How to transfer the loop from do i=1, lx1; do j=1, ly1; do k=1,lz1 to do l=1,lxyz.

X. G. Zhou

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Mar 29, 2026, 8:27:10 AM (3 days ago) Mar 29

to Nek5000

Dear community,

I see the derivatives calculated in Nek5000 with subroutines comp_gije and gradm1 use the following data structure:

parameter (lxyz=lx1*ly1*lz1)

common /myTest/ dudx(lxyz,lelv), dudy(lxyz,lelv), dudz(lxyz,lelv)

real dudx, dudy, dudz

call gradm1(dudx,dudy,dudz,u)

do e=1,nelv

do l=1,lxyz

dudx(l,e) = ....

enddo

However, i see the y location for velocity mesh can be accessed from ym1(lx1,ly1,lz1,lelv). Hence we usually write:

do e=1,nelv

do i=1,lx1

do j=1,ly1

do k=1,lz1

.... = ym1(i,j,k,e)

enddo

Now i would like to get the derivatives in loop of i j k. Can i write the code like:

do e=1,nelv

do i=1,lx1

do j=1,ly1

do k=1,lz1

dudx( i + lx1*(j-1) + lx1*ly1*(k-1), e ) = .... ! instead of dudx(l,e) in loop lxyz?

enddo

I need to get the derivates and y coordinates in the same loop, any suggestions are helpful. Thanks for advance.

Yours,

Zhou

Fischer, Paul

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Mar 29, 2026, 9:19:12 AM (3 days ago) Mar 29

to X. G. Zhou, Nek5000

Dear Zhou,

Here is a brief description of some options - I hope it will clarify.

Best,

Paul

integer e

real f(lx1,ly1,lz1,lelt)

...

lxyz = lx1*ly1*lz1

c Option 1

do e=1,nelv

do i=1,lxyz

du = dudx(i,e)

f(i,1,1,e) = ym1(i,1,1,e)*dudx(i,e)

enddo

c Option 2

n = lxyz*nelv

do i=1,n

du = dudx(i,1)

f(i,1,1,1) = ym1(i,1,1,1)*dudx(i,1)

enddo

c Your suggested, while technically correct will be about 10x slower

c because you are not using unit-stride memory access

c Option 3

do e=1,nelv

do i=1,lx1

do j=1,ly1

do k=1,lz1

dudx( i + lx1*(j-1) + lx1*ly1*(k-1), e ) = .... ! instead of dudx(l,e) in loop lxyz?

enddo

If you really want to use nested loops (not recommended because it relies

on the compiler to vectorize all of the nested loops, which they rarely do),

you should at least order them correctly

c Option 4

do e=1,nelv

do k=1,lz1

do j=1,ly1

do i=1,lx1

dudx( i + lx1*(j-1) + lx1*ly1*(k-1), e ) = .... ! instead of dudx(l,e) in loop lxyz?

enddo

But the fastest is Option 2.

From: nek...@googlegroups.com <nek...@googlegroups.com> on behalf of X. G. Zhou <starlig...@163.com>
Sent: Sunday, March 29, 2026 7:27 AM
To: Nek5000 <nek...@googlegroups.com>
Subject: [nek5000] How to transfer the loop from do i=1, lx1; do j=1, ly1; do k=1,lz1 to do l=1,lxyz.

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X. G. Zhou

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Mar 29, 2026, 8:34:31 PM (3 days ago) Mar 29

to Nek5000

Dear Paul

Thanks for your kind and detailed responses, which are really helpful.

Thank you for your time.

Yours,

Zhou

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