Modern Fortran
Building efficient parallel applications
Milan Curcic
  • MEAP began March 2018
  • Publication in November 2020 (estimated)
  • ISBN 9781617295287
  • 416 pages (estimated)
  • printed in black & white

If I had this book when writing my master thesis in astrophysics I would have spared a lot of time in learning how to write the code for my calculations.

Davide Cadamuro
Using Fortran, early and accurate forecasts for hurricanes and other major storms have saved thousands of lives.

Using Fortran, better designs for ships, planes, and automobiles have made travel safer, more efficient, and less expensive than ever before.

Using Fortran, low-level machine learning and deep learning libraries provide incredibly easy, fast, and insightful analysis of massive data for science, business, and industry.

Fortran is an amazingly powerful and flexible programming language that forms the foundation of high performance computing for research, science, and industry. And it's come a long, long way since starting life on IBM mainframes in 1956. Modern Fortran is natively parallel, so it's uniquely suited for efficiently handling problems like complex simulations, long-range predictions, and ultra-precise designs. If you're working on tasks where speed, accuracy, and efficiency matter, it's time to discover—or re-discover—Fortran.

About the Technology

Fortran is a general-purpose programming language that is dominant in engineering and scientific computing applications. It’s the leading language used in high performance (HPC) and parallel programming. While Fortran is the oldest high-level programming language, it is constantly improving. A recent major revision of the standard, Fortran 2008, added native support for parallel programming on shared-and distributed-memory architectures, and will be one of few languages used for next-generation exascale HPC applications. The latest edition, Fortran 2018, brings additional parallel features and improves interoperability with C.

A few Fortran facts:

  • If you read or listened to the weather forecast this morning, you used Fortran.
  • You’ll find Fortran at the lowest levels of most popular data analysis and machine learning frameworks
  • Fortran is the reference benchmark language used to measure the performance of the fastest supercomputers in the world (
  • Fortran is the only natively-parallel programming language steered by an International Standards Committee with scientists and engineers as its target audience
  • Fortran is the dominant language of the High Performance Computing, spanning industries from applied mathematics, fluid and aerodynamics, to weather, ocean, and climate prediction

Simply put, the future is in parallel, and Fortran is already there.

About the book

Modern Fortran: Building efficient parallel applications teaches you how to develop fast, efficient parallel applications with Fortran. To warm up, you'll start by building a simple application in modern Fortran, with a focus on reusable modules, functions, and subroutines. Next, you'll dive into Fortran's unique approach to parallel programming with coarrays. As you work through this practical guide, you'll systematically explore all the really interesting stuff: how to overload operators and generic functions, work with a variety of different inputs and outputs, and much more. Finally, you'll take on advanced parallelism with teams and events, and interoperability with C.

Packed with practical examples, tasks to practice, cookbook-style recipes, Modern Fortran is guaranteed to make you see this powerful language in a new light.
Table of Contents detailed table of contents

