# Strange set of errors

Stephen Webb sdavis.webb at gmail.com
Fri Aug 3 19:53:47 CEST 2007

```Greetings all,

I've recently begun using Python to do scientific computation, and I wrote
the following script to find approximate eigenvalues for a semi-infinite
matrix:

from pylab import *
from numpy import *
from scipy import *

def bandstructure(N,s):

b = s/4.0

jmax = 10 + N**2

spectrum1 = [0]*2*N
spectrum2 = [0]*2*N
spectrum3 = [0]*2*N

for k in arange(1, 2*N+1, 1):

A = zeros( (jmax,jmax) )

i = 0
while i <= jmax-1:
if i <= jmax-2:
A[i,i+1] = b
A[i+1,i] = b
A[i,i] = ((k + 2.0*i*N)/N)**2
i = i+1
else:
A[i,i] = ((k + 2.0*i*N)/N)**2
i = i+1

#This portion of the code builds a matrix twice as large to check
against

B = zeros( (2*jmax,2*jmax) )

i = 0
while i <= 2*jmax-1:
if i <= 2*jmax-2:
B[i,i+1] = b
B[i+1,i] = b
B[i,i] = ((k + 2.0*i*N)/N)**2
i = i+1
else:
B[i,i] = ((k + 2.0*i*N)/N)**2
i = i+1

x = linalg.eigvals(A)
y = linalg.eigvals(B)

j = 1
while j<=3:
if abs(y[j]-x[j]) <= 10.0**(-5):
j = j + 1
else:
print 'jmax not large enough to obtain accurate results'

spectrum1[k-1] = x[0] + 0.5*s
spectrum2[k-1] = x[1] + 0.5*s
spectrum3[k-1] = x[2] + 0.5*s

plot (k, spectrum1, k, spectrum2, k, spectrum3)

xlabel('k (energy level)')
ylabel('E/E_r')
title('Finite Size Band Structure, N = %d, s = %f' % (N, s))
grid(true)

When I run this script, I get the following message, which I can't figure
out:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "bandstruc.py", line 61, in bandstructure
plot (k, spectrum1, k, spectrum2, k, spectrum3)
File
"/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/matplotlib/pylab.py",
line 2028, in plot
ret =  gca().plot(*args, **kwargs)
File
"/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/matplotlib/axes.py",
line 2535, in plot
for line in self._get_lines(*args, **kwargs):
File
"/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/matplotlib/axes.py",
line 437, in _grab_next_args
for seg in self._plot_2_args(remaining[:2], **kwargs):
File
"/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/matplotlib/axes.py",
line 337, in _plot_2_args
x, y, multicol = self._xy_from_xy(x, y)
File
"/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/site-packages/matplotlib/axes.py",
line 266, in _xy_from_xy
nrx, ncx = x.shape
ValueError: need more than 0 values to unpack

Also, I've been having trouble with the plot function in matplotlib. For
example, I enter the following in the terminal:

>>> from pylab import *
>>> plot([1,2,3])

I'm reasonably sure that the issue in the first big error message is just an
indexing error on my part, but I have no idea what the second thing means.
Every time I run the plot([1,2,3]) I get a different ending number that
seems to vary randomly.