Trying to plot Lissajous curve to learn sympy but "TypeError: can't convert expression to float"

[Lawrence Tattrie]

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.

https://en.wikipedia.org/wiki/Lissajous_curve

[Amit Saha]

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.









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[Lawrence Tattrie]

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

[Aaron Meurer]

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

> https://groups.google.com/d/msgid/sympy/c25365a3-49a2-478e-a690-3f972a26188b%40googlegroups.com.