I put together a short Fortran code that show the basics. The code should compile and run, and from the code, its comments and the output, you may be able to divine what is going on.

I recommend using gfortran, which is open, free and available on all respectable operating systems using your package manager (search for gfortran, gcc-fortran or gcc). For M$ Windoze, the folks at Cygwin can help you to make your OS somewhat more respectable. Follow their instructions to install Cygwin, then install a recent version of the package called gcc-fortran (as of 2024, v13 is the most recent version).

In principle you can compile and run the example code with

gfortran -o fortran-example fortran-example.f90  # Compile
./fortran-example                                # Run

The first step compiles fortran-example.f90 and creates a binary executable called fortran-example. The second step executes that program.

However, the Fortran compiler is very good at optimising your code (to improve the speed) and at helping you to write good code, to avoid surprises. To get all the help you can get, I recommend you compile your code using the following options and fix any warnings you may find:

gfortran -std=f2018 -fall-intrinsics -pedantic -fwhole-file -pipe -funroll-all-loops -O2 -fPIC -g -Wall
-Wextra -o fortran-example fortran-example.f90

This is particularly important because many Fortran compilers are quite forgiving — they may do what you want even though you didn’t get the code quite right — until you run on a different system and suddenly nothing works anymore. Hence use these flags from the beginning, so that you know where you introduced the issue.

From time to time you should run your code with run-time checks enabled. This will slow things down dramatically, but can catch many issues in your code:

gfortran -O0 -fcheck=all -fbacktrace -g -ffpe-trap=invalid,zero,overflow,underflow,denormal
-finit-integer=-99999999 -finit-real=snan -Wall -Wextra -Wcharacter-truncation -Wunderflow -o fortran-example
fortran-example.f90

Note that optimisation options like -O3 and -Ofast may produce faster code, but also incorrect code. Use with care, or if you don’t care about round-off and reproducibility.

I may add some of my notes at some point.