Help! My first for-loop plot

Hello.

I'm trying to make a simple for-loop plot, but I can't get the line to show! Where did it go? I even specified it as dashed blue line.

x=[0:0.1:3*pi];

y=sin(x);

for i=0:0.5:3*pi
   if sin(i) > 0
       plot(i,1,'--bx')
   elseif sin(i) < 0
       plot(i,-1,'--bx')
   else
   end
end

plot(x,y);

Answer 1

[dpb]

"I can't get the line to show! Where did it go?"

It didn't "go" anywhere, you never actually plotted it--in

    if sin(i) > 0
       plot(i,1,'--bx')

you're only plotting a single point each call and there's also not a hold statement so each new call to plot draws as if was first and only point on the axes.

Try

plot(x,sign(y),'--b')
hold all
plot(x,y)

ADDENDUM

If you really are trying to do animation here is reason for loop rather than just unfamiliarity with Matlab and vector operations, look at

doc addpoints % for the animatedline object

[Magnarok]

Like this? Still no line, and I tried change it to plot(x, sin(y), '-b'); as you said, but it plotted wrong poits for me.

x=[0:0.1:3*pi];

y=sin(x);

hold all
for i=0:0.1:3*pi
   if sin(i) > 0
       plot(i,1,'-bo');
   elseif sin(i) < 0
       plot(i,-1,'-bo');
   else
   end
end

plot(x,y);

[dpb]

No "not like that"... :) That's the same as you see you don't get a line but a series of points. You don't need a loop at all; the three lines I showed are the entire code required.

"change it to plot(x, sin(y), '-b') as you said,..."

That is NOT what I wrote at all...read more carefully!

EWWW!!! My bad I should TYPE and PROOFREAD more carefully!!! :(

Fixed in Answer.

[Magnarok]

Thank you! Just started out MATLAB you see... :)

Got a hint elsewhere to use for-loop.