MATLAB: How to plot a triangle with its altitudes

plotplotting

Hi I'd just like to know whats wrong with my coding i have as when i run this it does not plot the altitudes correctly.
x = [0,2,4,0];
y = [0,3,2,0];
%Altitude CF
% Gradient Forumla
% m = (y2-y1)/(x2-x1)
m3 = (y(2)-y(1))/(x(2)-x(1));
altitudem3=-1/m3;
% c = y-(m*x)


base3_c = y(1)-(m3*x(1));
altitude3_c = y(3)-(altitudem3*x(3));
% y=mx+c


% mx+base_c=altitudemx+altitude_c


% x=((altitude_c)-(base_c))/(m-altitudem)


xintersection=(altitude3_c-base3_c)/(m3-altitudem3);
yintersection=(altitudem3*xintersection)+altitude3_c;
%Altitude BE
m2 = (y(3)-y(1))/(x(3)-x(1));
altitudem2=-1/m2;
% c = y-(m*x)
base2_c = y(1)-(m2*x(1));
altitude2_c = y(2)-(altitudem2*x(2));
% y=mx+c
% mx+base_c=altitudemx+altitude_c
% x=((altitude_c)-(base_c))/(m-altitudem)
xintersection2=(altitude2_c-base2_c)/(m2-altitudem2);
yintersection2=(altitudem2*xintersection)+altitude2_c;
%Altitude AD
m = (y(3)-y(2))/(x(3)-x(2));
altitudem=-1/m;
% c = y-(m*x)
base_c = y(2)-(m*x(2));
altitude_c = y(1)-(altitudem*x(1));
% y=mx+c
% mx+base_c=altitudemx+altitude_c
% x=((altitude_c)-(base_c))/(m-altitudem)
xintersection3=(altitude_c-base_c)/(m-altitudem);
yintersection3=(altitudem*xintersection)+altitude_c;
hold on
plot(x,y);
plot([xintersection,x(3)],[yintersection,y(3)])
plot([xintersection2,x(2)],[yintersection2,y(2)])
plot([xintersection3,x(1)],[yintersection3,y(1)])
hold off

Best Answer

If the question is: "I'd just like to know whats wrong with my coding", this is the answer:
x = [0,2,4,0];
y = [0,3,2,0];
% This is fine:  
m3 = (y(2)-y(1))/(x(2)-x(1));
altitudem3=-1/m3;
base3_c = y(1)-(m3*x(1));
altitude3_c = y(3)-(altitudem3*x(3));
xintersection=(altitude3_c-base3_c)/(m3-altitudem3);
yintersection=(altitudem3*xintersection)+altitude3_c;
m2 = (y(3)-y(1))/(x(3)-x(1));
altitudem2=-1/m2;
base2_c = y(1)-(m2*x(1));
altitude2_c = y(2)-(altitudem2*x(2));
xintersection2=(altitude2_c-base2_c)/(m2-altitudem2);
% yintersection2=(altitudem2*xintersection)+altitude2_c;
%                                         ^ Here is the problem  
% It must be xintersection2:
yintersection2=(altitudem2*xintersection2)+altitude2_c;
m = (y(3)-y(2))/(x(3)-x(2));
altitudem=-1/m;
base_c = y(2)-(m*x(2));
altitude_c = y(1)-(altitudem*x(1));
xintersection3=(altitude_c-base_c)/(m-altitudem);
yintersection3=(altitudem*xintersection)+altitude_c;
%                                     ^ Here is the problem  
% It must be xintersection3:
yintersection3=(altitudem*xintersection3)+altitude_c;
hold on
plot(x,y);
plot([xintersection,x(3)],[yintersection,y(3)])
plot([xintersection2,x(2)],[yintersection2,y(2)])
plot([xintersection3,x(1)],[yintersection3,y(1)])
This is a typical problem, when code is duplicated by copy&paste. Modifying all occurrences of the differences is tricky. A more reliable solution is to use a loop instead. See John BG's answer, which considers m==0 in addition. But a code as near as possible to your code:
x = [0,2,4,0];
y = [0,3,2,0];
hold on
plot(x,y);  
v = mod(1:5, 3) + 1;
for k = 1:3
  i1 = v(k);
  i2 = v(k+1);
  i3 = v(k+2);
  m                = (y(i2)-y(i1)) / (x(i2)-x(i1));
  altitudem        = -1/m;
  base_c           = y(i1) - m * x(i1);
  altitude_c       = y(i3) - altitudem * x(i3);
  xintersection(k) = (altitude_c - base_c) / (m - altitudem);
  yintersection(k) = (altitudem * xintersection(k)) + altitude_c;
  plot([xintersection(k), x(i3)], [yintersection(k), y(i3)])
end
Using loops instead of copy&past + modifications reduces the chance for typos.
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