Plotting a polynomial using Matplotlib and coeffiecients

Question

My code is:

import numpy as np
import matplotlib as plt
polyCoeffiecients = [1,2,3,4,5]
plt.plot(PolyCoeffiecients)
plt.show()

The result for this is straight lines that describe the points in 1,2,3,4,5 and the straight lines between them, instead of the polynomial of degree 5 that has 1,2,3,4,5 as its coeffiecients ( P(x) = 1 + 2x + 3x + 4x + 5x)

How am i suppose to plot a polynomial with just its coefficients?

Define values for x and use [`numpy.poly1d`](http://docs.scipy.org/doc/numpy-1.10.1/reference/generated/numpy.poly1d.html) — lanery, May 20 '16 at 17:08

moser · Accepted Answer · 2022-06-14T16:14:32.527

12

Eyzuky, see if this is what you want:

import numpy as np
from matplotlib import pyplot as plt

def PolyCoefficients(x, coeffs):
    """ Returns a polynomial for ``x`` values for the ``coeffs`` provided.

    The coefficients must be in ascending order (``x**0`` to ``x**o``).
    """
    o = len(coeffs)
    print(f'# This is a polynomial of order {o}.')
    y = 0
    for i in range(o):
        y += coeffs[i]*x**i
    return y

x = np.linspace(0, 9, 10)
coeffs = [1, 2, 3, 4, 5]
plt.plot(x, PolyCoefficients(x, coeffs))
plt.show()

edited Jun 14 '22 at 16:14

answered May 20 '16 at 17:15

moser

446
3
14

Thanks for the answers. I now understand my problem: I have multiplication of matrices: (np.linalg.pinv(XXt[i])*X[i]*Yt) Now this code returns an array of arrays : [[X1],[X2],....[Xm]] I want to convert this into [X1,X2,...,Xm] How can i do this? – Eyzuky May 20 '16 at 18:02
If you liked the answer, plz vote for it ;-) to flat a list, see http://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python . – moser May 21 '16 at 19:03

charlie1450 · Answer 2 · 2020-05-27T01:29:06.507

9

You could approximately draw the polynomial by getting lots of x-values and using np.polyval() to get the y-values of your polynomial at the x-values. Then you could just plot the x-vals and y-vals.

import numpy as np
import matplotlib.pyplot as plt

curve = np.array([1,2,3,4,5])
x = np.linspace(0,10,100)
y = [np.polyval(curve, i) for i in x]
plt.plot(x,y)

edited May 27 '20 at 01:29

answered May 24 '20 at 21:16

charlie1450

91
1
2

This answer is better and more elegant the the one chosen as "the" answer – Shaq Apr 19 '22 at 17:29

score 2 · Answer 3 · answered Nov 01 '19 at 23:08

A very pythonic solution is to use list comprehension to calculate the values for the function.

import numpy as np
from matplotlib import pyplot as plt

x = np.linspace(0, 10, 11)
coeffs = [1, 2, 3, 4, 5]
y = np.array([np.sum(np.array([coeffs[i]*(j**i) for i in range(len(coeffs))])) for j in x])
plt.plot(x, y)
plt.show()

score 2 · Answer 4 · answered Jun 13 '20 at 19:25

Generic, vectorized implementation:

from typing import Sequence, Union
import numpy as np
import matplotlib.pyplot as plt


Number = Union[int, float, complex]

def polyval(coefficients: Sequence[Number], x: Sequence[Number]) -> np.ndarray:
    # expand dimensions to allow broadcasting (constant time + inexpensive)
    # axis=-1 allows for arbitrarily shaped x
    x = np.expand_dims(x, axis=-1)
    powers = x ** np.arange(len(coefficients))
    return powers @ coefficients

def polyplot(coefficients: Sequence[Number], x: Sequence[Number]) -> None:
    y = polyval(coefficients, x)
    plt.plot(x, y)

polyplot(np.array([0, 0, -1]), np.linspace(-10, 10, 210))
plt.show()

Plotting a polynomial using Matplotlib and coeffiecients

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