python (sympy) implicit function: get values instead of plot? - python

I am new to sympy but I already get a nice output when I plot the implicit function (actually the formula for Cassini's ovals) using sympy:
from sympy import plot_implicit, symbols, Eq, solve
x, y = symbols('x y')
k=2.7
a=3
eq = Eq((x**2 + y**2)**2-2*a**2*(x**2-y**2), k**4-a**4)
plot_implicit(eq)
Now is it actually possible to somehow get the x and y values corresponding to the plot? or alternatively solve the implicit equation without plotting at all?
thanks! :-)

This is an answer addressing your
is it actually possible to somehow get the x and y values corresponding to the plot?
and I say "addressing" because it's not possible to get the x and y values used to draw the curves — because the curves are not drawn using a sequenc of 2D points… more on this later,
TL;DR
pli = plot_implicit(...)
series = pli[0]
data, action = series.get_points()
data = np.array([(x_int.mid, y_int.mid) for x_int, y_int in data])
Let's start with your code
from sympy import plot_implicit, symbols, Eq, solve
x, y = symbols('x y')
k=2.7
a=3
eq = Eq((x**2 + y**2)**2-2*a**2*(x**2-y**2), k**4-a**4)
and plot it, with a twist: we save the Plot object and print it
pli = plot_implicit(eq)
print(pli)
to get
Plot object containing:
[0]: Implicit equation: Eq(-18*x**2 + 18*y**2 + (x**2 + y**2)**2, -27.8559000000000) for x over (-5.0, 5.0) and y over (-5.0, 5.0)
We are interested in this object indexed by 0,
ob = pli[0]
print(dir(ob))
that gives (ellipsis are mine)
['__class__', …, get_points, …, 'var_y']
The name get_points sounds full of promise, doesn't it?
print(ob.get_points())
that gives (edited for clarity and with a big cut)
([
[interval(-3.759774, -3.750008), interval(-0.791016, -0.781250)],
[interval(-3.876961, -3.867195), interval(-0.634768, -0.625003)],
[interval(-3.837898, -3.828133), interval(-0.693361, -0.683596)],
[interval(-3.847664, -3.837898), interval(-0.673830, -0.664065)],
...
[interval(3.837895, 3.847661), interval(0.664064, 0.673830)],
[interval(3.828130, 3.837895), interval(0.683596, 0.693362)],
[interval(3.867192, 3.876958), interval(0.625001, 0.634766)],
[interval(3.750005, 3.759770), interval(0.781255, 0.791021)]
], 'fill')
What is this? the documentation of plot_implicit has
plot_implicit, by default, uses interval arithmetic to plot functions.
Following the source code of plot_implicit.py and plot,py one realizes that, in this case, the actual plotting (speaking of the matpolotlib backend) is just a line of code
self.ax.fill(x, y, facecolor=s.line_color, edgecolor='None')
where x and y are constructed from the list of intervals, as returned from .get_points(), as follows
x, y = [], []
for intervals in interval_list:
intervalx = intervals[0]
intervaly = intervals[1]
x.extend([intervalx.start, intervalx.start,
intervalx.end, intervalx.end, None])
y.extend([intervaly.start, intervaly.end,
intervaly.end, intervaly.start, None])
so that for each couple of intervals matplotlib is directed to draw a filled rectangle, small enough that the eye sees a continuous line (note the use of None to have disjoint rectangles).
We can conclude that the list of couples of intervals
l_xy_intervals = ((pli[0]).get_points())[0]
represents rectangular areas where the implicit expression you are plotting is
"true enough"

You can do this, even with interval math, if you try getting the mid point of each interval. Starting from your code, and slightly change it, by saving the plot_implicit object in a variable called g we have:
from sympy import plot_implicit, symbols, Eq, solve
x, y = symbols('x y')
k=2.7
a=3
eq = Eq((x**2 + y**2)**2-2*a**2*(x**2-y**2), k**4-a**4)
g = plot_implicit(eq)
Now let's save in a variable named ptos the intervals that were used to draw the plot.
ptos = g[0].get_points()[0]
This way ptos[0][0] will be the first interval in the x axis and ptos[0][1] will be its pair in the y axis. The intervals have a property called mid which gives the middle point of the interval. So you can suppose that ptos[0][0].mid, ptos[0][1].mid will be a pair x,y "true enough" to be one of our numerical solutions.
This way, a data frame composed of this middle point pairs can be generated with:
intervs = np.array(dtype='object')
meio = lambda x0:x0.mid
px = list(map(meio, intervs[:,0]))
py = list(map(meio, intervs[:,1]))
import pandas as pd
dados = pd.DataFrame({'x':px, 'y':px})
dados.head()
Which in this example would give us:
x y
0 -1.177733 0.598826
1 -1.175389 0.596483
2 -1.175389 0.598826
3 -1.173045 0.596483
4 -1.173045 0.598826
This idea of getting the intervals middle points can be used whenever one needs to move from "interval math" to "standard" point level math. Hope this helps. Regards.

