Fortran 90 kind parameter

I am having trouble understanding Fortran 90's kind parameter. As far as I can tell, it does not determine the precision (i.e., float or double) of a variable, nor does it determine the type of a variable.

So, what does it determine and what exactly is it for?

The KIND of a variable is an integer label which tells the compiler which of its supported kinds it should use.

Beware that although it is common for the KIND parameter to be the same as the number of bytes stored in a variable of that KIND, it is not required by the Fortran standard.

That is, on a lot of systems,

REAl(KIND=4) :: xs   ! 4 byte ieee float
REAl(KIND=8) :: xd   ! 8 byte ieee float
REAl(KIND=16) :: xq   ! 16 byte ieee float

but there may be compilers for example with:

REAL(KIND=1) :: XS   ! 4 BYTE FLOAT
REAL(KIND=2) :: XD   ! 8 BYTE FLOAT
REAL(KIND=3) :: XQ   ! 16 BYTE FLOAT

Similarly for integer and logical types.

(If I went digging, I could probably find examples. Search the usenet group comp.lang.fortran for kind to find examples. The most informed discussion of Fortran occurs there, with some highly experienced people contributing.)

So, if you can't count on a particular kind value giving you the same data representation on different platforms, what do you do? That's what the intrinsic functions SELECTED_REAL_KIND and SELECTED_INT_KIND are for. Basically, you tell the function what sort of numbers you need to be able to represent, and it will return the kind you need to use.

I usually use these kinds, as they usually give me 4 byte and 8 byte reals:

!--! specific precisions, usually same as real and double precision
integer, parameter :: r6 = selected_real_kind(6) 
integer, parameter :: r15 = selected_real_kind(15) 

So I might subsequently declare a variable as:

real(kind=r15) :: xd

Note that this may cause problems where you use mixed language programs, and you need to absolutely specify the number of bytes that variables occupy. If you need to make sure, there are enquiry intrinsics that will tell you about each kind, from which you can deduce the memory footprint of a variable, its precision, exponent range and so on. Or, you can revert to the non-standard but commonplace real*4, real*8 etc declaration style.

When you start with a new compiler, it's worth looking at the compiler specific kind values so you know what you're dealing with. Search the net for kindfinder.f90 for a handy program that will tell you about the kinds available for a compiler.

I suggest using the Fortran 2008 and later; INT8, INT16, INT32, INT64, REAL32, REAL64, REAL128. This is done by calling ISO_FORTRAN_ENV in Fortran 2003 and later. Kind parameters provides inconsistent way to ensure you always get the appropriate number of bit representation

Just expanding the other (very good) answers, specially Andrej Panjkov's answer:

The KIND of a variable is an integer label which tells the compiler which of its supported kinds it should use.

Exactly. Even though, for all the numeric intrinsic types, the KIND parameter is used to specify the "model for the representation and behavior of numbers on a processor" (words from the Section 16.5 of the standard), that in practice means their bit model, that's not the only thing a KIND parameter may represent.

A KIND parameter for a type is any variation in its nature, model or behavior that is avaliable for the programmer to choose at compile time. For example, for the intrinsic character type, the kind parameter represents the character sets avaliable on the processor (ASCII, UCS-4,...).

You can even define your own model/behaviour variations on you defined Derived Types (from Fortran 2003 afterwards). You can create a Transform Matrix type and have a version with KIND=2 for 2D space (in which the underlying array would be 3x3) and KIND=3 for 3D space (with a 4x4 underlying array). Just remember that there is no automatic kind conversion for non-intrinsic types.

From the Portland Group Fortran Reference, the KIND parameter "specifies a precision for intrinsic data types." Thus, in the declaration

real(kind=4) :: float32
real(kind=8) :: float64

the variable float64 declared as an 8-byte real (the old Fortran DOUBLE PRECISION) and the variable float32 is declared as a 4-byte real (the old Fortran REAL).

This is nice because it allows you to fix the precision for your variables independent of the compiler and machine you are running on. If you are running a computation that requires more precision that the traditional IEEE-single-precision real (which, if you're taking a numerical analysis class, is very probable), but declare your variable as real :: myVar, you'll be fine if the compiler is set to default all real values to double-precision, but changing the compiler options or moving your code to a different machine with different default sizes for real and integer variables will lead to some possibly nasty surprises (e.g. your iterative matrix solver blows up).

Fortran also includes some functions that will help pick a KIND parameter to be what you need - SELECTED_INT_KIND and SELECTED_REAL_KIND - but if you are just learning I wouldn't worry about those at this point.

Since you mentioned that you're learning Fortran as part of a class, you should also see this question on Fortran resources and maybe look at the reference manuals from the compiler suite that you are using (e.g. Portland Group or Intel) - these are usually freely available.