How to calculate >15 bit computing?

[fishwater00]

In fortran, the highest precision is 15bit, I wanna calculate around 20-bit calculation, how can I do that? Thank you

 

[fishwater00]

such as 15^15..

 

[FJacq]

The highest precision in Fortran is not 15.

- it exists real types having a higher precision than DOUBLE PRECISION
- it exists libraries or modules providing derived types with arbitrary precision (ARPREC for instance for C++ and F90 softwares)

The easiest example :

program test

  integer,parameter :: ep=selected_real_kind(20) ! at least 20 significant digits

  REAL(ep) :: pi
  pi=4*ATAN(1._ep)

  write(*,*) pi

end program

Result with the Intel fortran compiler (ifort (IFORT) 11.1 20100414) :

[lcoul@localhost test]$ ifort t58.f90
[lcoul@localhost test]$ ./a.out
   3.14159265358979323846264338327950

François Jacq

 

[fishwater00]

Thank you so much

 

[fishwater00]

Thanks for your response, new issue comes out:

The maximum real precision is selected_real_kind(31) (I got from other book).

For integer, the maximum precision is selected_int_kind(16), is it right?

 

[FJacq]

For integer, the maximum precision is selected_int_kind(16), is it right?

It may depend on the compiler but the usual limitation for integers is selected_int_kind(18) (the order of magnitude of 2**63)

François Jacq

 

[xwb]

Note that selected_int_kind specifies the number of significant digits: not the number of bits. If you use a 32 bit compiler on a 32 bit machine, you will get more than 20 bits accuracy.