Related

modeling 5 ordinary differential equations and plotting the model to show the 5 equations

hello I am newbie at python and coding for the most part and I have 5 ordinary differential equations.(non-linear) that I want to model and have them plot. I have the parameters that are given, my main issue has been setting the independent variables to be a function of z. As well as setting the 'S' parameters to be a function of time since they vary depending on the time of year.
edited CODE
I've been able to have the code run with set parameters. I now wonder how I could take these parameters and make them behave at different times. The parameters that are set on this code are for a specific amount of "days" during the year. They are not meant to be consistent throughout. How could I implement time to have them be dependent on it?
import numpy as np
from scipy.integrate import odeint
import matplotlib.pyplot as plt
import math
from math import e
def func(z,t):
xh, xf, y, m, n = z
v1,v2,v3 = 0.05,0.06,0.07
B1,B2,B3 = 0.1984,0.1593,0.04959
d1,d2,d3 = 0.02272,0.02272,0.2
o1,o2 = 0.25,0.75
S1=S2=S3=0.005
S4=S5=0.3
p = 0
u = 500
k = 0.000075
a = 0.4784
r = 0.0165
K = 8000
i = 2
H = e**(-m*k)
g = ((xh+xf)**i)/((K**i)+((xh+xf)**i))
R = o1-(o2*(xf/(xh+xf+.002)))
P1 =(xh+xf)/(xh+y+xf+.002)
P2 = 1-((m+n)/(a*(xh+y+xf+.002)))
P3 = y/(xh+y+xf+.002)
dxhdt = (u*g*H)-(B1*(m*(xh/(xh+y+xf+.002))))-((d1+S1)*xh)-((v1*(m+n))*xh)-(xh*R)
dxfdt = (xh*R)-(B1*(m*(xf/(xh+y+xf+.002))))-((p+d2+S2)*xf)-(v2*(m+n)*xf)
dydt = (B1*(m*P1))-((d3+S3)*y)-((v3*(m+n))*y)
dmdt =(r*(m*P2))+(B2*(n*P3))-(B3*(m*P1))-(S4*m)
dndt = (r*(n*P2))-(B2*(n*P3))+(B3*(m*P1))-(S5*n)
return [dxhdt,dxfdt,dydt,dmdt,dndt]
z0=[13000,11000,0,0,0]
t = np.linspace(0,100,1000)
xx=odeint(func,z0,t)
plt.figure(1)
plt.plot(t,xx[:,0],'b-',label = 'xh')
plt.plot(t,xx[:,1],'y-',label = 'xf')
plt.plot(t,xx[:,2],'g-',label = 'y')
plt.plot(t,xx[:,3],'r-',label = 'm')
plt.plot(t,xx[:,4],'m-',label = 'n')
plt.legend()
plt.ylabel('POPULATION')
plt.xlabel('TIME')
plt.show()
I though about creating two different functions and looping the plot. How do you makes "days" of function of t? just declaring it is? I get error code "TypeError: 'float' object cannot be interpreted as an integer"
z0=[13000,11000,0,0,0]
t = np.linspace(0,91.25,1000)
xx=odeint(func,z0,t)
xy=odeint(func2,z0,t)
plt.figure(1)
for t in range(1,91.25):
plt.plot(t,xx[:,0],'b-',label = '$x_h$')
plt.plot(t,xx[:,1],'y-',label = '$x_f$')
plt.plot(t,xx[:,2],'g-',label = 'y')
plt.plot(t,xx[:,3],'r-',label = 'm')
plt.plot(t,xx[:,4],'m-',label = 'n')
for t in range(91.25,182.50):
plt.plot(t,xy[:,0],'b-',label = '$x_h$')
plt.plot(t,xy[:,1],'y-',label = '$x_f$')
plt.plot(t,xy[:,2],'g-',label = 'y')
plt.plot(t,xy[:,3],'r-',label = 'm')
plt.plot(t,xy[:,4],'m-',label = 'n')
plt.legend()
plt.ylabel('POPULATION')
plt.xlabel('TIME')
plt.show()
I get what you mean by an ODE, but please expand it so others that are not cognizant of mathematics can understand.
If you want these to be a function of z, then you must declare a function something() and assign the variables this function. This way, your values will change with respect to changes in z.
Also by convention, I don't recommend using this much of variable declarations. Abstract these as much as possible. As an alternative, you can declare similar variables in the same line, like
v1, v2, v3 = 0.5, 0.6, 0.7
etc. This will make it much more readable.
If you don't have any syntax error due to multiple assignments in the first line, I recommend change each of this to be a function of z. Divide your bigger function to smaller chunks, make each of this a different function. This way you can manipulate results directly and code will be much more readable.
You prefer the state vector to be composed as
xh, xf, y, m, n
This interpretation of the state vector then needs to be applied everywhere, which means that you have to change the first line of the ODE function to
xh, xf, y, m, n = z
Also check that your fractions are implemented as they were in paper, esp. P1 appears suspicious. But without the genesis of the equation I can not say that it is wrong as it is.

