Difference between revisions of "Fortran issues"
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| − | But the -r8 flag can help. It's safer to specify the kind in the code |
+ | But the -r8 flag can help. It's safer to specify the kind in the code, especially if other people compile your code (and hack the makefiles). |
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1.23456000000000 |
1.23456000000000 |
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| + | Or you can just avoid REAL. ;-) |
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Revision as of 16:03, 25 June 2008
I thought it would be useful to have a page for noting Fortran problems which aren't well documented elsewhere or are common gotchas... --james 16:23, 25 June 2008 (BST)
Real intrinsic
Care needs to be taken with using REAL(X). The default behaviour for REAL differs if X is complex rather than integer or real. Some googling reveals this is an oddity, but it is the specification. I have found that DREAL isn't always portable (some compilers have issues when the argument to DREAL is the value of a derived type, e.g. DREAL(X%v) fails at compile time but REAL(X%v) doesn't).
keiko:~/src/test> more testkind.f90 && ifort testkind.f90 && ./a.out
program testkind
implicit none
integer,parameter :: r1=kind(0.0)
integer,parameter :: r2=kind(0.d0)
write (6,'(/a/)') 'Output:'
write (6,*) real(1)
write (6,*) real(1,r1)
write (6,*) real(1,r2)
write (6,*) real(1.23456d0)
write (6,*) real(1.23456d0,r1)
write (6,*) real(1.23456d0,r2)
write (6,*) real(dcmplx(1.23456d0,0.2155660d0))
write (6,*) real(dcmplx(1.23456d0,0.2155660d0),r1)
write (6,*) real(dcmplx(1.23456d0,0.2155660d0),r2)
end program testkind
Output:
1.000000
1.000000
1.00000000000000
1.234560
1.234560
1.23456000000000
1.23456000000000
1.234560
1.23456000000000
But the -r8 flag can help. It's safer to specify the kind in the code, especially if other people compile your code (and hack the makefiles).
jss43@keiko:~/src/test> more testkind.f90 && ifort -r8 testkind.f90 && ./a.out program testkind
implicit none
integer,parameter :: r1=kind(0.0)
integer,parameter :: r2=kind(0.d0)
write (6,'(/a/)') 'Output:'
write (6,*) real(1)
write (6,*) real(1,r1)
write (6,*) real(1,r2)
write (6,*) real(1.23456d0)
write (6,*) real(1.23456d0,r1)
write (6,*) real(1.23456d0,r2)
write (6,*) real(dcmplx(1.23456d0,0.2155660d0))
write (6,*) real(dcmplx(1.23456d0,0.2155660d0),r1)
write (6,*) real(dcmplx(1.23456d0,0.2155660d0),r2)
end program testkind
Output:
1.00000000000000
1.00000000000000
1.00000000000000
1.23456000000000
1.23456000000000
1.23456000000000
1.23456000000000
1.23456000000000
1.23456000000000
Or you can just avoid REAL. ;-)