Connecting to ARCHER2 and transferring data

  • We should all understand and follow the ARCHER2 Code of Conduct to ensure this course is conducted in the best teaching environment.
  • ARCHER2’s login address is login.archer2.ac.uk.
  • You have to change the default text password the first time you log in
  • MFA is mandatory in ARCHER2

Hello World

  • A Fortran program begins with a program statement and ends with an end program statement.
  • A print statement is a simple way to write output to the terminal.
  • A write statement provides more control including ways to write to file.
  • Modules can be loaded with the use statement.
  • The iso_fortran_env module provides symbols which can be used to help read from and write to the terminal.

Break

Variables

  • Fortran provides the intrinsic numeric types of integer, real and complex.
  • Without a kind, these types are implementation-defined. Use kind to specify the representation of variables.
  • The iso_fortran_env intrinsic module provides standard kinds such as real32 and real64.
  • Always use implicit none to prevent the accidental implicit declaration of new variables.
  • Use the parameter attribute to make the value associated with the name constant.

Logical and conditionals, character variables

  • Fortran provides two non-numeric intrinsic data types: logical and character.
  • A program’s flow can be directed using the results of logical operations used in conjunction with if and case constructs.

Lunch

Loops and loop control

  • Iteration in Fortran is based around the do construct (somewhat analogous to C for construct). There is no equivalent of the C++ iterator class.
  • Without any control, a do loop will execute forever.
  • A loop iteration can be skipped with a cycle statement.
  • A loop can be ended if an exit statement is encountered.
  • It is very common to control the execution of a loop with an integer variable.

Array declarations

  • Unlike C, which often uses pointers to handle array data, Fortran has arrays which are an intrinsic feature of the language.
  • The number of dimensions in an array is its rank.
  • The number of elements in one of an array’s dimensions is its extent.
  • The ordered sequence of extents in an array is its shape.
  • The total number of elements in an array, equal to the product of its shape, is its size.
  • In Fortran array indices begin at 1 by default, but this can be changed if required.
  • New arrays can be created by using the intrinsic reshape() function with an existing one.
  • An array with an unknown size can be allocated at runtime.
  • Compilers are typically able to help you debug issues with arrays if you ask them to.

Array expressions and assignments

  • Fortran allows flexible operations on arrays or subsets of array elements and provides numerous intrinsic functions for such operations.
  • Some of these, such as all() and any() can be useful in directing the logical flow of a program.

Break

Mini exercise: a choice of two

  • With a fairly small amount of the Fortran language, you can already solve some real problems.

Modules and compilation of modules

  • Modules in Fortran provide the way to structure collections of related definitions and operations, and make them available elsewhere.

Functions and subroutines

  • Functions and subroutines are referred to collectively as _procedures_.
  • Using intent for dummy variables allows control over whether updates to their values are permitted.
  • Procedures can be modified with prefixes such as pure and recursive which ensure respectively that the procedure has no side-effects and that it is able to directly or indirectly call itself.
  • Procedures may be defined as module sub-programs for which the compiler will automatically generate the contract block as part of the module file.

More on array dummy arguments

  • There are some additional considerations to think about when dummy arguments are arrays.
  • You may wish to pass the shape of the array explicitly, at the cost of providing more dummy arguments.
  • Arrays may have an assumed shape, but remember that they only receive the shape and not the original bounds of its dimensions.
  • Allocatable arrays can also be used as dummy arguments.
  • Dummy arguments can have the optional attribute. The corresponding actual arguments can be positional or provided as a keyword argument.

Break

More on characters and strings

  • String-handling can be problematic if there is no mechanism to keep track of the length of the string.
  • Remember that Fortran provides intrinsic functions such as trim() and operators such as concatenation // which can help with string manipulation.
  • Pay attention when using strings as dummy arguments or procedure results.

Formats and edit descriptors

  • Control over the appearance of output is via the format specifier, which is a string.
  • This similar in spirit to the format string provided to printf() in C.
  • The use of a format allows predictable tabulation of output for human readers.

Lunch

Operations on external files

  • Writing data to, or reading data from, an external file is an essential part of a useful application.
  • Unit numbers are integers which are used as handles to open files.
  • Errors from I/O can be handled with labels and by examining the returned message.

Using namelists

  • Namelists are declared using the namelist /namelist-group-name/ variable-name-list construct.
  • Read and write namelists by passing them directly to the read and write statements.
  • Remember that namelists cannot be read out of order from a file without first moving backwards through the file.

Break

Exercises

  • Gain more experience putting your new knowledge to use.

Structures: derived types

  • The ability to aggregate related data in a structure is important.
  • Fortran offers the _derived type_ in addition to intrinsic types.
  • In its simplest form, one may think of this as the analogue to a C struct.
  • Derived types also form the basis of aggregation of data and related operations or procedures (viz. object-oriented programming); however, this introductory course will only touch on this feature.

Pointers and targets

  • Pointers are extremely useful for certain types of operations: they provide a way to refer indirectly to other variables or names (they act as an _alias_), or can be used for establishing dynamic data structures.
  • Remember to always initialise a pointer, to null() if necessary.
  • If a pointer is allocated, memory for its type is allocated and the pointer becomes associated.
  • The move_alloc() intrinsic function moves a memory allocation from one allocatable variable to another.

Procedures again: interfaces

  • An interface provides important information about a procedure to the compiler. Without this information, the compiler may not be able to use it.
  • When using modules, interfaces are generated automatically and provided to the compiler.
  • If not in a module, a function can be made visible to the compiler by declaring it with the external attribute, but the compiler can go further by checking the argument types if you write a full, explicit interface block.
  • Interfaces can also be used to implement limited polymorphism and operator overloading for derived types.
  • An elemental function is one which can be applied to a scalar actual argument or element-by-element to an array actual argument.

Break

Miscellaneous

  • Fortran provides other utility procedures that can help with your program’s functionality.
  • These allow you to retrieve information from the calling command line and environment.
  • You can pass commands back to the operating system, but you should be very careful that your code remains portable.
  • Timing your code with cpu_time() can help you profile your code.

Lunch

Other things you may see

  • You may come across Fortran following an older standard; some will be understandable on sight, but you may need to consult a reference.
  • MPI and OpenMP allow you to write Fortran code that will execute in parallel.
  • NVIDIA supports CUDA in Fortran for execution on GPUs.
  • Writing tests is an important step in safe code development.
  • Several books and the Fortran Wiki are invaluable resources.

Exercises: conjugate gradient and matrices

  • Fortran performance and array handling make it ideal for the solution of intense problems.

Break

Exercises