How can I cut a piece away from a plot and set the point I need to zero?

In my work I have the task to read in a CSV file and do calculations with it. The CSV file consists of 9 different columns and about 150 lines with different values acquired from sensors. First the horizontal acceleration was determined, from which the distance was derived by double integration. This represents the lower plot of the two plots in the picture. The upper plot represents the so-called force data. The orange graph shows the plot over the 9th column of the CSV file and the blue graph shows the plot over the 7th column of the CSV file.
As you can see I have drawn two vertical lines in the lower plot in the picture. These lines represent the x-value, which in the upper plot is the global minimum of the orange function and the intersection with the blue function. Now I want to do the following, but I need some help: While I want the intersection point between the first vertical line and the graph to be (0,0), i.e. the function has to be moved down. How do I achieve this? Furthermore, the piece of the function before this first intersection point (shown in purple) should be omitted, so that the function really only starts at this point. How can I do this?
In the following picture I try to demonstrate how I would like to do that:
If you need my code, here you can see it:
import numpy as np
import matplotlib.pyplot as plt
import math as m
import loaddataa as ld
import scipy.integrate as inte
from scipy.signal import find_peaks
import pandas as pd
import os
# Loading of the values
print(os.path.realpath(__file__))
a,b = os.path.split(os.path.realpath(__file__))
print(os.chdir(a))
print(os.chdir('..'))
print(os.chdir('..'))
path=os.getcwd()
path=path+"\\Data\\1 Fabienne\\Test1\\left foot\\50cm"
print(path)
dataListStride = ld.loadData(path)
indexStrideData = 0
strideData = dataListStride[indexStrideData]
#%%Calculation of the horizontal acceleration
def horizontal(yAngle, yAcceleration, xAcceleration):
a = ((m.cos(m.radians(yAngle)))*yAcceleration)-((m.sin(m.radians(yAngle)))*xAcceleration)
return a
resultsHorizontal = list()
for i in range (len(strideData)):
strideData_yAngle = strideData.to_numpy()[i, 2]
strideData_xAcceleration = strideData.to_numpy()[i, 4]
strideData_yAcceleration = strideData.to_numpy()[i, 5]
resultsHorizontal.append(horizontal(strideData_yAngle, strideData_yAcceleration, strideData_xAcceleration))
resultsHorizontal.insert(0, 0)
#plt.plot(x_values, resultsHorizontal)
#%%
#x-axis "convert" into time: 100 Hertz makes 0.01 seconds
scale_factor = 0.01
x_values = np.arange(len(resultsHorizontal)) * scale_factor
#Calculation of the global high and low points
heel_one=pd.Series(strideData.iloc[:,7])
plt.scatter(heel_one.idxmax()*scale_factor,heel_one.max(), color='red')
plt.scatter(heel_one.idxmin()*scale_factor,heel_one.min(), color='blue')
heel_two=pd.Series(strideData.iloc[:,9])
plt.scatter(heel_two.idxmax()*scale_factor,heel_two.max(), color='orange')
plt.scatter(heel_two.idxmin()*scale_factor,heel_two.min(), color='green')#!
#Plot of force data
plt.plot(x_values[:-1],strideData.iloc[:,7]) #force heel
plt.plot(x_values[:-1],strideData.iloc[:,9]) #force toe
# while - loop to calculate the point of intersection with the blue function
i = heel_one.idxmax()
while strideData.iloc[i,7] > strideData.iloc[i,9]:
i = i-1
# Length calculation between global minimum orange function and intersection with blue function
laenge=(i-heel_two.idxmin())*scale_factor
print(laenge)
#%% Integration of horizontal acceleration
velocity = inte.cumtrapz(resultsHorizontal,x_values)
plt.plot(x_values[:-1], velocity)
#%% Integration of the velocity
s = inte.cumtrapz(velocity, x_values[:-1])
plt.plot(x_values[:-2],s)
I hope it's clear what I want to do. Thanks for helping me!
I didn't dig all the way through your code, but the following tricks may be useful.
Say you have x and y values:
x = np.linspace(0,3,100)
y = x**2
Now, you only want the values corresponding to, say, .5 < x < 1.5. First, create a boolean mask for the arrays as follows:
mask = np.logical_and(.5 < x, x < 1.5)
(If this seems magical, then run x < 1.5 in your interpreter and observe the results).
Then use this mask to select your desired x and y values:
x_masked = x[mask]
y_masked = y[mask]
Then, you can translate all these values so that the first x,y pair is at the origin:
x_translated = x_masked - x_masked[0]
y_translated = y_masked - y_masked[0]
Is this the type of thing you were looking for?

