MATLAB Answers


Draw a line between two points

Asked by Artyom
on 17 Jul 2013

Hi. I have three points [5.2 0 0 0 9.9 0 0 12.1]. How to draw a line through the first two points and find is the third point (12.1) above/below this line.


on 17 Jul 2013

Okay. You say you have three points, but your array seems to contain eight. Are you saying you only want to consider points where your y value is not zero? That is, you would plot a line through (1,5.2) and (5,9.9) then see if (9,12.1) is above or below the line?

I'll go ahead and submit a solution under this assumption. If I'm mistaken, just let me know any I'll modify it as needed.

on 17 Jul 2013

Actually I have an array like this:

[1 5.2
 2 4.3
 3 5.1
 4 7.8
 5 9.9

I just want to show that we take only 2 points (1 and 5) and draw a line through them.

on 17 Jul 2013

Ah, okay. I suppose you've been successful in modifying my below example code to be more in line with your needs?



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4 Answers

Answer by Evan
on 17 Jul 2013
Edited by Evan
on 17 Jul 2013
 Accepted answer

P = [1 5.2; 2 0; 3 0; 4 0; 5 9.9; 6 0; 7 0; 8 12.1];
posP = find(P(:,2)); %find where y values aren't zero.
C = polyfit(P(posP(1:2),1),P(posP(1:2),2),1); %fit line to first and second nonzero 
Y = polyval(C,P(posP(3),1)); %find value on line at x-value of third nonzero
if Y == P(posP(3),2)
  disp('The third point falls on the line')
elseif Y > P(posP(3),1)
  disp('The third point falls above the line')
elseif Y < P(posP(3),1)
  disp('The third point falls below the line')


Answer by Iain
on 17 Jul 2013

This is the raw maths...

 point1 = [x1 y1];
 point2 = [x2 y2];
 point3 = [x3 y3];
 m = (y2 - y1) / (x2 - x1);
 c = y2 - m*x2;
 y3_est = m*x3 + c;
 if y3_est > y3
  disp('point below line')
 elseif y3_est == y3
  disp('point above line')


Answer by Kiran Sagar
on 17 Jul 2013

Frame an equation between two of the points of the form y=mx+c. then replace x with the x-cordinate of the point to be checked, compare the resultant y with the point's y cordinate. that's it. supposing your two points forming the line are the first and last rows of the matrix A, and the test point as one of the in between points, here is a probable code:

A = [1 5.2;
2 0;
3 0;
4 0;
5 9.9;];
m = (A(5,2)-A(1,2))/(A(5,1)-A(1,1));
c = A(1,2)-m*A(1,1);
y_test = m*A(2,1)+c;
if y_test<A(2,2)
%%%%% your statements %%%%%
%%%%%your statements%%%


Answer by Artyom
on 17 Jul 2013

Thank you guys.


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