Plotting a function with matplotlib

Question

So I have defined a function, and for some reason the terminal is returning the following error:

TypeError: only length-1 arrays can be converted to Python scalars

I'm not sure what I have done wrong exactly?

Here is my self-contained function with corresponding plot:

import matplotlib
import math
import numpy
import matplotlib.pyplot as pyplot
import matplotlib.gridspec as gridspec

def rotation_curve(r):
    v_rotation = math.sqrt((r*(1.33*(10**32)))/(1+r)**2)
    return v_rotation

curve_range = numpy.linspace(0, 100, 10000)

fig = pyplot.figure(figsize=(16,6))

gridspec_layout = gridspec.GridSpec(1,1)
pyplot = fig.add_subplot(gridspec_layout[0])

pyplot.plot(curve_range, rotation_curve(curve_range))

matplotlib.pyplot.show()

Could anyone advise me where I have gone wrong?

please add the full error traceback.

cel
– cel

2015-10-21 16:16:55 +00:00
Commented Oct 21, 2015 at 16:16 — cel
– cel, Commented Oct 21, 2015 at 16:16

wflynny · Accepted Answer · 2015-10-21 16:31:39Z

1

The problem is in the definition of rotation_curve(r). You input and manipulate a numpy array (curve_range), but you do so using a non-vectorized function math.sqrt:

v_rotation = math.sqrt((r*(1.33*(10**32)))/(1+r)**2)

Instead, use numpy.sqrt which broadcasts the sqrt operation across every element in the array. The multiplication and exponentiation operators are overloaded in numpy arrays, so those should work fine.

def rotation_curve(r):
    v_rotation = numpy.sqrt((r*(1.33*(10**32)))/(1+r)**2)
    return v_rotation

answered Oct 21, 2015 at 16:31

wflynny

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2 Comments

GCien Over a year ago

Closer. It is now giving me the following ValueError: x and y must have same first dimension. Traces back to pyplot.plot(rotation_curve, rotation_curve(curve_range))

GCien Over a year ago

Fixed the problem, ignore previous comment.

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