function [data, list] = extr_by_var(datastack, variable)
% excludes the data that corresponds to the '0's of a binary variable. also
% makes a list of the indices of the excluded data. 
data = datastack;
list = zeros(length(variable(:,1)),1);
m = 1;
for i = length(variable(:,1)):-1:1
    if variable(i,1) == 0
       data(:,:,i) = [];
       list(m,1) = i;
       m = m+1; 
    end
end
for i = length(list(:,1)):-1:1
    if list(i,1) == 0
       list (i,:) = [];
    end
end
end