What's the syntax error in this code?

edit

I am afraid there is more than one syntax error ;-)

You forgot to declare t as variable.

@interact has to be followed by the definition of a function, not by print statement.

You have to repeat the @interact decorator for each interacting function.

missing ':' after else.

In parametric_plot( x(t), y(t) , ...) is wrong,

you need a tuple of functions parametric_plot( ( x(t), y(t) ), ...).

( My sage version (4.7.1) doesn't know ContiniousSlider ).

var('t')
print "Choose 1 for Lissajous curve and 2 for beat"
@interact
def choise(v =(1..2)):
  if v==1:
    @interact
    def lissa(delta =(-pi,pi),a = (1..10),b = (1,10)):
      p = parametric_plot((cos(a*t+delta),cos(b*t)),(t,0,5*pi))
      p.show()
  else:
    @interact
    def beat(a = (0,5), omega = (0,5),percent = (0,0.1)):
      domega = percent*omega;
      p = parametric_plot((2*a*cos(domega/2*t)*cos(omega*t),2*a*cos(domega/2*t)),(t,0,5*pi))
      p.show()

The following code produces the beats you are looking for.

var('t')
f1=110
f2=104
fs=(f1-f2)
fr=(f1+f2)/2
t1=2/fs
p = plot(a*cos(pi*fs*t)*sin(2*pi*fr*t),(t,0,t1),color='red')
p+=plot([a*cos(pi*fs*t),-a*cos(pi*fs*t)],(t,0,t1))
p.show()

The code below produces an interact that is probably more like want you want. Note that the slider command lets you initialize the sliders.

var('t')
print "Choose 1 for Lissajous curve and 2 for beat"
@interact
def choise(v =(1..2)):
  if v==1:
    @interact
    def lissa(delta =slider(-pi,pi,0.1,1),a = slider(1,10,1,6),b = slider(1,10,1,2)):
      p = parametric_plot((cos(a*t+delta),cos(b*t)),(t,0,5*pi))
      p.show()
  else:
    @interact
    def beat(a = slider(0,5,1,1), f1=slider(0,150,10,110),f2=slider(0,150,10,100)):
        fs=(f1-f2)
        fr=(f1+f2)/2
        t1=2/fs
        p = plot(a*cos(pi*fs*t)*sin(2*pi*fr*t),(t,0,t1),color='red')
        p+=plot([a*cos(pi*fs*t),-a*cos(pi*fs*t)],(t,0,t1))
        p.show()

Here's a link to the code using the Sage single-cell server.

@interact is a decorator to a function, not a print statement.