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technophobe · Joined: 28 May 2007 Posts: 29

Hi Guys,
I've been debugging a code now for several hours and have noticed something wierd (turns it out was the source of my error).

I have been using fortran on and off for several years now, I have a lot of experience with Matlab and thought that I understood programming structure pretty well until today. I was surprised by the output of the following few lines of code:

PaulLaidler · Posted: Wed Mar 12, 2008 6:05 pm Post subject:

I think that you will find that there is no normal behaviour in this situation.
The Fortran standard will say that the value of the DO index, on normal termination of the loop, is not defined (it can be anything).

You can store the index elsewhere (put k=i in the loop) and use the stored value on termination.

technophobe · Joined: 28 May 2007 Posts: 29

Thanks Paul,
I never knew that the DO index wasn't defined. I am perhaps too used to using Matlab where the FOR index is retained between loops.

Now that I know this I'll be more careful in future,
Cheers,
B

JohnCampbell · Joined: 16 Feb 2006 Posts: 2554 Location: Sydney

My understanding of this is that the fortran 9x standard does define the value of the index and in your case the value is 11. More generally :-

do i = 1,n
! code with cycle and exit options
end do

if (i <= n) then ! the loop terminated via an exit
if (i == n+1) then ! the loop ran to completion

This is useful when searching for a value in a list. "I" points to the found value or the next vacant entry beyond the defined list, such as:-

do i = 1,n
if (value == list_of_values(i)) exit
end do
if ( i > n) then ! new value

Regards John

PaulLaidler · Posted: Thu Mar 13, 2008 9:11 am Post subject:

The Fortran standard states "When an active DO construct becomes inactive, the do-variable, if any, retains its last defined value".

To me this is ambiguous. It depends on how the iteration is coded by the compiler.
In practise I think that you will find that most Fortran compilers behave in the same way as FTN95 but for complete portability you should not depend on this.

JohnCampbell · Joined: 16 Feb 2006 Posts: 2554 Location: Sydney

Paul,

I remember years ago when fortran 90 first came out and Tom Lahey did a tour promoting the improvements in the F90 standard. One that I (may have mistakenly) taken from his talk was the improved definition of how DO loops operate, including how the limits are defined on entry to the loop and the loop exits when the counter fails the test, for example
"DO I = 1,I"
should be valid if "I" had a value on entry to the loop. I continues to increment until it exceeds the value "I" had before the loop started.

For do I = 1,n, if I<=n on exit then the loop terminated internally, wheras the loop completed normally if I>n.

I've been using this assumption for 15 years !

John

weaverwb · Joined: 04 Aug 2005 Posts: 37 Location: Monterey

Hi all,

I've been using Fortran since Fortran II so I've seen a lot of compilers come and go. Most have claimed that the index is not defined after leaving the loop but all tested for 'exceed'; i.e., in your case it would be 11. Personally, I've always taken their word for it and not trusted the value of the index after completion of the loop.