chooseattribute.m

function [best_feature,best_threshold,left,right] = chooseattribute(ft_lb)
% function chooseattribute select the best feature and best threshold by
% calculating maximum gain
current_entropy = calentropy(ft_lb(:,end),1);

best_feature = -1;
best_threshold = -inf;
max_gain = -inf;
left = [];
right = [];

for i = 1:size(ft_lb,2)-1
    % sort the matrix according to the chosen column 
    tmp = ft_lb;
    [~,I] = sort(tmp(:,i));
    sorted = tmp(I,:);
    
    for j = 1:size(sorted,1)-1
       % try each threshold and select the best one
       th = (sorted(j,i)+sorted(j+1,i))/2; 
       left_ = sorted(1:j,:);
       right_ = sorted(j+1:end,:);
       remainder = calentropy(left_(:,end),size(left_,1)/size(sorted,1))...
           + calentropy(right_(:,end),size(right_,1)/size(sorted,1));
       gain = current_entropy-remainder;
       if gain > max_gain
           max_gain = gain;
           best_feature = i;
           best_threshold = th; 
           left = left_;
           right = right_;
       end
    end
end