Older versions of Fortran had strict guidelines on how a program was formatted. Fortran 90 lifted this restriction and would accept free format code as well as historical fixed format code.
Fixed format Edit
Prior to Fortran 90, source code followed a well-defined fixed format. Comments are indicated with a 'C' in the first column, columns 2-5 were reserved for an optional numerical statement label, a non-blank character in column 6 indicated the current line was a continuation from the previous one, and columns 7 through 72 were available for program statements. Columns 73 through 80 were ignored and often contained line sequence numbers. Blank lines were not allowed. This rigid formatting was the result of Fortran being developed in the era of batch computing and punched card input. The sequence number was used in the case a program 'deck' was dropped; program order could be recovered if the punch cards were placed in a card reader and sorted on columns 73-80. Compiler vendors offered extensions to this formatting, but it was rarely portable (for example, interpreting tab characters as 6 spaces.)
Note that while column position was significant, white space was not. The following program illustrates legal use of white space in fixed-format Fortran:
C2345678901234567890 PROGRAM Z GOTO11 11 CONTINUE GO TO 780 780 CONTINUE G OTO3 60 360 CONTINUE STOP END
While this code is technically legal, it is strongly encouraged to use white space to separate keywords, labels, and data to maintain readability.
Fortran was developed before the standardization of the ASCII character set and traditionally Fortran code has been written in all-caps. Variable names were limited to six characters, but this was often extended by compiler vendors.
Free format Edit
As of Fortran 90 and onward, source code does not require fixed column formatting. In this case, commands can freely start on any column. The 72 column limit has also been released. This allows for much more space for indentation.
program test implicit none integer four four = 4 write (*,*) four end program
Case sensitivity Edit
Fortran is not case sensitive. Fortran was typically used on systems that only supported capital letters. In fact, the language itself was called FORTRAN (in capitals). It remains customary, though completely unnecessary, to type Fortran commands in all capitals. This is useful to distinguish keywords in source code that is displayed on monocrome displays and print. These days, syntax highlighting is available to replace this. However it may be useful to visually distinguish older Fortran code from modern source code.
Whitespace and empty lines usually won't matter in Fortran 90 or above. Some statements require whitespace, for example,
subroutine require whitespace between the statement keyword and the program unit identifier.
However, unlike many other languages such as C, C++, and Java, the line delimiter ';' is optional, so each line of code may stay on its own line. However, the use of the command separator character ';' is discouraged.
Program units Edit
Fortran programs are made up of program units. A single source code file can contain several program units but it is conventional to place each program unit in its own separate source code file. At their most basic they consist of a series of Fortran statements and conclude with the
Main program Edit
Every executable program must have a main program unit. For example, the following is a complete compilable and executable program.
write (*,*) "Hello, world" end
However, it is much clearer to use the
program statement to indicate that it is the main program unit.
program main write (*,*) "Hello, world!" end program main
The main program is separated into sections. The first section should consist of module
use statements. This is followed by
implicit none statements that control whether undeclared variables are implicitly typed. This is followed by the declaration section where variables, types, interfaces and procedures are declared. Then comes the executable statements of the main program. The last section is the internal subprograms initiated by the
program main ! Use statements section use module_name ! Implicit none statement section implicit none ! Declarations section integer :: a real :: b ! Executable section write (*,*) "Blah, blah, blah..." end program main
Program units may also be subprograms: these can be procedures (functions and subroutines), block data, modules or submodules.
External subprograms Edit
The following code shows a main program and a function. The increment function is external to the main program, and therefore needs a declaration in the main program on line 4.
program main implicit none integer :: a integer, external :: increment a = increment(34) write (*,*) a end program main function increment(input) result (output) implicit none integer :: output integer :: input output = input + 1 end function increment
However, the interface of the function is still implicit. To explicitly declare an external procedure, one can use an interface that declares all the inputs and outputs of external procedures. In which case, the main program would be as follows.
program main implicit none integer :: a interface integer function increment(input) integer :: input end function end interface a = increment(34) write (*,*) a end program main
Internal subprograms Edit
Internal subprograms do not need an explicit interface or a declaration, because they are part of the parent program unit. A subprogram is internal if it is contained within the
contains section of a program unit.
program main implicit none integer :: a a = increment(34) write (*,*) a contains function increment(input) result (output) integer :: output integer :: input output = input + 1 end function increment end program main