# Extracting element index in reduction kernel **URL:** \ **Category:** Uncategorised\ **Created:** [21 August 2025 20:12 UTC](https://pyfr.discourse.group/t/extracting-element-index-in-reduction-kernel/1288 "2025-08-21T20:12:19Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Frankx9](https://avatars.discourse-cdn.com/v4/letter/f/3e96dc/32.png) [@Frankx9](https://pyfr.discourse.group/u/Frankx9)\ **Post date:** [21 August 2025 20:12 UTC](https://pyfr.discourse.group/t/extracting-element-index-in-reduction-kernel/1288/1 "2025-08-21T20:12:19Z")
Hi, a clarification regarding the element index in the reduction kernel. Specifically for OpenMP backend I can access the flattened index associated to a variable at a given quadrature point in a given element as: `idx = _y*BLK_SZ*${ncola} + ib*BLK_SZ*${ncola}*nrow + X_IDX_AOSOA(${i}, ${ncola});`but I have the need of accessing a value of an array that has just `nrow = 1 and ncola = 1 (1 variable)`. Is therefore correct to extract the associated element index as: `idx = ib*BLK_SZ*${ncola}*nrow + X_IDX_AOSOA(0, ${ncola});` and for CUDA backend I have a flattened index as: `ixdtype_t idx = j*ldim + SOA_IX(i, blockIdx.y, gridDim.y);` hence the element index should be `ixdtype_t i = ixdtype_t(blockIdx.x)*blockDim.x + tid;` Is this correct in both cases? Best ---
**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\ **Post date:** [21 August 2025 23:26 UTC](https://pyfr.discourse.group/t/extracting-element-index-in-reduction-kernel/1288/2 "2025-08-21T23:26:45Z")
In the OpenMP case I believe you can further simplify the `X_IDX_AOSOA(…)` macro to `_xi + _xj`. Regards, Freddie. ---
**Author:** ![Frankx9](https://avatars.discourse-cdn.com/v4/letter/f/3e96dc/32.png) [@Frankx9](https://pyfr.discourse.group/u/Frankx9)\ **Post date:** [22 August 2025 05:40 UTC](https://pyfr.discourse.group/t/extracting-element-index-in-reduction-kernel/1288/3 "2025-08-22T05:40:39Z")
I think something is still off on OpenMP The initial shape on host of my array is `(1, neles=52547)` then when blocked on OpenMP becomes `(3285, 1, neles=52547, 16).` I try to access the value (unique) at a given element with the flattened index `idx = ib\*BLK_SZ\*${ncola}\*nrow + X_IDX_AOSOA(0, ${ncola});` or `idx = ib\*BLK_SZ\*${ncola}\*nrow + _xi + _xj;` but in both cases it is wrong Any idea? Best ---
**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\ **Post date:** [22 August 2025 10:11 UTC](https://pyfr.discourse.group/t/extracting-element-index-in-reduction-kernel/1288/4 "2025-08-22T10:11:26Z")
The blocked OpenMP shape you’re using seems wrong. Can you confirm the ioshape and datashape’s of your array on the backend? To understand what is going on you may want to fill the matrix with integers counting up from 0. Regards, Freddie. ---
**Author:** ![Frankx9](https://avatars.discourse-cdn.com/v4/letter/f/3e96dc/32.png) [@Frankx9](https://pyfr.discourse.group/u/Frankx9)\ **Post date:** [22 August 2025 11:29 UTC](https://pyfr.discourse.group/t/extracting-element-index-in-reduction-kernel/1288/5 "2025-08-22T11:29:56Z")
Hi, the ioshape of the matrix is (1, 52547) and the datashape is [3285, 1, 16] Does it help? ---
**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\ **Post date:** [22 August 2025 12:27 UTC](https://pyfr.discourse.group/t/extracting-element-index-in-reduction-kernel/1288/6 "2025-08-22T12:27:52Z")
I do not believe the reduction code has ever been tested on a matrix with a 2D ioshape. It is possible that some indices (sizes) are being picked up incorrectly. Does anything change if you go for an ioshape of (1, 1, 52547)? Regards, Freddie. ---
**Author:** ![Frankx9](https://avatars.discourse-cdn.com/v4/letter/f/3e96dc/32.png) [@Frankx9](https://pyfr.discourse.group/u/Frankx9)\ **Post date:** [22 August 2025 12:29 UTC](https://pyfr.discourse.group/t/extracting-element-index-in-reduction-kernel/1288/7 "2025-08-22T12:29:13Z")
Wouldn’t this break the other kernels actually or not ensuring compatibility with the GPUs backend? ---
**Author:** ![fdw](https://avatars.discourse-cdn.com/v4/letter/f/a587f6/32.png) [@fdw](https://pyfr.discourse.group/u/fdw)\ **Post date:** [22 August 2025 12:43 UTC](https://pyfr.discourse.group/t/extracting-element-index-in-reduction-kernel/1288/8 "2025-08-22T12:43:13Z")
The reduction code is reimplemented by each backend on its own. Thus, the implementations are all slightly different and may behave differently with non-standard (i.e., 2D) matrices. If you use matrices with a 3D ioshape (which is the common case) everything should work. Regards, Freddie.