How can I plot a triangle with its altitudes?

조회 수: 13(최근 30일)
Jack Bason
Jack Bason 2018년 4월 30일
편집: John BG 2018년 5월 11일
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
  댓글 수: 4
John BG
John BG 2018년 5월 5일
Also, none of the supplied answers so far, including mine, consider negative coordinates of the input points
x =[ -6 -5 -4] y =[ -10 4 10]
.

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채택된 답변

Jan
Jan 2018년 5월 4일
편집: Jan 2018년 5월 4일
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.

추가 답변(2개)

sloppydisk
sloppydisk 2018년 4월 30일
Please clarify what you are trying to do. What would you like to see as output?
  댓글 수: 1
Jack Bason
Jack Bason 2018년 4월 30일
As an output I would like to see a plotted triangle with all 3 altitudes that intersect at one point. The coding needs to have the ability to change the co-ordinates and it still work.

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John BG
John BG 2018년 5월 1일
편집: John BG 2018년 5월 1일
Hi Jack Bason
I have rearranged your code to do what you asked for, please find attached copy of this script.
close all;clear all;clc
x=[0,2,4];y=[0,3,2];
x2=[x x x(1)];y2=[y y y(1)];
plot(x2,y2,'b')
hold on
x_intersect=zeros(1,3)
y_intersect=zeros(1,3)
for k=1:1:length(x)
p1=[x2(k) y2(k)]
p2=[x2(k+1) y2(k+1)]
pref=[x2(k+2) y2(k+2)]
% 1st segment slope: m offset: k0
m1=(p2(2)-p1(2))/(p2(1)-p1(1))
switch abs(m1)
case Inf % vertical, 90.00 degree up
yint=pref(2)
xint=p1(1) % = p2(1), xint
m2=0
case 0 % horizontal, completely flat, 0.00 degree
yint=p1(2) % =p2(2)
xint=pref(1) % = p2(1)
m2=0
otherwise
k01=p1(2)-m1*p1(1)
m2=-1/m1 % 2nd segment m k0
k02=pref(2)-m2*pref(1)
xint=(k01-k02)/(m2-m1) % intersect point
yint=m1*xint+k01
x_intersect(k)=xint % logging intersect point
yIntersect(k)=yint
end
plot([p1(1) p2(1)],[p1(2) p2(2)],'g*')
plot(pref(1),pref(2),'y*')
plot(xint,yint,'g*')
plot([xint pref(1)],[yint pref(2)],'r')
end
.
.
if you find this answer useful would you please be so kind to consider marking my answer as Accepted Answer?
To any other reader, if you find this answer useful please consider clicking on the thumbs-up vote link
thanks in advance for time and attention
John BG

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