the compiler supported recursion with local variables, but this was not always the case.)

Physical Structure

The external scoping rules of Fortran is similar to the scoping found in the C programming language. However Fortran 77 does not have a standard INCLUDE statement.

DATABLOCK EXTNM1
EXTERNAL EXTNM2
PROGRAM EXTNM3
FUNCTION EXTNM3
ENTRY EXTNM4
SUBROUTINE EXTNM5
DATA BLOCK
COMMON EXTNM6

Tips

Arrays are indexed starting with 1.

Web References

The Fortran Wikibook
The FAQ page for the comp.lang.fortran usenet group

Books and Articles