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data type and logarithm

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  data type and logarithm simona bellavista <afylot@gmail.com> - 2011-06-16 01:37 -0700
    Re: data type and logarithm Nobody <nobody@nowhere.com> - 2011-06-16 10:16 +0100
      Re: data type and logarithm "afylot@gmail.com" <antonella.garzilli@gmail.com> - 2011-06-16 02:27 -0700
    Re: data type and logarithm Peter Otten <__peter__@web.de> - 2011-06-16 13:55 +0200
    Re: data type and logarithm Terry Reedy <tjreedy@udel.edu> - 2011-06-16 13:20 -0400
    Re: data type and logarithm Robert Kern <robert.kern@gmail.com> - 2011-06-16 16:21 -0500

#7730 — data type and logarithm

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])
    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

Many thanks

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#7731

On Thu, 16 Jun 2011 01:37:08 -0700, simona bellavista wrote:

> print rho.dtype
> print entropy.dtype
> 
> I get  |S22 , what's that?

A string. You probably want to convert "columns" to floats before
appending its elements to the array.

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#7732

I tried to cast it to float by

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

but I get

TypeError: don't know how to convert scalar number to float

By the way, if I avoid to perform the logarithm and do a plot like

pylab.plot(rho, entropy)

it works!

Any idea?

On Jun 16, 11:16 am, Nobody <nob...@nowhere.com> wrote:
> A string. You probably want to convert "columns" to floats before
> appending its elements to the array.

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#7735

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]

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#7755

On 6/16/2011 4:37 AM, 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)

combine into one statement
file = open("sjlsjls.dat")

> rho = np.array([], dtype = 'float64')
> entropy = np.array([], dtype = 'float64')
> for line in file:
>      columns = line.split()

        r,e = line.split()

>      rho = np.append(rho,columns[0])
>      entropy = np.append(entropy,columns[1])

        rho = mp.append(rho, float(r)) # same with entropy)

does numpy really not let you write Python stype

        rho.append(float(r))
?

There are also numpy,scipy lists for numpy,scipy questions.

-- 
Terry Jan Reedy

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#7768

On 6/16/11 12:20 PM, Terry Reedy wrote:

> rho = mp.append(rho, float(r)) # same with entropy)
>
> does numpy really not let you write Python stype
>
> rho.append(float(r))
> ?

No. numpy arrays are not extensible in-place in general because we use view 
semantics for slices and similar operations like transpositions. We can't have 
the underlying memory change out from underneath us. This is one of the worst 
ways to accumulate values into a numpy array.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

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