parameter initialization expressions
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From: baf on 11 Mar 2010 15:42 Why does the following program produce 0.5 for c and 0 for d? program testdp implicit none integer, parameter::dp=selected_real_kind(14) real(dp),parameter::a=1_dp,b=2_dp,c=a/b,d=1_dp/2_dp write(*,*)c,d end program testdp
From: m_b_metcalf on 11 Mar 2010 15:47 On Mar 11, 9:42 pm, baf <b...(a)nowhere.com> wrote: > Why does the following program produce 0.5 for c and 0 for d? > > program testdp > implicit none > integer, parameter::dp=selected_real_kind(14) > real(dp),parameter::a=1_dp,b=2_dp,c=a/b,d=1_dp/2_dp > write(*,*)c,d > end program testdp Because a real literal constant must contain at least one of: a decimal point or an exponent part (MR&C, p. 15). Yours contain neither. Your constants are integers. Regards, Mike Metcalf
From: glen herrmannsfeldt on 11 Mar 2010 15:50 baf <baf(a)nowhere.com> wrote: > Why does the following program produce 0.5 for c and 0 for d? > program testdp > implicit none > integer, parameter::dp=selected_real_kind(14) > real(dp),parameter::a=1_dp,b=2_dp,c=a/b,d=1_dp/2_dp > write(*,*)c,d > end program testdp 1_dp and 2_dp are still integers, even if they have the KIND of dp. You would have a compile time error if no appropriate INTEGER KIND existed. It might have been nice if it was reqiured for KINDs not to overlap between different types. -- glen
From: baf on 11 Mar 2010 16:00 On 03/11/2010 12:42 PM, baf wrote: > Why does the following program produce 0.5 for c and 0 for d? > > program testdp > implicit none > integer, parameter::dp=selected_real_kind(14) > real(dp),parameter::a=1_dp,b=2_dp,c=a/b,d=1_dp/2_dp > write(*,*)c,d > end program testdp Both comments make perfect sense. I did a search and replace in a larger code for items like 4d0 changing it to 4_dp, but forgot I needed to add either an exponent or a decimal point. Trying to clean up old code and got bit. Thanks to both of you for your answers.
From: Richard Maine on 11 Mar 2010 17:32
glen herrmannsfeldt <gah(a)ugcs.caltech.edu> wrote: > baf <baf(a)nowhere.com> wrote: > > Why does the following program produce 0.5 for c and 0 for d? > > > program testdp > > implicit none > > integer, parameter::dp=selected_real_kind(14) > > real(dp),parameter::a=1_dp,b=2_dp,c=a/b,d=1_dp/2_dp > > write(*,*)c,d > > end program testdp > > 1_dp and 2_dp are still integers, even if they have > the KIND of dp. > > You would have a compile time error if no appropriate > INTEGER KIND existed. > > It might have been nice if it was reqiured for KINDs not > to overlap between different types. Yes. I personally think it would have been nicer still if kinds weren't numeric. F90 introduced features that programmers can use to get away from darn near everything being integer codes, but the language itself doesn't take good advantage of those features. I sometimes think that at least some of the people writing the language specs weren't yet themselves fuly comfortable with some of the newer features. (Other times I am sure of it instead of just thinking so. :-)). Anyway, spilt milk aside, the thing to understand is that dp above is just an integer parameter. Odds are that it is 8, though that will vary. In any case, it is a named parameter for some integer value. That is *ALL* it is. The fact that its value was computed using selected_real_kind does *NOT* restrict it to being used as a real kind parameter. That's the most obviously useful way to use it, but you can use it anywhere else that you could use an integer parameter with the value 8 (or whatever). Unfortunately, one of those other places that you can use a parameter with the value 8 is as a kind specifier for integer. Odds are that 1_dp and 2_dp are 8-byte integers. The reason I say it is unfortunate is that it invites exactly the kind of error that the OP had here. The code shown is perfectly valid; it is just highly unlikely that it does what the writer intended. -- Richard Maine | Good judgment comes from experience; email: last name at domain . net | experience comes from bad judgment. domain: summertriangle | -- Mark Twain |