# How to plot circle with direction?

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Megha on 10 Sep 2019
Commented: michael on 20 Jan 2021
I wish to plot a circle with direction of its motion.An exmaple is attached below.
I wish to get around 5-10 arrows per circle (that can be chnaged as per our need).
Below attached matlab.mat file is the data file.
The code goes like:
subplot(2,2,[1 3])
h1 = plot(x, y, '-','Color','b','LineWidth',2);
Now I dont know how to put 5-10 arrows showing direction.
Can anyone help?
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### Answers (1)

Bruno Luong on 10 Sep 2019
Edited: Bruno Luong on 10 Sep 2019
2D version. adapt to your need
% circle parameter
r = 5;
cx = 0;
cy = 0;
cfun = @(tt) [cx+r*cos(tt); cy+r*sin(tt)];
xy = cfun(linspace(0,2*pi,361));
close all
hold on
plot(xy(1,:),xy(2,:),'b');
% arrows parameters
m = 10; % number of arrows
h = 0.1*r; % height
w = 0.1*r; % width
dir = -1; % 1 anticlock, -1 clock
a = [-w/2 0 w/2;
-dir*h 0 -dir*h];
for k=1:m
tt = 2*pi*k/m;
R = [cos(tt) -sin(tt);
sin(tt) cos(tt)];
xy = cfun(tt)+ R*a;
plot(xy(1,:),xy(2,:),'b');
end
axis equal
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Megha on 11 Sep 2019
I have tried the following code, but arrows are too close to each other, in this case, to see the dirction of motion of circle.
rMag = 0.5;
lenTime = length(x);
% Indices of tails of arrows
vSelect0 = 1:(lenTime-1);
% Indices of tails of arrows
vSelect1 = vSelect0 + 1;
% X coordinates of tails of arrows
vXQ0 = x(vSelect0, 1); % x last removed
% Y coordinates of tails of arrows
vYQ0 = y(vSelect0, 1); % y last removed
% X coordinates of heads of arrows
vXQ1 = x(vSelect1, 1); % x first removed
% Y coordinates of heads of arrows
vYQ1 = y(vSelect1, 1); % y first remove
% vector difference between heads & tails
vPx = (vXQ1 - vXQ0) * rMag;
vPy = (vYQ1 - vYQ0) * rMag;
% add arrows
h2 = quiver (vXQ0,vYQ0, vPx, vPy, 0, 'b'); grid on; hold off
axis equal
michael on 20 Jan 2021
I'w suggest to set
X = 10; % this is step size - each 10th point will have a vector
vSelect0 = 1:X:(lenTime-1);

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