Select a point on the graph

Question

luca 2019-10-9

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/484366-select-a-point-on-the-graph

评论： luca 2019-10-9

Hi given the following code,

figure(1);
scatter(x(:,1),x(:,2));
hold on;
scatter(member_value(:,1),member_value(:,2),'r');
legend({'Data','Pareto Frontier'})

I obtain a graph like this

And I want to select the red point that is closest to the origin.

May someone help me with the code?

2 个评论
显示无隐藏无

Adam 2019-10-9

What do you mean by 'select' it? You can click on it and select it if you wish, but that depends what you want to do having 'selected' it.

If you mean programmatically find it then isn't it just a simple case of pythagoras, having subtracted your origin from all points? (Or some built-in distance function that does the maths for you anyway)

luca 2019-10-9

Yes the idea is to use something like pythagoras, that able me to find the point with the minimum distance from the origin. But do you know how can I implement it?

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Adam Danz 2019-10-9

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/484366-select-a-point-on-the-graph#answer_395487

编辑：Adam Danz 2019-10-9

在 MATLAB Online 中打开

To find the coordinate closest to the origin (0,0),

d = hypot(member_value(:,1),member_value(:,2)); 
[~, minIdx] = min(d); 
plot(member_value(minIdx,1),member_value(minIdx,2),'ks','MarkerSize',12);

hypot() method avoids potential under/overflow: https://www.mathworks.com/help/matlab/ref/hypot.html

8 个评论
显示 6更早的评论隐藏 6更早的评论

Adam Danz 2019-10-9

编辑：Adam Danz 2019-10-9

在 MATLAB Online 中打开

Let's say you have 4 coordinates (black dots, below).

Each of those coordinates forms a right-triangle where 1 side of the triangle is the 'x-coordinate and the other side of the triangle is the y-coordinate. The hypotenuse of the triangle is the line that extends from (0,0) to the dot.

hypot() inputs are the x and y coordinates which are the sides of each triangle. It outputs the length of the hypotenuse which is the distance of the dot from (0,0). The hypotenuse are the diagonal lines in the image below.

Hypotenuse demo code for the image above:

clf()
x = rand(4,2).*8+1;
scatter(x(:,1),x(:,2));
hold on;
axis equal
xlim([0,9])
ylim([0,9])
grid on
leg = plot(x(:,[1,1])', [x(:,2),zeros(size(x,1),1)]', '-','LineWidth',2);
hyp = plot([x(:,1),zeros(size(x,1),1)]', [x(:,2),zeros(size(x,1),1)]', '-','LineWidth',2);
set(hyp,{'Color'},{leg.Color}')
d = hypot(x(:,1),x(:,2)); 
labels = strsplit(sprintf('h=%.1f\n',d'));
t = text(x(:,1)-.2,x(:,2),labels(1:end-1),'HorizontalAlignment','right');

luca 2019-10-9

Thanks Adam ! now its clear how it works

请先登录，再进行评论。

Answer 2

Turlough Hughes 2019-10-9

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/484366-select-a-point-on-the-graph#answer_395486

在 MATLAB Online 中打开

You can do the following:

[~,ind]=min(sqrt(member_value(:,1).^2+member_value(:,2).^2)); %find index for point closest to origin
hold on; plot(member_value(ind,1),member_value(ind,2),'.k');

Note, that if x was arranged as a row vector this will not work, but this is not the case for you.