Index Error

[ErnestTBass]

I am trying to run a simple Python 3 program. I am getting at the moment an index error The code is as shown.

import numpy as np
import matplotlib.pyplot as plt
%matplotlib inline

topo = np.loadtxt('topo.asc',delimiter=',')

plt.plot(topo[0,:], label='North')

plt.plot(topo[-1,:],'r--', label='South')

plt.plot(topo[len(topo)/2,:], 'g:', linewidth=3, label='Mid')

plot.title('Topographic profiles')
plot.ylabel('Elevation (m)')
plot.xlabel('<-- West  East -->')
plot.legend(loc = 'lower left')

plt.show()

The error reads as below:

Error:
IndexError                                Traceback (most recent call last)
<ipython-input-4-fba3d9a19be8> in <module>
      9 plt.plot(topo[-1,:],'r--', label='South')
     10 
---> 11 plt.plot(topo[len(topo)/2,:], 'g:', linewidth=3, label='Mid')
     12 
     13 plot.title('Topographic profiles')

IndexError: only integers, slices (`:`), ellipsis (`...`), numpy.newaxis (`None`) and integer or boolean arrays are valid indices

Now I think I see what is wrong.I just do not know how to fix it. In line 11 there is a division by 2. There it it is possible that the index could not be an integer.

Thus the error occurs.

However, what do I do to fix it?

Thanks in advance.

Respectfully,

ErnestTBass