fortran: kind parameters

[ghost]

Hi,

no big deal but the way the kind parameters are used in the fortran code are not really standard conforming. That being said, every compiler I know of does use the kind parameters such that the code works as intended, but nevertheless it's basically a compiler dependent implementation and not strictly standard conforming fortran.

real(kind=X) with any integer X is a way to tell the compiler what type of floating point (and also integer, respectively) representation to choose. BUT it is up to the compiler which integer X corresponds to which representation. There is NO guarantee that e.g. 4 denotes a 4 byte float and 8 denotes an eight byte float.

The standard conforming way to do this is to ask for a kind parameter with the desired precision, then store this and use it to define variables, etc. instead of the X above. In its most generic form, this is done through selected_real_kind and selected_int_kind. However, a simpler form is to query for the kind parameter of a given constant and then use this. http://fortranwiki.org/fortran/show/Real+precision

[dmikushin]

Standard-conforming Fortran often tends to be too verbose -> harder to read. In this particular case, one could use simple real type everywhere, and add compiler switch -fdefault-real-8, if it is needed to be treated as double.

[bcumming]

This can be done properly with low verbosity overhead, and it would prefer to have things done "properly" for a pedagogical code.

We can define the following constant in one module

integer, parameter :: dp = kind(1.d0)

Then everywhere we declare a real type use

real (kind=dp) :: alpha, residual instead of real (kind=8) :: alpha, residual

Using standards is hopefully going to be more portable across compilers, so we don't have to find equivalent PGI, Cray and Intel flags for -fdefault-real-8.