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Fortran Syntax

Most languages these days are inspired from C, but Fortran was designed before C. It has started a different family of programming languages that use symilar keywords and concepts. Fortran is not a curly bracket language.

Define Program

In Fortran, you have to declare the main program using a special keyword that is "program". This can be the only block in the application but is mandatory.

program name_of_program
  ! variable declarations 
  ...
  ! program statements 
  ...
end program

Program Name

Notes: You can see the program has a name. Most of the time this name is the same as the file name that is containing the main program. Unfortunately the compiler do not check this and it could be a readability issue. We recommend to use the same name.

Name of program can start with any letter and can contain letters and numbers and underscore. We think the program name should be short but is up to you to give it a significant name. You can name your program "main".

Line Comments

In Fortran the comments start with "!" and can be used at beginning of line or at end of line for each statement. Is a good engineering practice to comment your code properly.

Indentation

What is a statement?

Fortran is a statement oriented language. Every line of code can represent an individual statement. Some statements can include other statements that are nested. This is called a block of code or block statement.

A statement usually start with a keyword and uses one or more expressions. Expressions can contain identifiers, operators, data-literals, function calls and delimiters. An expression that is not used in a statement does not make sense in Fortran.

continuation symbol

A statement can be extended on multiple lines of code. You can use continuation operator & that connect the lines. Operator & is not part of the statement and is ignored by the compiler.

Multiple Statements

You can have many statements on a single line. In this case you can separate the statement with semicolumn ";". Sometimes programmers add semicolumn to indicate "no statement" where you should have one.

Source Code

Source code is usually stored in a file. This has extension "f" or "f90" or "f95". Is depending on syntax version and compiler you are using. I recommand to use only "f95" version for this course.

The source code can represent a program or a program module. You can compile the source code into an executable object file. After you create several object files, you can link them together into an executable file.

Every source file can contain: declarations, statements or subprograms. Usually the main program is at the bottom in a source file. You can use only one main program for an application.

Hello World

Let's make the first example. This example is a little bit simplified to sho you how everything works together. In the console we compile the program and then we run it.

! main program
program main
  print  * , "Hello " // get_name()
contains
  function get_name() result(name)
     character (len=40) :: name
     character (len=20) :: name1, name2
     print *, 'Who are you? :'
     read *, name1, name2
     name = trim(name1) // " " // &
            trim(name2) 
  end function
end program

~>gfortran ./main.f95 -o main
~>./main
 Who are you? :
Elucian Moise
 Hello Elucian Moise               
~>

Note: Code examples are available on GitHub. This is an open source project. We run examples online on replit.com. Some of the examples will be available for you to modify, compile and run.

Homework: Log-in to replit.com, and create your won example. Do not use copy paste. Instead, try to type the code line by line. You will get use with the syntax this way. I hope it works!

History

Fotran has a long hystory of modifications. To be brief, we mention the older major versions: Fortran 66, Fortran 77. The new version are Fortran90, Fortran 95, Fortran 2003, Fortran 2008, Fortran 2018. In the future we will have Fortran 2023 and beyound. Important changes:

Fortran 90 features

ISO/IEC standard 1539:1991 in 1991 and an ANSI Standard in 1992. In addition to changing the official spelling from FORTRAN to Fortran, this major revision added many new features to reflect the significant changes in programming practice that had evolved since the 1978 standard:

• Inline comments
• Lowercase Fortran keywords
• Operator overloading
• Derived (structured) data types
• RECURSIVE procedures
• SELECT . . . CASE construct for multi-way selection
• Portable specification of numerical precision
• Free-form source input removed the need to skip the first six character positions before entering statements.
• Identifiers up to 31 characters in length (In the previous standard, it was only six characters).
• Ability to operate on arrays (or array sections) as a whole, thus greatly simplifying math and engineering computations.
• WHERE statement for selective array assignment
• Array-valued constants and expressions,
• User-defined array-valued functions and array constructors.
• Modules, to group related procedures and data together, and make them available to other program units, including the capability to limit the accessibility to only specific parts of the module.
• A vastly improved argument-passing mechanism, allowing interfaces to be checked at compile time
• User-written interfaces for generic procedures
• New data type declaration syntax, to specify the data type and other attributes of variables
• Dynamic memory allocation by means of the ALLOCATABLE attribute and the ALLOCATE and DEALLOCATE statements
• POINTER attribute, pointer assignment, and NULLIFY statement to facilitate the creation and manipulation of dynamic data structures
• Structured looping constructs, with an END DO statement for loop termination, and EXIT and CYCLE statements for terminating normal DO loop iterations in an orderly way
• New and enhanced intrinsic procedures. Free-form source input removed the need to skip the first six character positions before entering statements.

Fortran 95 extension

Fortran 95 introduce "High Performance Fortran". Is out of scope for this tutorial to present these features. You can read it yourself after you finish the fundamentals.

• Fortran_95_language_features
• High Performance Fortran - Extension

Deprecated features

It is good to know what deprecated features are yet presented in code or tutorials and may not work for your compiler. Do not try to use these features. If you do you will get an error: "deleted feature..." so you know you will not use it.

• Arithmetic IF-statement
• Non-integer DO parameters or control variables
• Shared DO-loop termination or termination with a statement other than END DO or CONTINUE
• Branching to END IF from outside a block
• Alternate return
• PAUSE statement
• ASSIGN statement and assigned GO TO statement
• Assigned statement numbers and FORMAT specifiers
• Computed GO TO statement
• Statement functions
• DATA statements among executable statements
• CHARACTER* form of CHARACTER declaration
• Assumed character length functions
• Fixed form source code

File Endings

Historically, the file endings of Fortran source files are .f and .for. These are associated with Fortran < 90, and therefore with fixed format. Even modern compilers assume fixed format for them, unless the free format command-line flag is used.

For this reason, file endings should be used that represent modern Fortran versions. We can either set endings that correspond with the actual language version (.f03 for 2003, .f08 for 2008, .f18 for 2018, and so on) or simply use .f90 as a default to indicate modern Fortran, with the latter being recommended. Be aware that you must set the file ending to either .f90 or .f95 to be recognised by CMake.

Read next: Data Types