Using matplotlib to plot the answer to ps1

Now that you know how to solve the first problem in Problem set 1, you have to graph the answer. I used matplotlib. The website for this plotting module can be found here.

Here's a copy of code to plot the data for ps 1:

#Plots x1 vs x2 for Problem 1 of the Stanford machine learning class

from pylab import *
import numpy as np
import numpy.linalg
from math import *

x1,x2=np.loadtxt('q1x.dat',unpack=True)
y=np.loadtxt('q1y.dat',unpack=True)

# we need to show when h=.5 as a line to separate the data. h=.5 when
# theta transpose x =0. so 0=theta0 +theta1*x1+theta2*x2
# solving for x2 give -theta0/theta2-theta1/theta2*x1

a=np.array([min(x1),max(x1),1,0.01])
b=-(-2.6205)/1.1719-0.7604/1.1719*a
plot(a,b)
# Use different colors and markers to plot x1 vs x2 if y=0 or y=1

for i in range(0,98):
    if y[i]==0:
        plot(x1[i],x2[i],'ro')
    if y[i]==1:
        plot(x1[i],x2[i],'bx')

xlabel('x1')
ylabel('x2')

show()

I used a for x1 and b as the calculated value for x2. Plotting a vs b gives that beautiful straight line across the plot.

One of my previous posts talked about expecting the unexpected in numpy. It happens again in this little short program. This line:
a=np.array([min(x1),max(x1),1,0.01])
was not what I though it was. I thought this gave me a list of values between the minimum of x1 and the maximum of x1. I was wrong. Here is what this actually gives:
  print a
[ 0.57079941  7.7054006   1.          0.01      ]

If you want a list of values, you need to use this code:

 a=np.linspace(min(z),max(z),25)

This gives 25 values between the minimum and maximum of your data set.

The rest of the code is very straight forward. Just set up a loop. If y=1, the data plots as a red circle. If y=0, the data plots as a blue x.

I know I have posted this graph before but it is so pretty, I'm going to post it again: