PROGRAM sample
C   this is how you put in a comment
C       CALL sumArray
C       CALL arrays
C       CALL erroneous
C       CALL recurse(0)
C       CALL setCommon
C       CALL showCommon
C       CALL useFCN
        CALL shortIF
        END

        SUBROUTINE sumArray
        WRITE(*,*), 'What is your limit?'
        READ(*,*), limit
        CALL directly(limit)
        CALL inArray(limit)
        END

        SUBROUTINE directly(howmany)
        INTEGER howmany
        REAL current, sum, bigsum
C       I never use current.  "g77 -Wall" warns me.
        sum = 0.0
C       bigsum is accidentally not initialized.  It has garbage.
        DO 10, I = 1, howmany
C          forgot to declare variable I
           bigsum = bigsum + (float(I)+0.1)**2;
           sum = sum + I**2;
 10     CONTINUE
        WRITE(*,*), 'Directly: the sum is ', sum,
      1   ' and the big sum is ', bigsum
        END

        SUBROUTINE inArray(howmany)
        INTEGER howmany
        REAL sum, bigsum
        DIMENSION intValues(howmany), realValues(howmany)
        REAL realValues
        INTEGER intValues
        DO 10, I = 1, howmany
           intValues(I) = I
           realValues(I) = I + 0.1
 10     CONTINUE
        sum = 0.0
        bigsum = 0.0
        DO 20, I = 1, howmany
           sum = sum + intValues(I)**2
           bigsum = bigsum + realValues(I)**2
 20     CONTINUE
        WRITE(*,*), 'inArray: the sum is ', sum,
      1   ' and the big sum is ', bigsum
        END
        
        SUBROUTINE arrays
C       demonstrates other features of Fortran 
        DIMENSION array3(10, 10, 10)
        INTEGER array3
        INTEGER row, col, plane

C       fill a 3-d array, using 3-way IF and GOTO statements
        DO 79 row = 1, 10
            DO 79 col = 1, 10
                DO 79 plane = 1, 10
                    IF (row + col - (2*plane)) 10, 20, 30
 10                 array3(row, col, plane) = -row;
                    GOTO 79
 20                 array3(row, col, plane) = 0;
                    GOTO 79
 30                 array3(row, col, plane) = col + plane;
 79     CONTINUE
C       print out the array, using formatted output and DO notation in
C       output list
        DO 179 row = 1, 10
            PRINT 1000, row
 1000       FORMAT (4HRow , I3)
            DO 179 col = 1, 10
                PRINT 2000, col, (array3(row, col, plane),plane = 1,10)
 2000           FORMAT ('  Column ', I3, ': ', 10I4)
 179    CONTINUE
        END

        SUBROUTINE modify(x)
        INTEGER x
        x = 4
        END

        SUBROUTINE erroneous
        WRITE(*,*) 'Three is ', 3
        CALL modify(3)
        WRITE(*,*) 'Three is ', 3
        END

        SUBROUTINE rhelp(depth)
C       direct recursion is not allowed, so we use indirect for "recurse()"
        INTEGER depth
        CALL recurse(depth)
        END

        SUBROUTINE recurse(depth)
        INTEGER depth
        WRITE(*,*) 'Depth is ', depth
        IF (depth - 5) 10, 10, 20
 10     CALL rhelp(depth+1)
 20     RETURN
        END

        SUBROUTINE setCommon
        COMMON A, B, C, I, J
C       using default types
        A = 1.0
        B = 2.1
        C = 3.2
        I = 10
        J = 20
        END

        SUBROUTINE showCommon
        COMMON X, Y, I, J, K
C       using default types; I is a mistake, because the value is float.
        WRITE(*,*) 'Contents of common: ', X, Y, I, J, K
        END

        REAL FUNCTION quad(X)
        INTEGER X
        quad = 4*X
        END

        SUBROUTINE useFCN
        triple(X) = 3 * X
        REAL val;
        val = 3.1;
        WRITE(*,*) 'Three times ', val, ' is ', triple(val)
        WRITE(*,*) 'Four times ', val, ' is ', quad(IFIX(val))
        END

        SUBROUTINE shortIF
        INTEGER X
        X = 30
        IF (X .GT. 40) GOTO 39
        RETURN
 39     WRITE(*,*) 'X is big'
        END