# Linear regression with matplotlib / numpy

### Question

I'm trying to generate a linear regression on a scatter plot I have generated, however my data is in list format, and all of the examples I can find of using `polyfit` require using `arange`. `arange` doesn't accept lists though. I have searched high and low about how to convert a list to an array and nothing seems clear. Am I missing something?

Following on, how best can I use my list of integers as inputs to the `polyfit`?

here is the polyfit example I am following:

``````from pylab import *

x = arange(data)
y = arange(data)

m,b = polyfit(x, y, 1)

plot(x, y, 'yo', x, m*x+b, '--k')
show()
``````
1
74
3/8/2017 1:45:27 PM

`arange` generates lists (well, numpy arrays); type `help(np.arange)` for the details. You don't need to call it on existing lists.

``````>>> x = [1,2,3,4]
>>> y = [3,5,7,9]
>>>
>>> m,b = np.polyfit(x, y, 1)
>>> m
2.0000000000000009
>>> b
0.99999999999999833
``````

I should add that I tend to use `poly1d` here rather than write out "m*x+b" and the higher-order equivalents, so my version of your code would look something like this:

``````import numpy as np
import matplotlib.pyplot as plt

x = [1,2,3,4]
y = [3,5,7,10] # 10, not 9, so the fit isn't perfect

fit = np.polyfit(x,y,1)
fit_fn = np.poly1d(fit)
# fit_fn is now a function which takes in x and returns an estimate for y

plt.plot(x,y, 'yo', x, fit_fn(x), '--k')
plt.xlim(0, 5)
plt.ylim(0, 12)
``````
159
6/18/2015 3:04:17 PM

This code:

``````from scipy.stats import linregress

linregress(x,y) #x and y are arrays or lists.
``````

gives out a list with the following:

slope : float
slope of the regression line
intercept : float
intercept of the regression line
r-value : float
correlation coefficient
p-value : float
two-sided p-value for a hypothesis test whose null hypothesis is that the slope is zero
stderr : float
Standard error of the estimate

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