Python: bisection method

I am trying to write a program to determine the zeros of the given function (f(x) := ln((sin(x**(1/2))**3) + 2) - 1, using the bisection method. The values a and b, which are the initial values used in the bisection method, are inserted on the program already. All it neeeds to do is show the plot and determine the zeros, but I can't get it to run (it stops on line 22). Can anyone spot the error?
import matplotlib.pyplot as plt
import numpy as np
import math
t = np.arange(0.5, 6.0, 0.01)
s = np.log((np.sin(np.sqrt(t)))**3+2)-1
z = len(t)*(0.0,)
plt.plot(t, s, t, z)
plt.xlabel('x')
plt.ylabel('f(x)')
plt.title('A procura do zero')
plt.grid(True)
plt.savefig("test.pdf")
plt.show()
def bisseçao(a,b):
z=(a+b)/2
while b-a>10**(-5):
if (math.log((math.sin(math.sqrt(a)))**3+2)-1)*(math.log((math.sin(math.sqrt(z)))**3+2)-1)<0:
b=(a+z)/2
if (math.log((math.sin(math.sqrt(b)))**3+2)-1)*(math.log((math.sin(math.sqrt(z)))**3+2)-1)<0:
a=(z+b)/2
return a
a1=1
b1=2
a2=4
b2=5
print("Os zeros são:",bisseçao(a1,b1),bisseçao(a2,b2))
Here is the first problem:
z=(a+b)/2
while b-a>10**(-5):
You need to cumpute a new z in every iteration not only at the beginning of the function.
Second problem part 1:
b=(a+z)/2
Second problem part 2:
a=(z+b)/2
Setting the upper/lower bound between lower/upper bound and center point is not correct. They should be set exactly at the center point.
Correct implementation (with a small simplification for clarity - no need to type the whole function five times over):
func = lambda x: np.log((np.sin(np.sqrt(x)))**3+2)-1
def bisseçao(a, b):
while b-a>10**(-5):
z = (a + b)/2
if func(a)*func(z)<0:
b = z
if func(b)*func(z)<0:
a = z
return a
P.S. The code will run into a problem if z happens to fall exactly at the root. You may want to check for this condition explicitly.
P.P.S. The code will also fail if the starting interval does not contain a root. You may want to check for this condition too.

Bifurcation diagram

I want to draw a Bifurcation diagram of quadratic map in python.
Basically its a plot of x_{n+1}=x_n^2-c and it should look like http://static.sewanee.edu/Physics/PHYSICS123/image99.gif
But I am newbie so I am not sure do I make it right.
My code
import numpy as n
import scipy as s
import pylab as p
xa=0.252
xb=1.99
C=n.linspace(xa,0.001,xb)
iter=100
Y=n.zeros((len(X),iteracje))
i=1
Y0=1
for Y0 in iter:
Y(i+1)=Y0^2-C
for Y0 in iter:
Y(i+1)=Y0^2-C
p.plot(C,Y)
p.show()
My problem is that I don't know how properly write these for loop properly.
Here is some modified code (partial explanation below)
import numpy as n
import scipy as s
import pylab as p
xa=0.252
xb=1.99
C=n.linspace(xa,xb,100)
print C
iter=1000
Y = n.ones(len(C))
for x in xrange(iter):
Y = Y**2 - C #get rid of early transients
for x in xrange(iter):
Y = Y**2 - C
p.plot(C,Y, '.', color = 'k', markersize = 2)
p.show()
First, the linspace command had the wrong format. help(s.linspace) will give you insight into the syntax. The first two arguments are start and stop. The third is how many values. I then made Y a numpy array of the same length as C, but whose values were all 1. Your Y0 was simply the number 1, and it never changed. Then I did some iteration to get past the initial conditions. Then did more iteration plotting each value.
To really understand what I've done, you'll have to look at how numpy handles calculations with arrays.

