differences for PR #33

Author: actions-user

01-trivial-program.md

@@ -0,0 +1,306 @@
+---
+title: "A Trivial Program"
+teaching: --
+exercises: --
+---
+
+::::::::::::::::::::::::::::::::::::: questions
+
+- How do you write programs in Fortran?
+- How do you compile the Fortran code to make an executable?
+- How do we run that executable and see any output?
+
+::::::::::::::::::::::::::::::::::::::::::::::::
+
+::::::::::::::::::::::::::::::::::::: objectives
+
+Be able to
+
+- write
+- compile
+- run
+
+a basic Fortran program.
+
+::::::::::::::::::::::::::::::::::::::::::::::::
+
+During the setup you compiled a simple `hello world` program.
+Let's look at that program in more detail:
+
+```bash
+cd ~/Desktop/intro-to-modern-fortran/01-trivial-program
+cat hello_world.f90
+```
+
+```fortran
+program hello_world
+
+    implicit none
+
+    print *, 'Hello world!'
+
+end program hello_world
+
+```
+
+The first statement `program hello_world` starts the program.
+It also defines the program name, `hello_world`.
+It is matched by the `end` statement (`end program hello_world`).
+The `end` statement is always the last statement in a program.
+
+::: spoiler
+
+### Style Guide - Labels
+
+`program` on its own marks the start of a program
+and `end` on its own will end the program.
+Words that follow `program` or `end` are labels which improve readability.
+Some legacy codes may not have labels.
+
+:::
+
+## Implicit None
+
+Some variables in Fortran have a default type.
+Variables with names beginning with letters `i-n`
+are implicitly of type `integer`.
+Anything else is of type `real`.
+
+**This is very bad practice and modern Fortran should not be used in this way.**
+
+To prevent implicit typing we add the:
+
+```fortran
+implicit none
+```
+
+statement to all `programs`
+(and `modules`, `functions`, and `subroutines` that you will encounter in later episodes).
+
+Now all variable names must be declared explicitly before they are referenced.
+
+The only executable line in this program is the `print` statement.
+We will cover I/O (Input/Output) in a later episode.
+For now, know that `print *,` will print what follows
+to standard output when the program runs.
+In this case, it will print the string `Hello world!` to our terminal.
+
+## Comments
+
+Comments start with an exclamation mark `!`.
+Comments can appear on their own line,
+or after any other Fortran statement.
+
+::::::::::::::::::::::::::::::::::::::::: spoiler
+
+### Legacy Fortran - F77 style comments
+
+You may see code that has comments written like this:
+
+```fortran
+c      this is a comment
+```
+
+with a `c` in the first column.
+In new code use the modern `!` comments.
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+::::::::::::::::::::::::::::::::::::::: challenge
+
+## Add comments to hello_world.f90
+
+Add two comments to your file:
+
+1. The first should explain what the program does
+2. The second should explain the purpose of `implicit none`
+
+Think about where the best place for these comments are.
+
+::::::::::::::: solution
+
+## Solution
+
+```fortran
+program hello_world
+    !! A simple hello world program
+
+    implicit none  ! prevent implicit typing (to integers) of variables
+                   ! whose name starts with the letters i-n
+
+    print *, 'Hello world!'
+
+end program hello_world
+
+```
+
+1. We have placed a comment describing the program under the `program` statement.
+   You might also see program descriptions before the `program` statement.
+   Note the double `!` at the start of the comment.
+   This allows the automatic documentation generator [FORD](https://forddocs.readthedocs.io/en/stable/index.html) to extract documentation from the comment.
+2. The second comment could have been placed before or after
+   the `implicit none` statement.
+   We have shown an _inline_ comment.
+   Note the two spaces between the Fortran and the start of the comment,
+   and the comment spans multiple lines with each `!` aligned vertically.
+
+You may have noticed comments at the top of the Fortran files
+you downloaded during the setup.
+Those comments provide licensing and authorship information.
+
+:::::::::::::::::::::::::
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+## Compiling
+
+Fortran is a compiled language (like C++).
+A compiler turns human-readable source code into machine code.
