# How can I randomly select a subset of logicals?

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Blake Mitchell on 31 Mar 2020
Commented: Blake Mitchell on 1 Apr 2020
Goal: I have a vector of logicals, 4203x1. These logicals correspond to trials with the conditions I want (in this example, about 48 out of 4203 are 1s, the rest are 0s). I would like to randomly downsample from these 48 to 20 to match the number of trials in another condition. How might I go about this?

James Tursa on 31 Mar 2020
Edited: James Tursa on 31 Mar 2020
T = your logical trials vector
n = 20; % number of trials to keep
f = find(T); % find the location of the 1's
f = f(randperm(numel(f))); % randomize the find results
T(f(n+1:end)) = false; % get rid of the other trials beyond the random 20
Or if you need to do this repeatedly for the same T, do the f = find(T) only once, then repeatedly do this
R = T;
f = f(randperm(numel(f))); % randomize the find results
R(f(n+1:end)) = false; % get rid of the other trials beyond the random 20
Alternatively, one could start with an all 0's matrix and then repeatedly randomly fill with 1's. E.g.,
R = false(size(T));
f = f(randperm(numel(f))); % randomize the find results
R(f(1:n)) = true; % randomly fill in the 1's
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Blake Mitchell on 1 Apr 2020
Thank you so much, this worked the way I was intending.

### More Answers (1)

dpb on 31 Mar 2020
Nmatch=20; % don't bury data in code; use variables for a variable factor
isM=find(v); % population of indices that match
ix=isM(randperm(numel(isM),Nmatch); % random selection of Nmatch out of total