# Randomly select an element from a vector satisfying a condition

21 views (last 30 days)
Amirhossein Moosavi on 1 Jul 2019
Commented: Arjun Siva S on 7 Dec 2020
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?
Amir

Amirhossein Moosavi on 1 Jul 2019
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));
##### 2 CommentsShowHide 1 older comment
Arjun Siva S on 7 Dec 2020
Thanks a lot! I've been thinking about the same problem for a long time.

Jos (10584) on 1 Jul 2019
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 on 1 Jul 2019
Thanks for your time. This only gives the value of an element; while I want its index.

David Goodmanson on 2 Jul 2019
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 on 2 Jul 2019
You are right. Many thanks for your attention.