Thanks for the update on range(). I was not aware of that. I was
actually using a different range and then modified it several times to
accommodate what I was actually wanting to iterate over and didn't
realize what I had ended up with when I posted my question.
However, I believe my error may have been coming from the U =
lambdify... line. Either way, I have been able to use the following line.
##
U = lambdify([x,t], u, 'numpy')
for t in range(7):
pylab.plot(xx, U(xx,t))
##
This does not produce quite what I'm looking for... yet. But I will
work with this some more today and see what I come up with.
Thanks again for the reply.
~~archery~~
Aaron S. Meurer wrote:
If I understand correctly, the problem is in the u = 1/2*… line:
You have to define t as a symbol to use it. See
http://docs.sympy.org/gotchas.html#id2. isympy automatically creates some
variables for you, but t is not one of them. So you need to have var('t'), t =
Symbol('t'), or something like that in there.
Also, on a related note, 1/2 will be evaluated by Python to 0.5 before SymPy
can make it a Rational. This should be fine as far as the plotting goes, but
if you would rather deal with exact fractions, you should do something like
S(1)/2, or just pass the whole thing to sympify (the same thing as S() ) as a
string:
u = sympify("1/2 * (sin(pi*(x+t/6)) + sin(pi*(x-t/6)))")
This will also make t a variable for you (though it will not put it into the
namespace).
I don't have pylab installed, so I can't test to see if there is some other
problem.
Also, apparently range(0, 6.6) is depreciated. Use range(0, 6) instead.
I hope this answers at least part of you question.
Aaron Meurer
On Feb 1, 2010, at 2:34 PM, Chad File wrote:
I have a question. It's more of a python question than a sympy question... but
here it is. I have this function that I'm trying to view/model. Basically
it's a sine function... but a traveling sine function. I'd like to plot it at
various stages of progression. I thought something of the following would be
acceptable, but I was wrong.
##
import pylab
xx = pylab.arange(0,1,1/1000)
u = 1/2 * (sin(pi*(x+t/6)) + sin(pi*(x-t/6)))
U = lambdify(x, u, 'numpy')
pylab.figure(1)
for t in range(0,6.6):
pylab.plot(xx, U(xx))
pylab.show()
##
Here's the output
##
NameError: global name 't' is not defined
##
I'm using isympy as interface, FYI.
I was thinking/hoping that sympy would "see" the t dependence and substitute
accordingly. However, I believe that numpy is taking over at the point of initializing
U. Hence my error of an undefined variable t.
I thought I would outsmart numpy by making a substitution of my own and modify
the for-loop as follows.
##
for i in range(0,6.6):
pylab.plot(xx, U(xx).subs(t,i/6))
##
However, that failed as well.
Does anybody know of a little trick that I could use to get the original
function u(x,t) to plot at various values of t... without hard coding each
interval independently?
~~archery~~
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