Interfaces and Optional Arguments

Keyword Arguments

Calling by Name

Fortran calls can name arguments explicitly. Keyword calls make the mapping clear and can appear in a different order.

Program

Play the program to call the same function with two keyword argument orders.

keyword_arguments.f90 
program keyword_arguments_demo
    implicit none
    integer :: subtotal, tax, total

    subtotal = 
    tax = add_tax(amount=subtotal, rate=2)
    total = add_tax(rate=3, amount=subtotal)
    print '(I0, A, I0)', tax, " ", total
contains
    function add_tax(amount, rate) result(out)
        integer, intent(in) :: amount, rate
        integer :: out
        out = amount + rate
    end function add_tax
end program keyword_arguments_demo

program keyword_arguments_demo
    implicit none
    integer :: subtotal, tax, total

    subtotal = 
    tax = add_tax(amount=subtotal, rate=2)
    total = add_tax(rate=3, amount=subtotal)
    print '(I0, A, I0)', tax, " ", total
contains
    function add_tax(amount, rate) result(out)
        integer, intent(in) :: amount, rate
        integer :: out
        out = amount + rate
    end function add_tax
end program keyword_arguments_demo

program keyword_arguments_demo
    implicit none
    integer :: subtotal, tax, total

    subtotal = 
    tax = add_tax(amount=subtotal, rate=2)
    total = add_tax(rate=3, amount=subtotal)
    print '(I0, A, I0)', tax, " ", total
contains
    function add_tax(amount, rate) result(out)
        integer, intent(in) :: amount, rate
        integer :: out
        out = amount + rate
    end function add_tax
end program keyword_arguments_demo
keyword argument `amount=subtotal` binds by dummy-argument name.
order independence Keyword arguments can be written in a different order.
explicit interface Contained procedures have an explicit interface, enabling keyword calls.