Fortran - Basic Syntax

A Fortran program is made of a collection of program units like a main program, modules, and external subprograms or procedures.

Each program contains one main program and may or may not contain other program units. The syntax of the main program is as follows −

program program_name
implicit none      

! type declaration statements      
! executable statements  

end program program_name

A Simple Program in Fortran

Let’s write a program that adds two numbers and prints the result −

program addNumbers

! This simple program adds two numbers
   implicit none

! Type declarations
   real :: a, b, result

! Executable statements
   a = 12.0
   b = 15.0
   result = a + b
   print *, 'The total is ', result

end program addNumbers

When you compile and execute the above program, it produces the following result −

The total is 27.0000000    

Please note that −

  • All Fortran programs start with the keyword program and end with the keyword end program, followed by the name of the program.

  • The implicit none statement allows the compiler to check that all your variable types are declared properly. You must always use implicit none at the start of every program.

  • Comments in Fortran are started with the exclamation mark (!), as all characters after this (except in a character string) are ignored by the compiler.

  • The print * command displays data on the screen.

  • Indentation of code lines is a good practice for keeping a program readable.

  • Fortran allows both uppercase and lowercase letters. Fortran is case-insensitive, except for string literals.


The basic character set of Fortran contains −

  • the letters A ... Z and a ... z
  • the digits 0 ... 9
  • the underscore (_) character
  • the special characters = : + blank - * / ( ) [ ] , . $ ' ! " % & ; < > ?

Tokens are made of characters in the basic character set. A token could be a keyword, an identifier, a constant, a string literal, or a symbol.

Program statements are made of tokens.


An identifier is a name used to identify a variable, procedure, or any other user-defined item. A name in Fortran must follow the following rules −

  • It cannot be longer than 31 characters.

  • It must be composed of alphanumeric characters (all the letters of the alphabet, and the digits 0 to 9) and underscores (_).

  • First character of a name must be a letter.

  • Names are case-insensitive


Keywords are special words, reserved for the language. These reserved words cannot be used as identifiers or names.

The following table, lists the Fortran keywords −

The non-I/O keywords
allocatable allocate assign assignment block data
call case character common complex
contains continue cycle data deallocate
default do double precision else else if
elsewhere end block data end do end function end if
end interface end module end program end select end subroutine
end type end where entry equivalence exit
external function go to if implicit
in inout integer intent interface
intrinsic kind len logical module
namelist nullify only operator optional
out parameter pause pointer private
program public real recursive result
return save select case stop subroutine
target then type type() use
Where While
The I/O related keywords
backspace close endfile format inquire
open print read rewind Write