Part 1: Getting started with modern Fortran

1 Introducing Fortran

1.1 What is Fortran?

1.2 Fortran features

1.3 Why learn Fortran?

1.4 Advantages and disadvantages

1.4.1 Side-by-side comparison with Python

1.5 Parallel Fortran, illustrated

1.6 What will you learn in this book?

1.7 Think parallel!

1.7.1 Copying an array from one processor to another

1.8 Running example: A parallel tsunami simulator

1.8.1 Why tsunami simulator?

1.8.2 Shallow water equations

1.8.3 What we want our app to do

1.9 Further reading

1.10 Summary

2 Getting started: Minimal working app

2.1 Compiling and running your first program

2.2 Simulating the motion of an object

2.2.1 What should our app do?

2.2.2 What is advection?

2.3 Implementing the minimal working app

2.3.1 Implementation strategy

2.3.2 Defining the main program

2.3.3 Declaring and initializing variables

2.3.4 Numeric data types

2.3.5 Declaring the data to use in our app

2.3.6 Branching with an if block

2.3.7 Using a do loop to iterate

2.3.8 Setting the initial water height values

2.3.9 Predicting the movement of the object

2.3.10 Printing results to the screen

2.3.11 Putting it all together

2.4 Going forward with the tsunami simulator

2.5 Answer Key

2.5.1 Exercise: Cold front propagation

2.6 New Fortran elements, at a glance

2.7 Further reading

2.8 Summary

Part 2 Core elements of Fortran

3 Writing reusable code with functions and subroutines

3.1 Toward higher app complexity

3.1.1 Refactoring the tsunami simulator

3.1.2 Revisiting the cold front problem

3.1.3 An overview of Fortran program units

3.2 Don’t repeat yourself, use procedures

3.2.1 Your first function

3.2.2 Expressing finite difference as a function in the tsunami simulator

3.3 Modifying program state with subroutines

3.3.1 Defining and calling a subroutine

3.3.2 When to use a subroutine over a function?

3.3.3 Initializing water height in the tsunami simulator

3.4 Writing pure procedures to avoid side effects

3.4.1 What is a pure procedure?

3.4.2 Some restrictions on pure procedures

3.4.3 Why are pure functions important?

3.5 Writing procedures that operate on both scalars and arrays

3.6 Procedures with optional arguments

3.7 Tsunami simulator: Putting it all together

3.8 Answer key

3.8.1 Exercise 1: Modifying state with a subroutine

3.8.2 Exercise 2: Writing an elemental function that operates on both scalars and arrays

3.9 New Fortran elements, at a glance

3.10 Further reading

3.11 Summary

4 Organizing your Fortran code using modules

4.1 Accessing a module

4.1.1 Getting compiler version and options

4.1.2 Using portable data types

4.2 Creating your first module

4.2.1 The structure of a custom module

4.2.2 Defining a module

4.2.3 Compiling Fortran modules

4.2.4 Controlling access to variables and procedures

4.2.5 Putting it all together in the tsunami simulator

4.3 Toward realistic wave simulations

4.3.1 A brief look at the physics

4.3.2 Update to the finite difference calculation

4.3.3 Renaming imported entities to avoid name conflict

4.3.4 The complete code

4.4 Answer key

4.4.1 Exercise 1: Using portable type kinds in the tsunami simulator

4.4.2 Exercise 2: Define the set_gaussian subroutine in a module

4.5 New Fortran elements, at a glance

4.6 Further reading

4.7 Summary

5 Analyzing time series data with arrays

5.1 Analyzing stock prices with Fortran arrays

5.1.1 Objectives for this exercise

5.1.2 About the data

5.1.3 Getting the data and code

5.2 Finding the best and worst performing stocks

5.2.1 Declaring arrays

5.2.2 Array constructors

5.2.3 Reading stock data from files

5.2.4 Allocating arrays of certain size or range

5.2.5 Allocating an array from another array

5.2.6 Automatic allocation on assignment

5.2.7 Cleaning up after use

5.2.8 Checking for allocation status

5.2.9 Catching allocation and deallocation errors

5.2.10 Implementing the CSV reader subroutine

5.2.11 Indexing and slicing arrays

5.3 Identifying risky stocks

5.4 Finding good times to buy and sell

5.5 Answer Key

5.5.1 Exercise 1: Convenience (de)allocator subroutines

5.5.2 Exercise 2: Reversing an array

5.5.3 Exercise 3: Computing the moving average over custom time period.

5.6 New Fortran elements, at a glance

5.7 Further reading

5.8 Summary

6 Reading, writing, and formatting your data

