# Draw a line between two points

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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.
Evan 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?

Evan on 17 Jul 2013
Edited: Evan on 17 Jul 2013
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

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

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)
else
end

Artyom on 17 Jul 2013
Thank you guys.