Python: x-y-plot with matplotlib

Question

I want to plot some data. The first column contains the x-data. But matplotlib doesn't plot this. Where is my mistake?

import numpy as np
from numpy import cos
from scipy import *
from pylab import plot, show, ylim, yticks
from matplotlib import *
from pprint import pprint

n1 = 1.0
n2 = 1.5

#alpha, beta, intensity
data = [
    [10,    22,     4.3],
    [20,    42,     4.2],
    [30,    62,     3.6],
    [40,    83,     1.3],
    [45,    102,    2.8],
    [50,    123,    3.0],
    [60,    143,    3.2],
    [70,    163,    3.8],
    ]

for i in range(len(data)):
    rhotang1 = (n1 * cos(data[i][0]) - n2 * cos(data[i][1]))
    rhotang2 = (n1 * cos(data[i][0]) + n2 * cos(data[i][1]))
    rhotang = rhotang1 / rhotang2
    data[i].append(rhotang) #append 4th value

pprint(data)
x = data[:][0]
y1 = data[:][2]
y3 = data[:][3]
plot(x, y1, x, y3)
show()

EDIT: http://paste.pocoo.org/show/205534/ But it doesn't work.

tom10 · Accepted Answer · 2010-04-23 15:12:55Z

5

You can do this by converting data to a numpy array:

data = np.array(data) # insert this new line after your appends

pprint(data)
x = data[:,0]    # use the multidimensional slicing notation
y1 = data[:,2]
y3 = data[:,3]
plot(x, y1, x, y3)

A few additional points:

You can do the calculation in a more clear and vectorized way using numpy, like this

data = np.array(data)
rhotang1 = n1*cos(data[:,0]) - n2*cos(data[:,1])
rhotang2 = n1*cos(data[:,0]) + n2*cos(data[:,1])
y3 = rhotang1 / rhotang2

As you wrote it, your calculation may not give what you want since cos etc take radians as their inputs and your numbers look like degrees.

edited Apr 23, 2010 at 15:12

answered Apr 23, 2010 at 14:45

tom10

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Comments

Oddthinking · Accepted Answer · 2010-04-23 14:56:46Z

2

x = data[:][0]
y1 = data[:][2]
y3 = data[:][3]

These lines don't do what you think.

First they take a slice of the array which is the whole array (that is, just a copy), then they pull out the 0th, 2nd or 3rd ROW from that array, not column.

You could try

x = [row[0] for row in x]

etc.

answered Apr 23, 2010 at 14:56

Oddthinking

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Comments

Hooloovoo · Accepted Answer · 2010-04-25 07:20:20Z

Try this:

#fresnel formula

import numpy as np
from numpy import cos
from scipy import *
from pylab import plot, show, ylim, yticks
from matplotlib import *
from pprint import pprint

n1 = 1.0
n2 = 1.5

#alpha, beta, intensity
data = np.array([
    [10,    22,     4.3],
    [20,    42,     4.2],
    [30,    62,     3.6],
    [40,    83,     1.3],
    [45,    102,    2.8],
    [50,    123,    3.0],
    [60,    143,    3.2],
    [70,    163,    3.8],
    ])

# Populate arrays
x = np.array([row[0] for row in data])
y1 = np.array([row[1] for row in data])
rhotang1 = n1*cos(data[:,0]) - n2*cos(data[:,1])
rhotang2 = n1*cos(data[:,0]) + n2*cos(data[:,1])
y3 = rhotang1 / rhotang2

plot(x, y1, 'r--', x, y3, 'g--')
show()

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