> Here is a more generic find() you could use. It will work on any valid
> logical expression.
>
> Usage Example:
> integer,allocatable :: i(:)
>
> allocate( i(count(x>3.0) )
>
> i = find(x > 3.0)
>
> .... do stuff with i...
>
> deallocate(i)
> ...
>
> Matlab does some of this for you behind the scenes. Fortran is more like
> C -- you get to deal with the wonderful :-) allocations yourself.
>
> function find(mask) result(f)
>         logical,intent(in) :: mask(:)
>
>   ! Note x and mask are of the same size by definition. The > operator is
>   ! broadcast along the rank of x and a vector of boolean values is created.
>
>      integer :: f(size(count(mask==.true.)) ! or just count(mask).

I lost count of who wrote this    ^^^^^^^^^^^^
Anayway, to compare logicals == should be replaced by .EQV.  :-()
I am astonished that compilers often don't flag this error.
And, indeed, COUNT(MASK) is the way to write it.

>         integer :: i,k
>         i = 1
>         do k = 1,size(mask)
>                 if (mask(k)) then
>                         f(i) = k
>                         i = i + 1
>                 end if
>         end do
>
> end function find
> I just dashed this off, so please let me know if I goofed this up. :-)
>
> Somebody who knows more F90 than I, tell me: could I do this:
>
> i(1:n) = find(x>3.0,n) ???
>
> I think not, but it would simplify the usage.
>
>
> Alvaro Fernandez
> Vice President
> Athena Technology Consultants, Inc.
>   WWW: www.athena-eng.com
> Email: [log in to unmask]
> Voice: 281-648-3994
>   Fax: 281-648-4015
>
> |
> ||-----Original Message-----
> ||From: Fortran 90 List [mailto:[log in to unmask]]On Behalf
> ||Of Malcolm Cohen
> ||Sent: Thursday, January 24, 2002 9:02 AM
> ||To: [log in to unmask]
> ||Subject: Re: Find indices of nonzero elements
> ||
> ||
> ||Choru 'n Moru said:
> ||> I am wondering if there is a way in F90 to get the indices of non-zero
> ||> elements.
> ||
> ||Hmm, why do you want the indices?
> ||
> ||You can operate on the non-zero elements without knowing the indices,
> ||by using the WHERE statement or construct.  You can also operate on
> ||corresponding elements of other arrays (the same way).
> ||
> ||E.g.
> ||
> ||  REAL A(...),B(...)
> ||  ...
> ||  WHERE (A.NE.0)
> ||    B = 1/A
> ||    A = SQRT(A)
> ||  ELSEWHERE
> ||    B = 0
> ||  ENDWHERE
> ||
> ||> For those familiar with MATLAB, there is function called find.
> ||> For example, I = FIND(A>100), returns the indices of A where A
> |is greater
> ||> than 100.  The caveat to this is that I'd like to do it without loops or
> ||> IF-THEN_ELSE statements.
> ||
> ||The most efficient way is probably to use loops and IF-THEN.
> ||Something like:
> ||
> ||FUNCTION find(a)
> ||  REAL a(:)
> ||  REAL find(COUNT(a.NE.0))
> ||  j = 1
> ||  DO i=1,SIZE(a)
> ||    IF (a(i).NE.0) THEN
> ||      find(j) = i
> ||      j = j + 1
> ||    END IF
> ||  END DO
> ||END FUNCTION
> ||
> ||> I'd like to exploit F90's vector operations.  Is
> ||> this even possible in F90?
> ||
> ||Sure, but why would you want to?
> ||
> ||Here is an obfuscated but "vector" version of FIND:
> ||
> ||    PACK((/(i,i=LBOUND(a,1),UBOUND(a,1))/),a.NE.0)
> ||
> ||This will probably be slower than the procedure using loops, as many
> ||compilers will manifest the array constructor before masking and packing
> ||it down.  [Yes, I'm sure some compilers can optimise it ok, but not all.]
> ||
> ||But mostly you can just use WHERE to operate on masked vectors already,
> ||rather than manifesting a index for the mask.
> ||
> ||Cheers,
> ||--
> ||...........................Malcolm Cohen, NAG Ltd., Oxford, U.K.
> ||                           ([log in to unmask])
> ||
> ||_____________________________________________________________________
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> |
>