Randomly select an element from a vector satisfying a condition

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

Answer 1

[Amirhossein Moosavi]

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));

[Bruno Luong]

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]

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

Answer 2

[Jos (10584)]

This is a two-step process:

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

You can combine the two steps in a single command:

randsample(X(X>2), 1)

[Amirhossein Moosavi]

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

Answer 3

[David Goodmanson]

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)));

[Amirhossein Moosavi]

Thanks for your answer. The problem here is a same index could be selected more than once if we want to select two or more indices.

[David Goodmanson]

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

[Amirhossein Moosavi]

You are right. Many thanks for your attention.