How to pick points under the curve?

What I'm trying to do is make a gaussian function graph. then pick random numbers anywhere in a space say y=[0,1] (because its normalized) & x=[0,200]. Then, I want it to ignore all values above the curve and only keep the values underneath it.
import numpy
import random
import math
import matplotlib.pyplot as plt
import matplotlib.mlab as mlab
from math import sqrt
from numpy import zeros
from numpy import numarray
variance = input("Input variance of the star:")
mean = input("Input mean of the star:")
x=numpy.linspace(0,200,1000)
sigma = sqrt(variance)
z = max(mlab.normpdf(x,mean,sigma))
foo = (mlab.normpdf(x,mean,sigma))/z
plt.plot(x,foo)
zing = random.random()
random = random.uniform(0,200)
import random
def method2(size):
ret = set()
while len(ret) < size:
ret.add((random.random(), random.uniform(0,200)))
return ret
size = input("Input number of simulations:")
foos = set(foo)
xx = set(x)
method = method2(size)
def undercurve(xx,foos,method):
Upper = numpy.where(foos<(method))
Lower = numpy.where(foos[Upper]>(method[Upper]))
return (xx[Upper])[Lower],(foos[Upper])[Lower]
When I try to print undercurve, I get an error:
TypeError: 'set' object has no attribute '__getitem__'
and I have no idea how to fix it.
As you can all see, I'm quite new at python and programming in general, but any help is appreciated and if there are any questions I'll do my best to answer them.
The immediate cause of the error you're seeing is presumably this line (which should be identified by the full traceback -- it's generally quite helpful to post that):
Lower = numpy.where(foos[Upper]>(method[Upper]))
because the confusingly-named variable method is actually a set, as returned by your function method2. Actually, on second thought, foos is also a set, so it's probably failing on that first. Sets don't support indexing with something like the_set[index]; that's what the complaint about __getitem__ means.
I'm not entirely sure what all the parts of your code are intended to do; variable names like "foos" don't really help like that. So here's how I might do what you're trying to do:
# generate sample points
num_pts = 500
sample_xs = np.random.uniform(0, 200, size=num_pts)
sample_ys = np.random.uniform(0, 1, size=num_pts)
# define distribution
mean = 50
sigma = 10
# figure out "normalized" pdf vals at sample points
max_pdf = mlab.normpdf(mean, mean, sigma)
sample_pdf_vals = mlab.normpdf(sample_xs, mean, sigma) / max_pdf
# which ones are under the curve?
under_curve = sample_ys < sample_pdf_vals
# get pdf vals to plot
x = np.linspace(0, 200, 1000)
pdf_vals = mlab.normpdf(x, mean, sigma) / max_pdf
# plot the samples and the curve
colors = np.array(['cyan' if b else 'red' for b in under_curve])
scatter(sample_xs, sample_ys, c=colors)
plot(x, pdf_vals)
Of course, you should also realize that if you only want the points under the curve, this is equivalent to (but much less efficient than) just sampling from the normal distribution and then randomly selecting a y for each sample uniformly from 0 to the pdf value there:
sample_xs = np.random.normal(mean, sigma, size=num_pts)
max_pdf = mlab.normpdf(mean, mean, sigma)
sample_pdf_vals = mlab.normpdf(sample_xs, mean, sigma) / max_pdf
sample_ys = np.array([np.random.uniform(0, pdf_val) for pdf_val in sample_pdf_vals])
It's hard to read your code.. Anyway, you can't access a set using [], that is, foos[Upper], method[Upper], etc are all illegal. I don't see why you convert foo, x into set. In addition, for a point produced by method2, say (x0, y0), it is very likely that x0 is not present in x.
I'm not familiar with numpy, but this is what I'll do for the purpose you specified:
def undercurve(size):
result = []
for i in xrange(size):
x = random()
y = random()
if y < scipy.stats.norm(0, 200).pdf(x): # here's the 'undercurve'
result.append((x, y))
return results

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