Intel® Fortran Compiler
Build applications that can scale for the future with optimized code designed for Intel® Xeon® and compatible processors.
27417 Discussions

## strange behavior while copying an array

Beginner
188 Views

Hi,

I'm experiencing strange behavior of the program while copying an array.

I made a simple test code to reproduce it.

```program test

integer :: ni, nj, nk
integer, allocatable :: id(:)
real*8, allocatable :: a(:,:,:), b(:,:,:)

ni = 50
nj = 50
nk = 1

allocate(a(ni,nj,nk))
allocate(b(ni,nj,nk))
allocate(id(nk))

a = 1d0

do n = 1, nk
id(n) = nk+1-n
enddo

call copy(a,b)

print*, sum(a), sum(b)

contains

subroutine copy(aa,bb)

real*8, intent(in)  :: aa(ni,nj,nk)
real*8, intent(out) :: bb(ni,nj,nk)

do n = 1, nk
bb(:,:,id(n)) = aa(:,:,n)
enddo

end subroutine copy

end program test```

I think the program should print out the sum value of 2500 for both array a and b.

However, the program prints out 2500 for array a, but 1250 for array b.

In this test, I used intel fortran compiler 2017 initial release.

Could you give me an advice for this problem?, or is there anything I'm doing wrong?

Thanks,

5 Replies
Black Belt
188 Views

You are addressing beyond the end of the arrays a and b.

the 3rd subscript of a and b have a dimension of 1. IOW dimensions are (50,50,1).

Your copy subroutine is specifying 3rd index values of 50:1 and 1:50.

I am surprised you did not crash your program.

Jim Dempsey

Beginner
188 Views

Hi.

The copy subroutine is specifying 3rd index value of 1, not 1 to 50.

The value of nk is just 1.

So, copy subroutine just copies aa(:,:,1) to bb(:,:,1).

Am I wrong?

Black Belt
188 Views

You are wrong:

```do n = 1, nk ! nk == 1
bb(:,:,id(n)) = aa(:,:,n)
! n = 1  bb(1:50,1:50,50) = aa(1:50,1:50,1) ! bb out of allocation range
enddo
```

Edit *** my error, id(1) should contain nk-1+1=1

! n = 1  bb(1:50,1:50,1) = aa(1:50,1:50,1) ! bb inside of allocation range

mea culpa

Jim Dempsey

Beginner
188 Views

I don't understand you.

```do n = 1, nk
bb(:,:,id(n)) = aa(:,:,n)
enddo```

in the above code, nk is 1 and id(1) is 1.

Do loop will be executed just once, not 50 times as you mentioned.

So I think, the above code is same as below code.

`bb(:,:,1) = aa(:,:,1)`

Isn't it?

Valued Contributor I
188 Views

Indeed the code is correct. I was first puzzled whether the subroutine copy knows the actual values of ni, nj, nk, but it actually does via host association. That seems to be a bug in the first ifort 17 release. The latest update (update 4, 17.0.4) works fine. I remember that there were a few issues with arrays in the ifort 17 initial release, at least a submitted some bug reports. As I said, the newest update works, as do gfortran 4.8., 4.9., 5.x, 6.x, 7.x, 8.x, nagfor v6 and PGF v17.