12. Introduction to FORTRAN90/95

You fill find that there are many similarities between FORTRAN90/95 and MATLAB. In this section we will highlight several of the differences so that you can begin writing programs in FORTRAN90/95 right away. The first difference is that all FORTRAN90/95 programs must begin and end with the statements PROGRAM myprog (where myprog is the name of your program) and END PROGRAM. Note that FORTRAN90/95 is not case-sensitive so there is no difference between capitalizing and not capitalizing. My personal preference is not to capitalize at all since common text editiors such as emacs and vim perform syntax highlighting and use different colors for keywords, variables, etc. (much the same way the m-file editor for MATLAB does). Each FORTRAN90/95 program you write should also have the statement IMPLICIT NONE as the second line. This command forces the programmer to explicitly ``declare'' all the variables (and their types) that are to be used. MATLAB let us use implicit typing but this can be dangerous. With explicit typing, you must declare all your variables in the first part of your program before the first executable statement (DO, IF/THEN, etc.). For example, if you want to use the counter variables i and j in a couple of DO loops in your program, you would declare them as integer, scalar variables by including the declaration statement INTEGER i, j in the beginning of your program. To declare an array of 100 floating point values named $f$, use REAL, DIMENSION(100) :: f. Table 12.1 lists some common fortran keywords and a simple example of how each is used.

Table 12.1: Common keywords in FORTRAN90/95. Keywords are shown in capital letters. Arguments and variables are shown in lower case.

Statement	Description	Examples
Assignment	assigns a value to a variable	pi = 3.1415
statement		text = 'abcedf'
Block IF	Branching conditional construct	IF (x > 0.) THEN
construct		result = SQRT(x)
		ELSE IF (x==0.) THEN
		result = 0.
		ELSE
		result = SQRT(-x)
		END IF
CALL	Calls a subroutine	CALL numerical_sort(data)
CASE	Branching among mutually	SELECT CASE (j)
	exclusive choices	CASE (1,2,5)
		block 1 commands
		CASE (-3:0)
		block 2 commands
		CASE DEFAULT
		block 3 commands
		END SELECT
CHARACTER	Declares variables or named	CHARACTER(len=10) :: string
	constants of character type
CLOSE	Closes a file	CLOSE(10)
COMPLEX	Declares variables or named constants	COMPLEX xypair
	of complex type	COMPLEX, DIMENSION(10) :: carray
CYCLE	Branches to top of loop	CYCLE
DO (counting	A loop that repeats a block of	DO j = 1, 10, 2
loop) construct	statements a specified number of times	write(*,*) ``Root is '', SQRT(j)
		END DO
DO (while	A loop that repeats a block of	DO
loop) construct	statements until a condition is met	IF (j==1) EXIT
		...
		END DO
END FUNCTION	Last statement of a function	END FUNCTION myfunc
END MODULE	Last statement of a module	END MODULE mymod
END PROGRAM	Last statement of a program	END FUNCTION myprog
END SUBROUTINE	Last statement of a subroutine	END SUBROUTINE mysub
EXIT	Branches out of a loop to the	IF (val < 0.) EXIT
	first statement after the loop
FORMAT	Defines the format for	12 FORMAT('Int = ', I4)
	input or output of data	WRITE(*,12) counter
FUNCTION	Declares the beginning of a	INTEGER FUNCTION factorial(n)
	user-defined function subprogram
IF	Executes or skips the associated	IF (y < 0.) y = SQRT(-y)
	statement depending on the
	conditional.
IMPLICIT NONE	Forces explicit typing	IMPLICIT NONE
INTEGER	Declares variables or named	INTEGER :: i, j, k
	constants of integer type	INTEGER, DIMENSION(25) :: i
LOGICAL	Declares variables or named	LOGICAL test1
	constants of logical type	LOGICAL, DIMENSION(25) :: flag
OPEN	Opens a file	OPEN(10)
PROGRAM	Defines a program and its name	PROGRAM my_prog
READ	Reads in data	READ(*,*) key_input
		READ(myfile,'(I5)')
		READ (12, 50) a(1), a(2)
REAL	Declares variables or named	REAL fvalue
	constants of floating-point type	REAL, DIMENSION(3,3) :: avecs
RETURN	Returns control from a procedure	RETURN
	to the routine that invoked the
	procedure
STOP	Stops execution of the program	STOP
SUBROUTINE	Declares the beginning of a	SUBROUTINE inverse(matrix)
	user-defined subroutine
WHERE	Masked array assignment	WHERE (x > 0.)
		x = SQRT(x)
		END WHERE
WRITE	Writes data	WRITE(*,*) screen_out
		WRITE(myfile,'(6F8.3)') vector(1:6)
		WRITE (12, 50) a(1:3)