2

I have a class that generates lists of data from mathematical equations I formulated to model a system over time. The class can take different numbers for the different variables in the equations, therefore you can create objects from the class that have different model output. I would like to plot the output from different objects on the same plot, but when I try to do so I get the following error: ValueError: x and y must have same first dimension, but have shapes (5,) and (10,). I am plotting using matplotlib. An example of what the code looks like is here:

import matplotlib.pyplot as plt

class Foo:
    def __init__(self, x=[1,2,3,4,5], y=[], var1=None, var2=None):
        self.x = x
        self.y = y
        self.var1 = var1
        self.var2 = var2

    def calculate_y(self, index):
        return (self.var1 + self.var2) * self.x[index]

    def run_calculation(self):
        for i in range (0, len(self.x)):
            self.y.append(self.calculate_y(i))

obj1 = Foo(var1 = 1, var2 = 2)
obj2 = Foo(var1 = 3, var2 = 4)

obj1.run_calculation()
obj2.run_calculation()

plt.figure(1)
plt.plot(obj1.x, obj1.y)
plt.plot(obj2.x, obj2.y)
plt.show()

I am using the lists generated by the class methods from the individual objects to plot. Any idea why I'm getting this error and am unable to plot? Thanks in advance.

psychedelic-snail
  • 347
  • 1
  • 3
  • 10
  • 1
    Second plot is `plt.plot(obj1.x, obj2.y)` instead of `plt.plot(obj2.x, obj2.y)` – Mad Physicist Mar 03 '20 at 13:22
  • Given that your code doesn't correspond to the error you are describing, it's very difficult to provide meaningful insight. The whole point of an MCVE is to generate the same type of error. Otherwise, what are you reproducing? – Mad Physicist Mar 03 '20 at 13:24
  • Sorry, I edited my post so that the error is the exact same when you run the code. Also, I thought plotting obj2.x instead of obj1.x shouldn't make a difference since they are the same values. Indeed, when I tried this, I still get the same error. – psychedelic-snail Mar 03 '20 at 13:30
  • 1
    Still learning Python, but it seems that with `obj2.run_calculation()` - the `self.y.append` - it seems that `y`'s size is 5 and after the append becomes 10. Seems like an issue within your class. – BigBen Mar 03 '20 at 13:56
  • 1
    For some reason, it seems both instances are appending the same list – Nicolas Gervais Mar 03 '20 at 20:23

1 Answers1

0

Your default argument of an empty list, for some obscure reason, makes it so the same list is appended (get the explanation here). Change this line so both instances work on separate lists: 

self.y = y.copy()

Here's your output: enter image description here

Nicolas Gervais
  • 33,817
  • 13
  • 115
  • 143