Deleting elements from vector

I am removing some elements from a (long) vector. I am wondering which is the faster method in general (or better per se). Removing elements directly or by assigning "truncated" vector to the old one? This is what I mean (idx_remove are logical indices):

1.

vec(idx_remove) = [];

or 2.

vec = vec(~idx_remove);

Thanks.

Answer 1

[Guillaume]

I would think that matlab's JIT compiler would reduce both to the same instructions, but the only way to know for sure is to time it on your machine with your version of matlab.

>>vlength = 200000;
>>idx_remove = logical(randi([0 1], 1, vlength));
>>srcvector = rand(1, vlength);
>>tic; vec = srcvector; vec(idx_remove) = []; toc
Elapsed time is 0.004964 seconds
>>tic; vec = srcvector; vec = vec(~idx_remove); toc
Elapsed time is 0.005631 seconds
>>tic; vec = srcvector; vec(idx_remove) = []; toc
Elapsed time is 0.006492 seconds
>>tic; vec = srcvector; vec = vec(~idx_remove); toc
Elapsed time is 0.006448 seconds

In the end though, does it matter? You're chasing after micro-optimisations here. I would favor code clarity to a minuscule gain in performance. For that reason, I would choose option 1 when modifying the vector, and option 2 to assign the shortened vector to a new variable.

Answer 2

[Jan]

Try FEX: CopyMask

vec = CopyMask(vec, ~idx_remove);

The next version will allow to apply the not() implicitly, such that the calculation of a large inverted logical vector can be avoided.