A Brief Review of FORTRAN 77 by John H. Mathews
(to accompany "NUMERICAL METHODS: FORTRAN Programs", Prentice Hall, 1995).

Specified Columns

     Each line of FORTRAN code must be written using the following
     column conventions.

Column     Contents
    1     "C"  or  "*"  for a comment line
   1-5     Statement number or label (non-signed integer).
    6      Blank or character for continuation line.
  7-72     FORTRAN statement
 73-80     Sequence number (usually omitted).

Form of a FORTRAN Program

     PROGRAM <program-name>
          {<specification-statements>}
          {<executable-statements>}
     END

Form of a FORTRAN Subroutine

     SUBROUTINE <subroutine-name>[<actual-argument-list>]
          {<specification-statements>}
          {<executable-statements>}
     RETURN
     END

Form of a Subroutine call.

     CALL <subroutine-name>[<actual-argument-list>]

Form of a FORTRAN function

     [<type>] FUNCTION <function-name> [(<dummy-assignment list>)]
          {<specification-statements>}
          {<executable-statements>}
     RETURN
     END

Specification Statements (or Type Declaration Statements)

     INTEGER  I,J,Row
     REAL  A0,B0,Max,X,Y
     DOUBLE PRECISION
     LOGICAL  A,B,C
     COMPLEX  Z,W0
     CHARACTER*10  Name
     IMPLICIT  F,DF
     PARAMETER  (Pi=3.1415926535)
     DATA  E/2.178281828/
     COMMON  A0,B0
     COMMON  /BlockA/ A,B,X,Y


Variables

     Variable names can be from one to six characters long.
     Variables beginning with I,J,K,L,M,N are presumed to be integers unless
     they are declared otherwise.

Assignment Statements

     X = A(1) + 3
     Y = A*X**2 + B*X + C
     Flag = .TRUE.
     Name = 'Fran'

Warning. Mixed Mode arithmetic might result in some frustrating surprises.  For example, when two integers are divided the result is an unexpected integer value. To avoid errors use explicit decimal points for real numbers and the intrinsic functions REAL, INT, DBLE, and CMPLX to get the required type conversion.

Arrays

     DIMENSION A(1:50), M(1:10,1:10)

Arithmetic Operations

     +     Addition
     -     Subtraction
     *     Multiplication
     /     Division
     **    Exponentiation

Relational Operators

     .EQ.     Equal to
     .NE.     Not equal to
     .LT.     Less than
     .GT.     Greater than
     .LE.     Less than or equal to
     .GE.     Greater than or equal to

Logical Operators

     .NOT.    Complement
     .AND.    True if both operands are true
     .OR.     True if either (or both) operands are true

Logical Constants

     .TRUE.
     .FALSE.

Remark Statement

     C        This is a comment.

Place-holder Statement (or label)

     10     CONTINUE

Unconditional Transfer

     GOTO 10


Computed Control Statement

Transfers control to a specified statement, depending on the value of an integer expression, e.g. <IJUMP>.

     GOTO(100,200,300,400), IJUMP

Warning. Do not use the  GOTO  statement to transfer into a DO, IF, ELSE, or ELSEIF block from outside the block.

IF (Arithmetic) Control Statement

Transfers control to a statement depending on whether the value of the (<arithmetic-expression>) is positive, negative or zero.

     IF (<arithmetic-expression>) 100, 200, 300

IF (Logical) Control Statement
     Executes a single statement only if the (<logical-expression>) is true.

     IF (<logical-expression>) GOTO 100
     IF (<logical-expression>) WRITE (*,*) 'Yes'
     IF (<logical-expression>) X = A+B

IF (Block) Control Statement

Performs the series of {<executable-statement>} following it or transfers control to an ELSEIF, ELSE, or ENDIF statement, depending on the value of the (<logical-expression>).

     IF (<logical-expression>) THEN
          {<executable-statements>}
     ENDIF

     IF (<logical-expression>) THEN
          {<executable-statements>}
     ELSE
          {<executable-statements>}
     ENDIF

     IF (<logical-expression-#1>) THEN
          {<executable-statements>}
     ELSEIF (<logical-expression-#2>) THEN
          {<executable-statements>}
     ELSEIF (<logical-expression-#3>) THEN
          {<executable-statements>}
                    

     ELSE
          {<executable-statements>}
     ENDIF

     IF (ABS(P3-P1).LT.ABS(P3-P0)) THEN
          U=P1; P1=P0; P0=U
          V=Y1; Y1=Y0; Y0=V
     ENDIF

     IF (Df.EQ.0) THEN
          Dp=P1-P0
          P1=P0
     ELSE
          Dp=Y0/Df
          P1=P0-Dp
     ENDIF

     IF (YC.EQ.0) THEN
          A=C
          B=C
     ELSEIF ((YB*YC).GT.0) THEN
          B=C
          YB=YC
          KR=KR+1
     ELSE
          A=C
          YA=YC
          KL=KL+1
     ENDIF

DO (Block) Control Statement

     DO  K = M1, M2
          {<executable-statements>}
     ENDDO

     DO  K = M1, M2, Mstep
          {<executable-statements>}
     ENDDO

     DO  (M  TIMES)
          {<executable-statements>}
     ENDDO

     SUM = 0
     DO K=0,100,2
          SUM = SUM + K
     ENDDO

     SUM = 0
     DO K=100,-1,1
          SUM = SUM + 1.0/REAL(K)
     ENDDO

     DO J=1,5
          DO K=1,5
               A(J,K) = 1.0/FLOAT(1+J+K)
          ENDDO
     ENDDO

     SUM = 0
     DO K=1,10000
          SUM = SUM + 1.0/REAL(K)
          IF (SUM.GT.5) EXIT
     ENDDO

WHILE (Block) Control Statement

     WHILE (<logical-expression>)
          {<executable-statements>}
     REPEAT

     SUM = 0
     WHILE (K.LT.10000)
          SUM = SUM + 1.0/REAL(K)
          IF (SUM.GT.5) EXIT
          K = K+1
     REPEAT

Input and Output

     READ  *,<input-variable-name-list>
     READ  <format>,<input-variable-name-list>
     READ  <N>,<input-variable-name-list>
          where <N> is a FORMAT statement number, e.g.
111     FORMAT(5X,I10,4F15.5)

     PRINT  *
     PRINT  *,<output-expression-list>
     PRINT  <format>,<output-expression-list>
     PRINT  <N>,<output-expression-list>
          where <N> is a FORMAT statement number, e.g.
999     FORMAT(5X,'X = ',F15.5)

     WRITE  (*,*)
     WRITE  (*,*) <output-expression-list>
     WRITE  (*,N) <output-expression-list>
          where <N> is a FORMAT statement number, e.g.
999     FORMAT(5X,'X = ',F15.5)

Pause Statement

     PAUSE

Stop Statement

     STOP

Mathematical Functions

     COS(X)      cosine (radians)
     SIN(X)      sine (radians)
     TAN(X)      tangent (radians)
     EXP(X)      exponential  exp(x)
     ACOS(X)     inverse cosine (radians)
     ASIN(X)     inverse sine (radians)
     ATAN(X)     inverse tangent (radians)
     ALOG(X)     natural logarithm base  e
     LOG10(X)    common logarithm base  10
     SQRT(X)     square root
     ABS(X)      absolute value
     INT(X)      conversion to integer
     FLOAT(I)    conversion to real number type
     REAL(I)     conversion to real number type
     DBLE(X)     conversion to double precision type
     CMPLX(X)    conversion to double precision type