Randomly select an element from a vector satisfying a condition

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Amirhossein Moosavi 2019-7-1

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评论： Arjun Siva S 2020-12-7

Dear experts,

I would like to randomly select an element from a vector satisfying a condition. In fact, I want to know what is the fastest way. For example, suppose vector X defined as follows:

X = [1 2 3 4 5 2 3 6 7 8 8 7 9 10 0 1 2 3 8 5 6 4];

How should I randomly select and identify the index of an element in this vector, which is greater than 2?

Thanks for your help,

Amir

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Answer 1

Amirhossein Moosavi 2019-7-1

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I already have an answer to this question as follow (but I am seeking for faster solutions):

Ind = find(X > 2); Ind = Ind(randsample(1:numel(Ind), 1));

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Bruno Luong 2019-7-2

The solution seems fine and pretty optimal to me. If you insist on faster move away from MATLAB or buy a faster computer.

Arjun Siva S 2020-12-7

Thanks a lot! I've been thinking about the same problem for a long time.

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Answer 2

Jos (10584) 2019-7-1

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This is a two-step process:

create an intermediate array with all elements of X satisfying your condition
select a single element from that

You can combine the two steps in a single command:

randsample(X(X>2), 1)

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Amirhossein Moosavi 2019-7-1

Thanks for your time. This only gives the value of an element; while I want its index.

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Answer 3

David Goodmanson 2019-7-2

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Relative speeds are going to depend on the length of X and the value N that the elements have to be greater than, (2 in the example). The following is generally faster, by a factor of 2 or so.

f = find(X > M);       % M = 2
Ind = f(randi(length(f)));

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David Goodmanson 2019-7-2

I agree, although the question did specify a single draw.

Amirhossein Moosavi 2019-7-2

You are right. Many thanks for your attention.

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