6.1 Your first I/O: Input from keyboard and output to screen

6.1.1 The simplest I/O

6.1.2 Reading and writing multiple variables at once

6.1.3 Standard input, output, and error

6.2 Formatting numbers and text

6.2.1 Designing the aircraft dashboard

6.2.2 Formatting strings, broken down

6.2.3 Format statements in legacy Fortran code

6.3 Writing to files on disk: A minimal note-taking app

6.3.1 Opening a file and writing to it

6.3.2 Opening a file

6.3.3 Writing to a file

6.3.4 Appending to a file

6.3.5 Opening files in read-only or write-only mode

6.3.6 Checking if a file exists

6.3.7 Error handling and closing the file

6.4 Answer Key

6.4.1 Exercise: Redirect stdout and stderr to files

6.5 New Fortran elements, at a glance

6.6 Summary

Part 3: Advanced Fortran use

7 Going parallel with Fortran coarrays

7.1 Why write parallel programs

7.2 Processing real-world weather buoy data

7.2.1 About the data

7.2.2 Getting the data and code

7.2.3 Objectives

7.2.4 Serial implementation of the program

7.3 Parallel processing with images and coarrays

7.3.1 Fortran images

7.3.2 Getting information about the images

7.3.3 Telling images what to do

7.3.4 Gathering all data to a single image

7.4 Coarrays and synchronization, explained

7.4.1 Declaring coarrays

7.4.2 Allocating dynamic coarrays

7.4.3 Sending and receiving data

7.4.4 Controlling the order of image execution

7.5 Toward the parallel tsunami simulator

7.5.1 Implementation strategy

7.5.2 Finding the indices of neighbor images

7.5.3 Allocating the coarrays

7.5.4 The main time loop

7.6 Answer key

7.6.1 Exercise 1: Finding the array sub-ranges on each image.

7.6.2 Exercise 2: Writing a function that returns the indices of neighbor images

7.7 New Fortran elements, at a glance

7.8 Further reading

7.9 Summary

8 Working with abstract data using derived types

8.1 Recasting the tsunami simulator with derived types

8.2 Defining, declaring, and initializing derived types

8.2.1 Defining a derived type

8.2.2 Instantiating a derived type

8.2.3 Accessing derived type components

8.2.4 Positional vs. keyword arguments in derived type constructors

8.2.5 Providing default values for derived type components

8.2.6 Writing a custom type constructor

8.2.7 Custom type constructor for the Field type

8.3 Binding procedures to a derived type

8.3.1 Your first type-bound method

8.3.2 Type-bound methods for the Field type

8.3.3 Controlling access to type components and methods

8.3.4 Bringing it all together

8.4 Extending tsunami to two dimensions

8.4.1 Going from 1-d to 2-d arrays

8.4.2 Updating the equation set

8.4.3 Finite differences in x and y

8.4.4 Passing a class instance to diffx() and diffy()

8.4.5 Derived type implementation of the tsunami solver

8.5 Answer Key

8.5.1 Exercise 1: Working with private components

8.5.2 Exercise 2: Invoking a type-bound method from an array of instances

8.5.3 Exercise 3: Computing finite difference in y-direction.

8.6 New Fortran elements, at a glance

8.7 Further reading

8.8 Summary

9 Generic procedures and operators for any data type

9.1 Analyzing weather data of different types

9.1.1 About the data

9.1.2 Objectives

9.1.3 Strategy for this exercise

9.2 Type systems and generic procedures

9.2.1 Static versus strong typing

9.3 Writing your first generic procedure

9.3.1 The problem with strong typing

9.3.2 Writing the specific functions

9.3.3 Writing the generic interface

9.3.4 Results and complete program

9.4 Built-in and custom operators

9.4.1 What’s an operator?

9.4.2 Things to do with operators

9.4.3 Fortran’s built-in operators

9.4.4 Operator precedence

9.4.5 Writing custom operators

9.4.6 Redefining built-in operators

9.5 Generic procedures and operators in the tsunami simulator

9.5.1 Writing user-defined operators for the Field type

9.6 Answer key

9.6.1 Exercise 1: Specific average function for a derived type

9.6.2 Exercise 2: Defining a new string concatenation operator

9.7 New Fortran elements, at a glance.

9.8 Summary

10 User-defined operators for derived types

10.1 Happy Birthday! A countdown app

10.1.1 Some basic specification

10.1.2 Implementation strategy

10.2 Getting user input and current time

10.2.1 Your first datetime class

10.2.2 Reading user input

10.2.3 Getting current date and time

