Question

I am trying to read some Fortran code, but can not determine what the % (percentage sign) does.

It is in a line like:

   x = a%rho * g * (-g*a%sigma + m%gb * m%ca * (1.6 * a%rho+g))

What does it do?

Answer 1

In Fortran 90 they allow you to create structures like in C++. It basically acts as the dot (.) operator.

From http://www.lahey.com/lookat90.htm :

Structures (Derived Types)

You can group your data using derived types. This enables users to combine intrinsic types (including arrays and pointers) into new types whose individual components can be accessed using the percent sign as a delimiter. (Derived types are known as records in VAX Fortran.)
! Example using derived types and modules.

module pipedef
   type pipe                          ! Define new type 'pipe', which
     real diameter                    ! is made up of two reals, an
     real flowrate                    ! integer, and a character.
     integer length
     character(len=10) :: flowtype
   end type pipe
end module pipedef

program main
   use pipedef                ! Associate module pipedef with main.
   type(pipe) water1, gas1    ! Declare two variables of type 'pipe'.
   water1 = pipe(4.5,44.8,1200,"turbulent") ! Assign value to water1.
   gas1%diameter = 14.9                     ! Assign value to parts
   gas1%flowrate = 91.284                   ! of gas1.
   gas1%length = 2550
   gas1%flowtype = 'laminar'
   .
   .
   .
end program