How to move a circle on a sine curve?

I tried to move the circle on the sine curve by changing the centre of the circle and did the following:

 theta =30:330
x=5*cosd(theta)
y=5*sind(theta)
x(301)=0
y(301)=0
x(302)=5*cosd(30)
y(302)=5*sind(30)
h=fill(x,y,'y')
axis ([0,200,0,50])
axis equal
for i=0:200
    x1=i+5*cosd(theta)
    y1=sind(i)+5*sind(theta)
    x1(301)=i
    y1(301)=sind(i)
    x1(302)=i+5*cosd(30)
    y1(302)=sind(i)+5*sind(30)
    set(h,'xdata',x1,'ydata',y1)
    pause(0.01)

end

But the circle keeps on moving on a straight line although y component of the changing circle should be changing.

Answer 1

[Mike Garrity]

Actually the Y component is changing. But it's only changing in the range -1 to 1. Your YLim goes from something like -80 to 80, so it's hard to see that small motion. If you scale it way up you can see the limits change.

But there's something interesting going on here. That "axis equal" is a little strange here. It's saying that you want the scale to be the same on the X & Y axes. To do that, it overrides the YLim you chose (i.e. [0 50]) and computes a new YLim with the correct scale. But the we it computes that new YLim is that it centers it around whatever is in the axes. What's in the axes is the pacman that you're trying to move up and down. So it keeps changing the YLim in such a way as to move the pacman into the center. Because your Y motion was small, it was hard to see this happening.

As Mischa said, what you need to do is to call "axis manual" after calling "axis equal". That will say "now that I have equal scales, stop changing the limits". Does that make sense?

[Airas Akhtar]

Thanks.It does make sense and I found another way a much simpler way.I just called axis equal before the limits of my axes.And alternatively the axis manual jist worked out fine. The concept you gave is very helpful.

Answer 2

[Mischa Kim]

Airas, use something like

 ...
 h = fill(x,y,'y');
 axis ([0,200,0,50])
 axis equal
 axis manual
 for ii = 0:200
    x = x + 1*cosd(ii);
    y = y + 1*sind(ii);
    set(h,'Vertices', [x(:) y(:)])
    pause(0.01)
    drawnow
 end

[Airas Akhtar]

Using your technique my code got short.Thanks