10.3 Calculating the difference between two times

10.3.1 Modeling a time interval

10.3.2 Implementing a custom subtraction operator

10.3.3 Time difference algorithm

10.3.4 The complete program

10.4 Overriding operators in the tsunami simulator

10.4.1 A refresher on the Field class

10.4.2 Implementing the arithmetic for the Field class

10.4.3 Syncing parallel images on assignment

10.5 Answer key

10.5.1 Exercise 1: Validating user input

10.5.2 Exercise 2: Leap year in the Gregorian calendar

10.5.3 Exercise 3: Implementing the addition for the Field type

10.6 New Fortran elements, at a glance

10.7 Summary

Part 4 The final stretch

11 Interoperability with C: Exposing your app to the web

11.1 Interfacing C: Writing a minimal TCP client and server

11.1.1 Introducing networking to Fortran

11.1.2 Installing libdill

11.2 TCP server program: Receiving network connections

11.2.1 IP address data structures

11.2.2 Initializing the IP address structure

11.2.3 Checking IP address values

11.2.4 Intermezzo: Matching compatible C and Fortran data types

11.2.5 Creating a socket and listening for connections

11.2.6 Accepting incoming connections to a socket

11.2.7 Sending a TCP message to the client

11.2.8 Closing a connection

11.3 TCP client program: Connecting to a remote server

11.3.1 Connecting to a remote socket

11.3.2 Receiving a message

11.3.3 The complete client program

11.4 Some interesting mixed Fortran-C projects

11.5 Answer Key

11.5.1 Exercise 1: The Fortran interface to ipaddr_port

11.5.2 Exercise 2: Fortran interfaces to suffix_detach and tcp_close

11.6 New Fortran elements, at a glance

11.7 Further Reading

11.8 Summary

12 Advanced parallelism with teams, events, and collectives

12.1 From coarrays to teams, events, and collectives

12.2 Grouping images into teams with common tasks

12.2.1 Teams in the tsunami simulator

12.2.2 Forming new teams

12.2.3 Changing execution between teams

12.2.4 Synchronizing teams and exchanging data

12.3 Posting and waiting for events

12.3.1 A push notification example

12.3.2 Posting an event

12.3.3 Waiting for an event

12.3.4 Counting event posts

12.4 Distributed computing using collectives

12.4.1 Computing minimum and maximum of distributed arrays

12.4.2 Collective subroutines syntax

12.4.3 Broadcasting values to other images

12.5 Answer Key

12.5.1 Exercise 1: Hunters and gatherers

12.5.2 Exercise 2: Tsunami time step logging using events

12.5.3 Exercise 3: Calculating the global mean of water height

12.6 New Fortran elements, at a glance

12.7 Further Reading

12.8 Summary


Appendix A: Setting up the Fortran development environment

A.1 Editing Fortran source files

A.2 Setting up the Fortran compiler

A.2.1 Linux

A.2.2 Mac OS

A.2.3 Windows

A.3 Setting up the MPI library

A.4 Setting up OpenCoarrays

A.4.1 Linux

A.4.2 Mac OS

A.4.3 Using OpenCoarrays

A.5 Building a Docker image

Appendix B: From calculus to code

B.1 The advection equation explained

B.1.1 Discretizing the derivatives

B.1.2 Casting the derivatives into code

Appendix C: Concluding remarks

C.1 Tsunami simulator: The complete code

C.1.1 Main program: tsunami.f90

C.1.2 The Field module: mod_field.f90

C.1.3 The I/O module: mod_io.f90

C.1.4 The parallel module: mod_parallel.f90

C.2 Going forward with the tsunami simulator

C.3 Neural networks and deep learning

C.4 Online resources

C.5 Compilers

C.6 Books

What's inside

  • Fortran’s place in the modern world
  • Parallel programming from the ground up
  • Working with variables and types
  • Module development in Fortran
  • Interoperating Fortran with C
  • Object-oriented and functional programming styles

About the reader

Written for developers, engineers, and computational scientists. No prior experience with Fortran is needed.

About the author

Milan Curcic is a scientist and founder. A Fortran programmer since 2006, he has worked with teams from the United States Navy and NASA to develop and improve weather and ocean prediction models. Milan has authored several Fortran libraries and is a founding member of the fortran-lang ( community project. He is currently a researcher at the University of Miami and is also building a startup that ports Fortran to the cloud for weather and ocean prediction.

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