Problem with fortran 90 Present() function

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SUMMARY

The Fortran 90 Present() function behaves unexpectedly when checking for optional variables in certain contexts. In the provided example, the subroutine test_present incorrectly returns TRUE for the optional variable 'c' when it is not passed in the call. This issue arises because the subroutine must be defined within a module, and the module must be used in the main program for Present() to function correctly. When the variables 'a' and 'b' are of type integer, Present() returns FALSE as expected, highlighting a type-specific behavior.

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This discussion is beneficial for Fortran developers, particularly those working with optional arguments and subroutine definitions, as well as educators teaching Fortran programming concepts.

sghan
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Hi all,
When I use Present(x) to check for the presence of an optional variable, I am getting TRUE even when the subroutine call clearly does NOT contain the optional variable. See here the call to test_present(a,b):

program options
implicit none

real*8 a,b
real*8 c
a = 10.d0
b = 5.d0
c = 3.d0
call test_present(a,b)

end program options

! In a separate file:
subroutine test_present(a,b,c)
implicit none

real*8 a,b
real*8, optional :: c
logical value

value = present(c)
write(*,*)"is c present?", value !THIS RETURNS TRUE!

end subroutine test_present


Why does present(c) return TRUE even though c is not given in the call? Strangely, if variables a,b are of type integer, present(c) returns false as it should.
Thanks!
 
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There are some examples and discussion here. Hope this helps.
 
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Thanks. Found the answer: in the program above, I'd have to put the subroutine in a module, then "use module" in the program. Only then does Present(x) work properly. If anyone wants to explain why this is necessary, I would appreciate it!
 
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