Vectorized nested indexing

Question

I have a for-loop to do indexing:

for (int i=0; i<N; i++){
    a[i] = b[c[i]]
}

c are the indices of interest and are int *, while b and a are float * and the manipulated values.

But, this takes a long time (and it can't take that long). I'd like to have some vectorizing version, most likely found in BLAS/LAPLACK/etc.

I'm looking for nested_indexing(float * output_vector, float * input_vector, int * input_indices).

I've tried looking through the docs, but have not found anything.

What you need is a gather instruction - stackoverflow.com/questions/16193434/… . Not sure if there are supporting libs — Leeor
– Leeor, Commented Nov 5, 2013 at 23:02
Have you looked at ?lapmt and ?laswp from LAPACK? They do something similar (if you don't mind the fact that the matrix is permuted in-place, not copied.) — finnw
– finnw, Commented Nov 5, 2013 at 23:30

Scott · Accepted Answer · 2013-11-06 15:22:56Z

1

vDSP_vgathr does exactly this. It takes in two float *'s and one int *. It does the equivalent of for (i=0; i<N; i++) a[i] = b[c[i]].

The wording they used was

Uses elements of vector B as indices to copy selected elements of vector A to sequential locations in vector C

It could be sequential indexing too, perhaps. I've noticed that the hardest part about finding these obscure functions is finding the right words to use in your searches.

answered Nov 6, 2013 at 15:22

Scott

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