how to plot a periodic function fro negative to positive?

Dear friend,

How to plot a periodic function for example 2*x from negative to positive?

I know a method to plot periodic function using mod but, It seems can not plot the negative range.

t = 10; % total simulation time
T = 3; % period
x = 0:0.1:t;
% lets say your original function is y=2*x which repeated every T second, then
y = 2*(mod(x,T));
plot(x,y)

However, because the mod of negative is a little strange, I don't know how to modify the code.

Your help would be highly appreciated

[V Sairam Reddy]

Please check this MATLAB Answer

[Daniel Niu]

Dear Mr Reddy,

The answer is not what I want. for example. I want my function at -pi:pi periodic. But the method only can plot function in 0:2pi.

I am looking forward to your reply.

Best regards.

Answer 1

[Torsten]

T = pi; % period
x = -10:0.1:10;
% lets say your original function is y=2*x which repeated every T second, then
y = 2*(mod(x-pi,2*pi)-pi);
plot(x,y)
grid on

[Daniel Niu]

Wow, thank you, Torsten

Answer 2

[Sam Chak]

Hi @Daniel Niu

Are you looking for this?

Edit: Made changes as per shown in your diagram. Because the amplitude is unknown, then you need to multiply the function by a certain factor to get the desired amplitude.

t = 3*pi;       % total simulation time
T = 2*pi;       % period (from -π to +π) --> π - (-π) = 2π
x = -t:0.01:t;
y = 2*(mod((x - pi), T)) - 2*T/2;   % also need to shift x by pi
plot(x/pi, y), grid on, 
xlabel('{$\frac{x}{\pi}$}', 'interpreter', 'latex', 'fontsize', 16)
ylabel('{$y$}', 'interpreter', 'latex', 'fontsize', 16)

[Sam Chak]

Or directly use the sawtooth() menthod.

T = 3*pi;

fs = 1000;
t  = 0:1/fs:T-1/fs;

a  = pi;      % amplitude
y  = a*sawtooth(2*t);

plot(t/pi, y), grid on, 
xlabel('{$\frac{t}{\pi}$}', 'interpreter', 'latex', 'fontsize', 16)
ylabel('{$y$}', 'interpreter', 'latex', 'fontsize', 16)

[Daniel Niu]

Dear Mr Chak,

I am looking something like this.

Thank you so much for your answers.

[Sam Chak]

Don't mention it @Daniel Niu. I have edited the code to display the sawtooh wave as per your sketched diagram. If you find the solution is helpful, please consider accepting ✔ and voting 👍 the Answer. Thanks!