Fortran/structures

Structures, structured types, or derived types(DT) were first introduced in Fortran 90. Structures allow the user to create data types that hold multiple different variables.

Derived types are often implemented within modules such that one can easily reuse them. They might also hold type-bound procedures which are intended to process the structure. The arguments pass(name), nopass indicate whether the object should be passed as the first argument.

Similar to the character data type, structures can be parameterized by two different parameter types: kind, len. The kind parameters must be known at compile type (consist of constants) whereas the len parameters can change at runtime.

Simple example

As an example, we can define a new structure type, 'Fruit' which stores some basic fruit variables:

type fruit
    real      :: diameter  ! in mm
    real      :: length    ! in mm
    character :: colour
end type

We can declare two 'fruit' variables, and assign them values:

type (fruit) :: apple, banana
apple = fruit(50, 45, "red")
banana%diameter = 40
banana%length   = 200
banana%colour   = "yellow"

And we can then use the fruit variables and their child values in normal Fortran operations.

Example: type-bound procedures

!> show the usage of type-bound procedures (pass/nopass arguments)
module test_m
    implicit none
    private
    public test_type
    type test_type
        integer :: i
    contains
        procedure, nopass :: print_hello
        procedure         :: print_int
    end type
contains
    !> do not process type specific data => nopass
    subroutine print_hello
        print *, "hello"
    end subroutine

    !> process type specific data => first argument is "this" of type "class(test_type)"
    !! use class and not type below !!!!
    subroutine print_int(this)
        class(test_type), intent(in) :: this

        print *, "i", this%i
  end subroutine
end module

program main
    use test_m
    implicit none
    type (test_type) :: obj

    obj%i = 1
    call obj%print_hello
    call obj%print_int
end program

Example: parameterized type

! testing types with params: kind + len
program main
    implicit none
    type matrix(rows, cols, k)
        integer, len  :: rows, cols
        integer, kind :: k = kind(0.0)    ! optional/default value
        real (kind=k), dimension(rows, cols) :: vals
    end type 
    type (matrix(rows=3, cols=3)) :: my_obj
end program