In general, don't define symbols for variables and then later on
reassign the same variable to something else, like
x = symbols('x')
# Some stuff
x = 1
This is a relatively common mistake that people make. There is no need
to create a symbol for a variable if you never intend to use that
variable as a SymPy symbol, but are going to just assign the Python
variable directly to some value. It's worth making sure you understand
the difference between SymPy symbols and Python variables (see
http://docs.sympy.org/latest/tutorial/gotchas.html for a writeup about
it).
Aaron Meurer
On Mon, Mar 7, 2016 at 6:39 PM, Lawrence Tattrie
<[email protected]> wrote:
> Thank you. The plot works. I should have seen the tmin and tmax. That
> correction seemed to do the trick. The frictionx and frictiony do not seem
> to be needed in the dictionaries varx, vary.
> Thank you again.
> - Lawrence
>
> On Monday, March 7, 2016 at 4:08:28 PM UTC-7, Amit Saha wrote:
>>
>> On Tue, Mar 8, 2016 at 9:54 AM, Lawrence Tattrie
>> <[email protected]> wrote:
>> > I am trying to learn sympy by plotting Lissajous curves. I am familiar
>> > with
>> > python and know some math. I downloaded and installed the Anaconda
>> > package
>> > for Windows. I have been able to get a simple curve plotted but the
>> > attached
>> > text file shows the few statements and a plot command which gives a
>> > "TypeError: can't convert expression to float" I have tried defining the
>> > symbols as real=true but that did not help. Is the plotting converting a
>> > float to a complex then complaining that it cannot convert a complex to
>> > float? What is the problem with my code?
>> >
>> > Attached is a short text file with statements and error. Thanks for
>> > your
>> > help.
>>
>> You basically had couple of things which were not right. The first was
>> the dictionaries:
>>
>> varx = {ampx:1.0, freqx:3.0, theta:pi/4.0, frictionx: frictionx}
>> vary = {ampy:2.0, freqy:1.0, frictionx: frictionx}
>>
>>
>> If you compare these to what you tried, you will see that the keys in
>> the dictionary are the symbol objects themselves, not the symbol
>> strings. So, the substitution you were attempting didn't work
>> correctly.
>>
>> The second was you needed the following earlier in the code before you
>> were using them:
>>
>> tmin = 0.0
>> tmax = 4.0 * pi
>>
>> The complete working code is as follows:
>>
>>
>> from sympy import *
>> x, y, z, t = symbols('x y z t')
>> k, m, n = symbols('k m n', integer=True)
>> f, g, h = symbols('f g h', cls=Function)
>> init_printing()
>> from sympy.plotting import (plot, plot_parametric,
>> plot3d_parametric_surface, plot3d_parametric_line,
>> plot3d)
>> from sympy.polys.polyfuncs import interpolate
>>
>>
>> ampx, ampy, freqx, freqy, theta = symbols('ampx ampy freqx freqy
>> theta', real=true)
>> frictionx, frictiony = symbols('frictionx, frictiony', real=true)
>> tmin, tmax = symbols('tmin, tmax', real=true)
>>
>> tmin = 0.0
>> tmax = 4.0 * pi
>>
>> frictionx = interpolate([(tmin, 0.0), (tmax, ln(0.5))], t)
>> frictiony = interpolate([(tmin, 0.0), (tmax, ln(0.5))], t)
>>
>>
>> x = ampx * frictionx * sin(freqx * t + theta)
>> y = ampy * frictiony * sin(freqy * t)
>>
>>
>> varx = {ampx:1.0, freqx:3.0, theta:pi/4.0, frictionx: frictionx}
>> vary = {ampy:2.0, freqy:1.0, frictionx: frictionx}
>> print x.subs(varx)
>>
>> plot_parametric(x.subs(varx), y.subs(vary), (t, tmin, tmax))
>>
>>
>>
>> Hope that helps you.
>>
>> Best Wishes,
>> Amit.
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> >
>> > https://en.wikipedia.org/wiki/Lissajous_curve
>> >
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>>
>>
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