graphing an equation with matplotlib

Question

I m trying to make a function that will graph whatever formula I tell it to   import numpy as np   import matplotlib pyplot as plt   def graph formula  x range         x   np array x range        y   formula       plt plot x  y        plt show       When I try to call it the following error happens  I believe it s trying to do the multiplication before it gets to y   formula    graph x  3 2 x-4  range -10  11    Traceback  most recent call last       File   lt pyshell 23 gt    line 1  in  lt module gt        graph x  3 2 x-4  range -10  11     NameError  name  x  is not defined

User · Answer

To plot an equation that is not solved for a specific variable (like circle or hyperbola):

import numpy as np  
import matplotlib.pyplot as plt  
plt.figure() # Create a new figure window
xlist = np.linspace(-2.0, 2.0, 100) # Create 1-D arrays for x,y dimensions
ylist = np.linspace(-2.0, 2.0, 100) 
X,Y = np.meshgrid(xlist, ylist) # Create 2-D grid xlist,ylist values
F = X**2 + Y**2 - 1  #  'Circle Equation
plt.contour(X, Y, F, [0], colors = 'k', linestyles = 'solid')
plt.show()

More about it: http://courses.csail.mit.edu/6.867/wiki/images/3/3f/Plot-python.pdf

User · Answer

This is because in line   graph x  3 2 x-4  range -10  11     x is not defined    The easiest way is to pass the function you want to plot as a string and use eval to evaluate it as an expression    So your code with minimal modifications will be   import numpy as np   import matplotlib pyplot as plt   def graph formula  x range         x   np array x range        y   eval formula      plt plot x  y        plt show     and you can call it as   graph  x  3 2 x-4   range -10  11

User · Answer

Your guess is right  the code is trying to evaluate x  3 2 x-4 immediately   Unfortunately you can t really prevent it from doing so   The good news is that in Python  functions are first-class objects  by which I mean that you can treat them like any other variable   So to fix your function  we could do   import numpy as np   import matplotlib pyplot as plt    def graph formula  x range         x   np array x range        y   formula x      lt - note now we re calling the function  formula  with x     plt plot x  y        plt show      def my formula x       return x  3 2 x-4  graph my formula  range -10  11     If you wanted to do it all in one line  you could use what s called a lambda function  which is just a short function without a name where you don t use def or return   graph lambda x  x  3 2 x-4  range -10  11     And instead of range  you can look at np arange  which allows for non-integer increments   and np linspace  which allows you to specify the start  stop  and the number of points to use

[python] graphing an equation with matplotlib

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