Draw a line between two points

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.

[Evan]

How are your points arranged in the vector you provide? Is this a 3d line? Are they arranged [x1 y1 z1 x2 y2 z2...]?

[Artyom]

They are arranged like this:

[1 5.2;
 2 0;
 3 0;
 4 0;
 5 9.9; 
 ... ]

[Evan]

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.

[Artyom]

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.

[Evan]

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

Answer 1

[Evan]

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')
end

Answer 2

[Iain]

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('colinear')
 else
  disp('point above line')
 end

Answer 3

[Kiran Sagar]

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 %%%%%
else
%%%%%your statements%%%
end

Answer 4

[Artyom]

Thank you guys.