Plotting multiple straight lines using random number loop

Hi,

I'm trying to plot several lines of different gradient from the same point on a graph. The gradient is determined by the angle phi, which has a mean angle of 45 degrees and a standard deviation of 3.

Considering the equation of a straight line y=mx+c, c is determined by the gradient and coordinate the line passes through.

I am having a problem with getting multiple plots and I'm not sure why. Currently the plot is only a single line.

My code is:

n=100;
x0=3;
y0=4;
x=linspace(0,20,n);

for i=1:n;
  phi(i)=45+3*randn;
  m(i)=tand(phi(i));
  c(i)= y0-m(i)*x0;

end

y=m(i)*x+c(i);

plot(x,y)

Can someone please help me find a solution for this problem?

Thanks.

[Walter Roberson]

This appears to be the same question as http://www.mathworks.com/matlabcentral/answers/35433-looping-a-program-or-multiple-variables ?

[Stuart]

Yes it is. I thought I'd present the basic problem rather than all the malarky that came with it!

I apologise if its agaisnt the boards rules / etiquette, it was driving me mad and needed help!

[Anshuman Tiwari]

Stuart Try This :

n=100;
x0=3;
y0=4;
x=linspace(0,20,n);
for i=1:n;
  phi(i)=45+3*randn;
  m(i)=tand(phi(i));
  c(i)= y0-m(i)*x0;
  y=m(i)*x+c(i);
  hold on 
plot(x,y)
hold off 
end

This will work

Answer 1

[Thomas]

I guess this is what you want..

n=100;
x0=3;
y0=4;
x=linspace(0,20,n);
y=[];
for i=1:n;
  phi(i)=45+3*randn;
  m(i)=tand(phi(i));
  c(i)= y0-m(i)*x0;
y(i,:)=m(i)*x+c(i);
plot(x,y(i,:))
hold on
end

[Stuart]

You've nailed it!

Yes! Cheers for that!

[Stuart]

Hi Thomas,

Thanks for your help.

After modifying the code slightly it has stopped working. Can you see why?

(Answer has been posted below as another answer)

Your help is much appreciated.

Answer 2

[Stuart]

Hi, I've slightly modified the code you've helped me with and now it doesn't seem to work. x seems to be changing from what I specified and so the plot only works to a certain x value.

The new code is:

%Finding the maximum and minimum points of entering the hoop
%Initial launch data
x0=20;
y0=4;

n=50;
x=linspace(0,27,n);
y=[];
mean_phi=atand((7.5-y0)/(26.425-x0));
sd_phi=1;

for i=1:n;
%Find the mean angle phi and then actual phi.

    phi(i)=mean_phi+sd_phi*randn;
    m(i)=tand(phi(i));
    c(i)= y0-m(i)*x0;

    y(i,:)=m(i)*x+c(i);
    plot(x,y(i,:))
    hold on
    axis equal
    Courtlay
    hring2= rectangle('position',[26.324 7.399 0.202 0.202], 'curvature',[1,1],'EdgeColor',[0 0 0],'LineWidth',0.02);

end

What have I done wrong?

[Thomas]

what is Courtlay?

It is not a function defined in MATLAB

[Stuart]

Oh, I apologise, its basically a birdseye view of a basketball court. All the markings and positions of the hoops.

Quite a long bit of code, probably to big for this comment box.

It doesn't have a function so to speak.

[Thomas]

You do not have to call it and the rectangle code in the for loop.

You can call it outside..

[Stuart]

Once again, you've nailed it.

Thank you again.

Answer 3

[Gaspar Cid]

Hey there guys,

Sorry to bring back this question, but i'm trying to plot multiple RADIAL straight lines from x0 and y0 and i can't do it (not random radial lines, the idea is that they fill a circle), how should i modify this code to make this happen? I guess that must be some change in phi...

I would really appreciate some help,

Thanks

Answer 4

[Gaspar Cid]

Well, i finally did this

N=20000
n=100;
x0=3774;
y0=-352;
x=linspace(-N,N,n);
y=[];
for i=1:n;
phi(i)=(360./n).*i;
m(i)=tand(phi(i));
c(i)= y0-m(i)*x0;
y(i,:)=m(i)*x+c(i);
plot(x,y(i,:))
hold on
end

It's seems to work, so i guess i answered myself haha