Avoid evaluation in vectorised conditional expressions

Question

0 个投票

Dear community,

It is well known how to vectorise for-loops that contain if-conditions, see, e.g. https://de.mathworks.com/matlabcentral/answers/17737-how-can-i-vectorize-function-with-if-statement, resulting in functions like (taken from above link):

function result = myfunc(x);

result = zeros(size(x));

idx1 = x>3;

idx2 = (x.^2 + x) > 9;

result(idx1) = 1;

result(idx2) = 2;

result(~(idx1|idx2)) = 3;

This principle works very well. However, in my case, result(idx1) and result(idx2) would be quite lengthy and expensive expressions. The problem is that both expressions are always evaluated for the entire input x, no matter if idx1 or idx2 are true or not. My question is if there is a way to avoid the evaluation of, e.g., the right hand side of result(idx2) if idx2 is false. Just as it would be in an if-elseif-construct.

I'm aware of alternative constructs with arrayfun or a "classical" for-loop with nested if-elseif-construct, but these workarounds are not efficient.

Any help is much appreciated, thank you!

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Follow Question

Answer 1

Torsten 2023-1-31

移动：Torsten 2023-1-31

1 个投票

If your right-hand side contains expressions with x, you must replace x by x(idx1) resp. x(idx2).

2 个评论
显示无隐藏无

Christian B 2023-1-31

Thanks for the answer. I should have specified the code in the question. I know about the indexing, as for example in

function result = myfunc(x)

result = zeros(size(x));

idx1 = x==2;

idx2 = x>2;

result(idx1) = sin(x(idx1));

result(idx2) = cos(x(idx2));

end

Due to the Matlab Profiler, each line is always executed. So my misunderstanding was that (here) also sin and cos on the right hand sides are evaluated for the entire input vector x and the false indices are just dropped. However, some further checks showed me that the right hand sides are only evaluated for the respective indices, which is the behaviour I was asking for.

Thanks!

Walter Roberson 2023-1-31

For future reference:

There are situations in which the cost of extracting a subset of data, computing with it, and expanding the results back into the appropriate locations, can exceed the savings of not performing the operation on the entire matrix.

sin() is implemented using hardware instructions. Those are potentially slower than a conditional memory move, but since the conditional move is going to need to be a loop at some level (you cannot know the relative offset without having evaluated all previous tests), sometimes it can be faster to just do the instruction and ignore or overwrite the results later. sin applied over a block of consecutive locations and be unrolled into several SIN hardware instructions in a row.

请先登录，再进行评论。

Avoid evaluation in vectorised conditional expressions

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

采纳的回答

2 个评论
显示无隐藏无

更多回答（0 个）

类别

标签

Community Treasure Hunt

Avoid evaluation in vectorised conditional expressions

0 个评论 显示 -2更早的评论 隐藏 -2更早的评论

采纳的回答

2 个评论 显示 无 隐藏 无

更多回答（0 个）

类别

标签

另请参阅

Community Treasure Hunt

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

2 个评论
显示无隐藏无