Simple plotting of log function in python

Question

I wrote a simple function to plot log in python:

import matplotlib.pyplot as plt
import numpy as np

x = list(range(1, 10000, 1))
y = [-np.log(p/10000) for p in x]

plt.scatter(x, y) # also tried with plt.plot(x, y)

plt.show()

I just want to see how the plot looks.

fn.py:5: RuntimeWarning: divide by zero encountered in log
  y = [-np.log(p/10000) for p in x]

I get the above error and on top of that I get a blank plot with even the ranges wrong.

It is strange why there is divide by zero warning, when I am dividing by a number?

How can I correctly plot the function?

Your code works as is, outputs this snaps.api-d.com/2019-01-30_19-37-27_wwxpv.png — WayToDoor
– WayToDoor, Commented Jan 30, 2019 at 18:37

Sheldore · Accepted Answer · 2019-01-30 19:46:01Z

3

Although you have tagged python-3.x, it seems that you are using python-2.x where p/10000 will result in 0 for values of p < 10000 because the division operator / performs integer division in python-2.x. If that is the case, you can explicitly use 10000.0 instead of 10000 to avoid that and get a float division.

Using .0 is not needed in python 3+ because by default it performs float division. Hence, your code works fine in python 3.6.5 though

import matplotlib.pyplot as plt
import numpy as np

x = list(range(1, 10000, 1))
y = [-np.log(p/10000.0) for p in x]

plt.scatter(x, y)
plt.show()

On a different note: You can simply use NumPy's arange to generate x and avoid the list completely and use vectorized operation.

x = np.arange(1, 10000)
y = -np.log(x/10000.0)

edited Jan 30, 2019 at 19:46

answered Jan 30, 2019 at 18:37

Sheldore

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4 Comments

WayToDoor Over a year ago

Woah good catch. I wouldn't have tought of that. For anyone wondering, this is because in python 2, deviding ints would output an int, while divividing floats & ints would return a float

Sheldore Over a year ago

@WayToDoor: Yes, indeed

Rafael Over a year ago

@Bazingaa python --version Python 2.7.10. Caught again in python versions, thanks a lot! I am trying to avoidpython 2.7 but somehow in many places people still use it and a lot of older code is there sue to which I have to create python 2 environments and ...it goes on.

Sheldore Over a year ago

@Rafael: Let me tag python 2 in your question for sake of readers

cdlane · Accepted Answer · 2019-01-30 19:09:09Z

1

Why import numpy and then avoid using it? You could have simply done:

from math import log
import matplotlib.pyplot as plt

x = xrange(1, 10000)
y = [-log(p / 10000.0) for p in x]

plt.scatter(x, y)
plt.show()

If you're going to bring numpy into the picture, think about doing things in a numpy-like fashion:

import matplotlib.pyplot as plt
import numpy as np

f = lambda p: -np.log(p / 10000.0)
x = np.arange(1, 10000)

plt.scatter(x, f(x))
plt.show()

answered Jan 30, 2019 at 19:09

cdlane

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