+This machine code can then be executed by the computer.
+Languages like Python are interpreted.
+This means Python source code is executed directly
+without being compiled into machine code first.
+The program is parsed, interpreted, and executed each time it is run.
+Compiled programs are usually more efficient than interpreted programs.
+This is a major reason that compiled languages like Fortran remain popular.
+The draw back is that there is an extra step in building Fortran programs.
+
+There are several steps[^compiler-process] that occur during compilation.
+Your compiler takes care of each step for you.
+To test your compiler in the setup episode you ran:
+
+::: group-tab
+
+### GFortran
+
+```bash
+gfortran hello_world.f90
+```
+
+### Intel
+
+```bash
+ifx hello_world.f90
+```
+
+### Flang
+
+```bash
+flang hello_world.f90
+```
+
+### Cray
+
+```bash
+ftn hello_world.f90
+```
+
+:::
+
+This created a file named `a.out`.
+This is the default executable name if no name is specified.
+To tell the compiler the name of the executable use the `-o` flag:
+
+::: group-tab
+
+### GFortran
+
+```bash
+gfortran -o hello_world hello_world.f90
+```
+
+### Intel
+
+```bash
+ifx -o hello_world hello_world.f90
+```
+
+### Flang
+
+```bash
+flang -o hello_world hello_world.f90
+```
+
+### Cray
+
+```bash
+ftn -o hello_world hello_world.f90
+```
+
+:::
+
+Now run `ls` to see the new executable:
+
+```bash
+ls
+```
+
+```output
+hello_world  hello_world.f90
+```
+
+We now have an executable called `hello_world`.
+Sometimes code can compile but fail when executed[^runtime-error].
+Let's run this executable to check it works:
+
+```bash
+./hello_world
+```
+
+```output
+ Hello world!
+```
+
+::::::::::::::::::::::::::::::::::::::: challenge
+
+## Compiler Documentation and Flags
+
+Take a moment to find and bookmark the documentation for your compiler.
+Find the correct flags to:
+
+- Turn on debugging
+- Specify the default optimisation level
+
+::::::::::::::: solution
+
+## Solution
+
+The links below may not match your compiler version.
+With any documentation make sure it's for your compiler version.
+
+- [GNU gfortran command options](https://gcc.gnu.org/onlinedocs/gcc-4.3.6/gfortran/Invoking-GNU-Fortran.html#Invoking-GNU-Fortran)
+- [Intel ifx docs](https://www.intel.com/content/www/us/en/docs/fortran-compiler/developer-guide-reference/2025-0/overview.html)
+- [LLVM Flang command line reference](https://flang.llvm.org/docs/FlangCommandLineReference.html)
+- [Cray ftn docs](https://cpe.ext.hpe.com/docs/latest/cce/man1/crayftn.1.html)
+
+:::::::::::::::::::::::::
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+## Filenames
+
+Fortran files normally end in `.f90`, although they can have different
+[file extensions](https://fortranwiki.org/fortran/show/File+extensions).
+The `.f90` extension is the most widely recognised across compilers.
+You may see Fortran files with an upper-case extension `.F90`.
+This tells the compiler to [pre-process](https://fortranwiki.org/fortran/show/Preprocessors)
+the file before compiling.
+
+In the next episode we will introduce variable declaration,
+and you will write your first Fortran program from scratch.
+
+:::::::::::::::::::::::::::::::::::::::: keypoints
+
+- Fortran programs start with the `program <label>` statement.
+- Fortran programs end with the `end program <label>` statement.
+- Always use `implicit none` to prevent implicit typing for variables.
+- Fortran comments start with `!`.
+- The `-o` flag specifies the name of the compiled executable:
+  `<compiler_command> -o <executable_name> <source_file.f90>`.
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
+[^compiler-process]: Compilation steps for a C program: <https://www.geeksforgeeks.org/compiling-a-c-program-behind-the-scenes/>, the steps are the same for Fortran!
+[^runtime-error]:
+    If a program fails to compile that's a _compiler error_.
+    If a program compiles but fails to run that's a _runtime error_.

config.yaml

@@ -60,7 +60,7 @@ contact: 'd.theodorakis@metoffice.gov.uk'
 
 # Order of episodes in your lesson
 episodes: 
-- introduction.Rmd
+- 01-trivial-program.md
 
 # Information for Learners
 learners: 
@@ -78,4 +78,3 @@ profiles:
 sandpaper: astroDimitrios/sandpaper
 pegboard: astroDimitrios/pegboard
 varnish: astroDimitrios/varnish
-