How to plot binary sine function?

binary sine function change its amplitude to minus after a peirod

for example) T=3, Range 0~6

y=sin(t/3) for 0<=t<3

y=-sin(t/3) for 3<=t<6

it reculsively occurs for whole range

6~9, 15~18, ....

9~12, 18~21, ....

how to plot this graph?

Answer 1

[Akira Agata]

How about modulating a phase with 0 <-> pi ?

The following is an example:

T = 3;
time = linspace(0,T*4,1000);

% Create a phase 0 <-> pi
binary = mod(floor(time/3),2) == 1;
phi = pi*binary;

% Plot a phase modulated signal
figure
plot(time,sin(2*pi*(1/T)*time + phi))

Answer 2

[David Hill]

t=0:.01:30:
y=sin(2*pi*t/3).*(-1).^floor(t/3);%I think you are missing the 2*pi
plot(t,y);

[NoYeah]

I did that expression before the question. but something went crazy depends on frequency and time interval. if any term of floor(blah) is containg fractional exponent, it brings whole results into complex plane and distort whole graph into something strange thing

for example) below yields complex value. not -1

(-1)^(99/10) 

[David Hill]

It works fine for me, floor is never fractional.

[NoYeah]

yes but if you set floor set fractional, the result may not correct