Make a point move up and down

I am trying to code for a point inserted on an image that is stationary on the x direction but is moving up and down in the y direction. I have set px = 0 but I am not sure what to do about py.

Here is what I have:

x = 0:.01:20; %linspace of x
y = sin(x); %wave equation
px = 0; %initial x plot 
py = 0; %initial y plot
img =imread('AvgBscan.tiff'); %read in image 
for i=1:100 %loop
    imshow(img);
    set(gcf,'DoubleBuffer', 'on');
    h = patch([0 1 0 1], [0 1 1 0], 'r');
    hold on
    figure(100); %so code will replot over the previous figure, and not make a new one.
    hold off
    plot(x,y, px, py,'o'); %circle point
    pause(0.02); %speed of moving point
    px = 0; 
    py = sin(px); %get that point to move
end

Answer 1

[Walter Roberson]

x = 0:.01:20; %linspace of x
y = sin(x); %wave equation
px = 0; %initial x plot 
py = 0; %initial y plot
img =imread('AvgBscan.tiff'); %read in image 
for i=1:100 %loop
    imshow(img);
    set(gcf,'DoubleBuffer', 'on');
    h = patch([0 1 0 1], [0 1 1 0], 'r');
    hold on
    figure(100); %so code will replot over the previous figure, and not make a new one.
    hold off
    py = y(i);
    plot(x,y, px, py,'o'); %circle point
    pause(0.02); %speed of moving point
end

[SS]

Thank you! To make it move down would I adjust the increments?

[Walter Roberson]

It should already be moving down, according to sin(x). You could modify the amount by using a multiplier times y(i), or you could change frequency by

py = sin(2*pi*frequency*x(i))

[SS]

It does not seem to be moving down. It goes up from 0 and stops at 1. If I make it -1*y(i), it just goes down

[Walter Roberson]

Your loop is to 100 which gets you to x=0+99*0.01 which is 0.99 radians which is not enough to have started moving down again.

[SS]

Got it, thank you once again