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From: Peter Otten <__peter__@web.de>
Newsgroups: comp.lang.python
Subject: Re: data type and logarithm
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Date: Thu, 16 Jun 2011 13:55:52 +0200
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simona bellavista wrote:

> Hi, I am quite new to python and I am trying to do some simple plots.
> I am using python Python 2.6.4 and numpy/1.5.1
> I have an ASCII  data file that I am reading with the following lines
> of code:
> 
> import pylab
> import numpy as np
> 
> filename='something.dat'
> file = open(filename)
> 
> rho = np.array([], dtype = 'float64')
> entropy = np.array([], dtype = 'float64')
> for line in file:
>     columns = line.split()
>     rho = np.append(rho,columns[0])

You have to convert the string to a float, e. g.

     rho = np.append(rho, np.float64(columns[0]))

>     entropy = np.append(entropy,columns[1])
> 
> and rho and entropy are apparently read correctly, but when I look to
> the data type
> 
> print rho.dtype
> print entropy.dtype
> 
> I get  |S22 , what's that?
> Then I want to plot a logarithmic plot and I do
> 
> pylab.plot(np.log(rho), entropy)
> 
> and I get
> 
> NotImplementedError: Not implemented for this type
> 
> Does anybody has a clue? I do not know how to proceed

It should be easier to use numpy.loadtxt():

with open(filename) as f:
    a = np.loadtxt(f)
rho = a[..., 0]
entropy = a[..., 1]